Deodorization of Spirulina (Arthrospira platensis) to Improve the Sensory Attributes of Spirulina-Enriched Yogurt

Abstract

The incorporation of Arthrospira platensis into dairy products offers health benefits but is limited by its undesirable aroma and flavor. This study evaluated three deodorization strategies—adsorption by activated carbon, extraction with ethanol, and fermentation with Saccharomyces cerevisiae—to improve the sensory profile of A. platensis and enhance its acceptability in yogurt. Deodorized powders were characterized for proximal and volatile composition and used to formulate yogurts at five concentrations (0.5–2.5% w/v). Texture, aroma volatile profile, and sensory attributes were assessed after yogurt production, while shelf-life quality attributes were monitored over 29 days of refrigerated storage. Yogurts containing fermented A. platensis showed higher sensory scores (>8.7/10), the greatest purchase intent (>71.4%), and improved texture, remaining acceptable at an addition level of 2.5%. In contrast, yogurts with untreated or carbon-activated treated A. platensis were only acceptable at low addition levels (≤0.5%) due to off-flavors and textural issues. Ethanol effectively reduced aldehydes and ketones (such as Safranal and β-Ionone), while fermentation eliminated pyrazines and generated desirable alcohols and acids (such as 1-Pentanol and Butanoic acid). These findings highlight fermentation as a promising strategy to deodorize A. platensis and improve its integration into dairy matrices, enabling the development of functional yogurts with enhanced sensory quality and nutritionally relevant microalgae content.

1. Introduction

The development of new food products that incorporate functional or nutraceutical ingredients is a growing trend aiming to improve consumer health [1]. In this context, Arthrospira platensis is considered one of the most promising sources of functional food ingredients due to its high content of bioactive compounds, such as high biological value proteins (60–70%), polyunsaturated fatty acids (8–19% of linoleic), antioxidant pigments (1–4% of C-phycocyanin, 1.7 mg/g of carotenoids and 4.8 mg/g dry weight of chlorophyll), sulfated polysaccharides (SPP), essential minerals (Ca, Fe, K, Na, Mg), and vitamins (A and B-complex). These compounds offer significant health benefits (Neuroprotective, anti-inflammatory, antioxidative, hepatoprotective, antiaging, antitumor), contributing to disease risk reduction and improved nutritional status [2,3,4,5,6]. Previous studies have demonstrated that incorporating A. platensis biomass into various food products, such as pasta, bread, cookies, and beverages, enhances their nutritional profile and functional properties [7,8,9,10].

Yogurt is a widely consumed product recognized as a source of health-promoting principles. The physicochemical properties of yogurt and its manufacturing process facilitate the incorporation of adjuncts. As a result, it has been extensively explored as a vehicle to increase the content of bioactive ingredients in the human diet [11]. Recent studies have evaluated the addition of A. platensis in yogurt, reporting an increase in protein, fat, and ash content, enhanced antioxidant capacity, higher viscosity, improved water and oil retention capacity, reduced syneresis, as well as increased support for the development of probiotic bacteria such as Lactobacillus bulgaricus, Streptococcus thermophilus, and Bifidobacterium bifidum. Unfortunately, it has also been reported that the sensory acceptability of yogurt decreases as the addition of A. platensis exceeds 0.5% (w/w) due to the presence of “fish”, “sea”, “algae”, and “herbaceous” notes [12,13,14,15,16,17,18].

The FAO/WHO suggests a daily intake of A. platensis of 3 to 10 g, recommending between 0.5 and 3 g per serving [19,20,21]. Clinical studies on childhood malnutrition have reported an effective daily dose of 8 g [22]. The current sensory limit of 0.5% (w/w) A. platensis in yogurt implies the consumption of about 1.2 g of A. platensis per serving. Therefore, implementing strategies that minimize the undesirable sensory impact of A. platensis addition on yogurt is critical. Removing volatile compounds responsible for unpleasant odors may significantly improve the acceptability of A. platensis-enriched yogurt [23], allowing for the formulation of yogurt with increased amounts of algae.

Deodorization is a key process to reduce key volatile compounds such as aldehydes (hexanal), ketones (β-ionone), and sulfur compounds (dimethylsulfide) in algae used as food ingredients. In this regard, Cuellar-Bermúdez et al. [24] evaluated using solvents to eliminate the fishy odor from A. platensis, identifying ethanol-based extraction as the most effective treatment, capable of eliminating 18 volatile compounds. Additionally, Khalafu et al. [25] studied the elimination of fucoidan from brown algae through activated-carbon adsorption, successfully eliminating odor-causing compounds such as 2,4-Bis(1,1-dimethylethyl) phenol, one of the active odorants in fucoidan. Finally, in another study, Zhu et al. [26] evaluated the fermentation of Laminaria japonica with different microorganisms, achieving the most significant reduction in volatile compounds through Saccharomyces cerevisiae fermentation, eradicating 1-octen-3-one and lowering the concentration of unsaturated aldehydes responsible for the fishy odor by 91%.

Therefore, this study aimed to compare the effect of deodorizing A. platensis using either activated carbon, ethanol, or fermentation with S. cerevisiae on the sensory acceptance of A. platensis-enriched yogurt, as well as evaluating the impact of these deodorizing methods on the overall quality of the manufactured product.

2. Materials and Methods

In the present study, A. platensis powder was subjected to three different deodorization treatments: Activated carbon, Ethanol, and Fermentation. The deodorized powders were then used to formulate stirred yogurt at five addition levels: 0.5–2.5 g of A. platensis per 100 mL of yogurt (% w/v). Yogurt formulated at the same addition levels with non-deodorized A. platensis and yogurt with no A. platensis addition were also studied as control treatments.

The proximal and volatile composition of the studied A. platensis powders was determined. Produced yogurts were assessed for volatile compounds profile, texture profile parameters, and sensory evaluation, including texture in the spoon, texture in the mouth, odor, flavor, overall acceptability, and purchase intention. Color, pH, acidity, and syneresis were measured weekly for a month of refrigerated storage at 4 °C to evaluate the product’s stability.

2.1. Deodorization Treatments

Arthrospira platensis powder from Nature’s Heart^® (Tultitlán de Mariano Escobedo, Mexico) was used for all deodorization treatments.

2.1.1. Deodorization with Activated Carbon

Cylindrical pelletized activated carbon derived from coconut shell (Boyu^®, Zhejiang, China; Ø~4.5 mm) was used for deodorization. Activated carbon cylindrical pellets (325 g) were washed with running distilled water to remove dust residues and fine particles. Subsequently, the material was transferred to a beaker, and 750 mL of a 4% (w/v) aqueous suspension of A. platensis in distilled water was added. The mixture was stirred at 115 rpm for 17 h at room temperature, following the protocol described by Khalafu, Aida, Lim, and Maskat [25].

After treatment, the mixture was filtered through a 1 mm² sieve to remove activated carbon residues. The A. platensis was recovered by centrifugation at 4000× g for 10 min. The resulting paste was oven-dried at 65 °C for 16 h, ground using a mortar and pestle, and sieved through a No. 60 mesh (250 µm). Finally, the deodorized A. platensis powder was stored at 4 °C in the dark until use. A yield of 73% relative to the initial weight of the powder was obtained.

2.1.2. Deodorization with Ethanol

The ethanol deodorization process followed the procedure described by Cuellar-Bermúdez, Barba-Davila, Serna-Saldivar, Parra-Saldivar, Rodriguez-Rodriguez, Morales-Davila, Goiris, Muylaert and Chuck-Hernández [24]. 40 g of A. platensis powder was suspended in 1 L of absolute ethanol (MEYER, QUIMICA SUASTES S.A. de C.V., Alcaldia Tláhuac, Mexico). The mixture was stirred in a flask using a magnetic stirrer at 300 rpm for 60 min at room temperature.

After the contact period, the A. platensis was separated from the ethanol by filtration using Whatman No. 1 filter paper. The resulting paste was oven-dried at 65 °C for 16 h and then sieved through a No. 60 mesh (250 µm) without grinding. Finally, the deodorized A. platensis powder was stored at 4 °C in the dark until use. A yield of 90% relative to the initial weight of the powder was obtained.

2.1.3. Fermentation with Saccharomyces cerevisiae

The fermentation of A. platensis powder was conducted following the protocol proposed by Sahin et al. [27], with some modifications. A. platensis powder was suspended in distilled water at a concentration of 4% (w/v). To ensure complete cell disruption, it was subsequently subjected to four freeze–thaw cycles at −20 °C.

Subsequently, 5% (w/v) sucrose was added as a carbon source, and the A. platensis mixture was pasteurized at 65 °C for 30 min. Upon completion, it was cooled to 25 °C, and the pH was adjusted to 6.6. The mixture was inoculated with 1% (w/v) of baker’s instant dry yeast Tradi Pan^® (SAFMEX S.A. de C.V., Toluca, Mexico; S. cerevisiae). The 750 mL sample was placed in a 1 L flask and incubated at 28 °C in an orbital incubator at 130 RPM for 24 h.

At the end of the fermentation process, the liquid phase was separated by centrifugation at 4000× g for 10 min, and the resulting A. platensis paste was oven-dried at 65 °C for 16 h. Subsequently, it was ground using a mortar and pestle and sieved through a No. 60 mesh (250 µm). Finally, the deodorized A. platensis powder was stored at 4 °C in the dark until use. A yield of 80% relative to the initial weight of the powder was obtained.

2.2. Yogurt Formulation

Stirred yogurt was formulated following the procedure proposed by Chandan et al. [28], using the average amounts of additives commonly added to yogurt, ultra-pasteurized cow’s milk (Santa Clara, Pachuca, Mexico; 3 g/100 mL of protein, 3 g/100 mL of fat and 11.6 g/100 mL of total solids) was used as the base to which the following ingredients were added: 6% (w/v) sucrose, 3.5% (w/v) powdered milk (Alpura, Cuautitlán Izcalli, Mexico; 25.5 g/100 g of protein, 25.9 g/100 g of fat and 89 g/100 g of total solids) and 1.5% (w/v) corn starch (Unilever, Tultitlán, Mexico), to improve texture and flavor.

Once the mixture was homogenized, it was divided into different batches to manufacture the different designated treatments as follows: control yogurt (without A. platensis addition) (YT); yogurt added with non-deodorized A. platensis (YA); yogurt added with A. platensis deodorized using activated carbon (YC); yogurt added with A. platensis deodorized using ethanol (YE); and yogurt added with A. platensis fermented by S. cerevisiae (YF). Each A. platensis-enriched yogurt formulation was prepared at concentrations of 0.5, 1.0, 1.5, 2.0, and 2.5 g of A. platensis powder per 100 mL yogurt base (% w/v).

The mixture was heated to 85 °C for 30 min, then cooled to 40 °C and inoculated with Chr Hansen YF-L811 starter cultures (L. delbrueckii subsp. bulgaricus and S. thermophilus), following the manufacturer’s recommendations. Fermentation was monitored until the pH reached 4.7, to make post-acidification more evident. Subsequently, the yogurt was cooled to 20 °C and gently mixed, then placed in 20 g portions in one-ounce polypropylene cups with lids, which were then refrigerated for two days at 4 °C for sensory analysis.

Additionally, portions of approximately 10 g were placed in 15 mL conical tubes and stored at 4 °C for syneresis determination. For volatile profile analysis, 5 g of yogurt was transferred into 40 mL EPA vials (Supelco^®, cat. 23188), kept at 4 °C for two days, and stored at −80 °C until analysis.

2.3. Proximal Analysis

The proximal analysis of the deodorized A. platensis powders and the plain yogurt was performed only once, the day after manufacture (day 1). For this purpose, the following parameters were determined: moisture content using the gravimetric method, protein content by the micro Kjeldahl assay, and ash content by calcination, according to the standard methods of the AOAC [29]. Fat content was determined using the Roese–Gottlieb method [29] for plain yogurt and the modified Bligh and Dyer method [30] for A. platensis powders.

2.4. Analysis of Color, pH, and Acidity of Yogurts

Color measurements were performed using the CIELAB scale with a colorimeter (Minolta CR-300, Konica Minolta Sensing, Inc., Tokyo, Japan) on days 1, 8, 15, 22, and 29, following the protocol described by Barkallah, Dammak, Louati, Hentati, Hadrich, Mechichi, Ayadi, Fendri, Attia and Abdelkafi [15]. The equipment was adjusted to produce L*, a*, and b* readings. In this scale, L* represents lightness (ranging from 0 for black to 100 for white), a* denotes the red (+) to green (−) color space, and b* corresponds to the blue (−) to yellow (+) axis.

The pH of both non-fortified and A. platensis-fortified yogurt samples was determined on days 1, 8, 15, 22, and 29, using a pH meter (Hi 5222, HANNA) previously calibrated with standard buffer solutions at pH 4.0, 7.0, and 10.0. Titratable acidity (TA) was simultaneously assessed by titrating 3 g of the yogurt sample with 0.1 N NaOH, using phenolphthalein as an indicator. TA results were expressed as the percentage of lactic acid in the sample [15].

2.5. Analysis of Yogurt Syneresis

Yogurt syneresis was assessed on days 1, 8, 15, 22, and 29, following the method proposed by Robertson et al. [31], with some modifications. Approximately 10 g of yogurt samples were collected at the time of production and kept at 4 °C until analysis. They were centrifuged at 3000× g for 10 min at 4 °C. Syneresis was calculated from the whey volume (supernatant) generated after centrifugation, expressed as a percentage by weight, using Equation (1).

$$Syneresis \left(\%\right)=\left( {\displaystyle \frac{Weight of whey}{Weight of sample}}\right)\times 100$$

(1)

2.6. Sensory Evaluation of Yogurts Enriched with Deodorized A. platensis

Sensory evaluation was conducted on the second day after yogurt production, with six panelists participating in each test. Panelists were selected based on their knowledge of yogurt’s sensory characteristics. Four independent tests were performed, evaluating the following formulations [32]: yogurt with non-deodorized A. platensis (YA), yogurt with A. platensis deodorized with activated carbon (YC), yogurt with ethanol-deodorized A. platensis (YE), and yogurt with fermented A. platensis (YF).

In each test, panelists evaluated five yogurt samples with increasing concentrations of A. platensis (0.5, 1.0, 1.5, 2.0, and 2.5% w/v). Each sample was assigned a random code. Panelists were asked to rate the samples on a scale from 1 to 10 based on the following attributes: texture in the spoon, odor, flavor, and texture in the mouth. The obtained scores were averaged to determine an overall acceptance score. Additionally, panelists were asked whether they would purchase the evaluated yogurt. To minimize sensory fatigue, water was provided for palate cleansing between samples [32].

2.7. Texture Analysis of Yogurts

The texture of the yogurt was evaluated one day after production using a back extrusion test with a TA-XT plus texture analyzer (Texture Technologies Corp., New York, NY, USA) equipped with a 30 kg load cell. A sample was placed in a beaker for the test, ensuring a uniform depth of 30 mm. The samples were then refrigerated at 4 °C for one hour before analysis. The test was conducted at room temperature (25 °C) using a cylindrical probe with a diameter of 25 mm positioned at the center of the beaker containing the yogurt.

The instrument parameters were set as follows: pre-test speed of 1 mm/s, test speed of 1 mm/s, and post-test speed of 2 mm/s, with a trigger force detection of 0.005 kg. The probe compressed the sample to 50% of its original height (30 mm). The assessed texture parameters were hardness, adhesiveness, cohesiveness, gumminess, and elasticity [33].

2.8. Analysis of Volatile Organic Compounds

To quantify volatile organic compounds (VOCs) in A. platensis powders, a 2 g aliquot from each A. platensis powder treatment was placed in 10 mL vials (Agilent, Santa Clara, CA, USA) and heated at 50 °C for 10 min. The headspace was then exposed to a 75 μm Carboxen/polydimethylsiloxane fiber (SUPELCO, 57318) for 10 min at 50 °C for extraction. Subsequently, the fiber was introduced into the gas chromatograph (GC) injector at 250 °C for 5 min.

VOCs were detected using a gas chromatography system (GC 7890B; Agilent Technologies) coupled with a mass spectrometry detector (MS, Varian Saturn 2100D). Compound separation was performed using a DB-WAX capillary column (60 m × 0.25 mm × 0.25 µm; Agilent, Cat. 122-7062) with helium as the carrier gas (42 cm·s⁻¹). The column temperature was initially set at 40 °C, followed by an increase to 120 °C (3 °C min⁻¹) and then to 220 °C (7 °C min⁻¹). The mass range monitored by the analyzer was 35 to 400 m/z. Compounds were identified by comparing their spectra with data from the NIST database (MAINLIB, REPLIB, and TUTORIAL) [34].

In the case of yogurts, yogurt samples were thawed overnight at 4 °C. Subsequently, they were heated to 50 °C for 10 min. The headspace of each sample was then exposed to a 75 μm Carboxen/polydimethylsiloxane fiber (SUPELCO, 57318) at 50 °C for 10 min for extraction. The fiber was then introduced into the gas chromatograph (GC) injector at 250 °C for 5 min. The analysis followed the same steps used for the A. platensis powder samples described by Güler [35].

2.9. Statistical Analysis

All treatments were conducted in triplicate. A one-way analysis of variance (ANOVA) was performed to compare the data obtained, followed by Tukey’s test for multiple mean comparisons, with a significance level of 0.05. Sensory panelists’ purchase intent was analyzed using a non-parametric approach, specifically Pearson’s Chi-square test (p < 0.05). All statistical analyses were conducted using Minitab software, version 18.1.

3. Results and Discussion

3.1. Proximal Analysis

3.1.1. A. platensis Powder

The proximal analysis revealed significant differences in the ash, protein, lipid, and moisture content among the studied A. platensis powders subjected to different deodorization treatments (

Table 1. Proximal composition of deodorized A. platensis powder.

A. platensis Treatment	Ash 1		Protein 1		Fat 1		Moisture (%)
A	8.46 ± 0.10	A	60.92 ± 0.86	B	17.83 ± 0.64	A	6.62 ± 0.49	B
C	6.30 ± 0.04	B	60.20 ± 1.10	B	17.89 ± 0.50	A	6.89 ± 0.25	AB
E	8.53 ± 0.10	A	65.38 ± 1.23	A	10.29 ± 1.59	B	4.94 ± 0.43	C
F	5.29 ± 0.50	C	60.61 ± 1.53	B	10.61 ± 0.28	B	7.76 ± 0.09	A

¹ Results expressed in % (w/w) on a wet basis. A: Non-deodorized; C: Activated-carbon; E: Ethanol; F: Fermented. All data are presented as the mean of at least three replicates ± standard deviation (SD). Different letters within the same column indicate statistically significant differences (p < 0.05).

).

Some of the studied deodorization treatments significantly affected the ash content of A. platensis powders. While ethanol treatment caused no statistically significant change, the ash content in the activated carbon-treated and fermented samples was reduced by 25% and 37%, respectively. Previously reported ash content values range from 5.27% to 9.87% [24,36]. Differences in ash content can be attributed to the nature of the employed deodorization processes. The activated carbon and fermentation treatments involved immersion in aqueous solutions and prolonged agitation, which may have facilitated the leaching of minerals [37]. In the case of the ethanol treatment, the shorter processing time may have minimized mineral extraction. Additionally, some mineral salts exhibit low solubility in ethanol, which could explain the limited ash loss observed in this treatment. The more significant mineral loss observed in fermented A. platensis could also be explained by cell disruption induced by freezing and thawing, promoting the solubilization of cytoplasmic minerals into the process water [38].

The lipid content of A. platensis varied significantly depending on the deodorization treatment applied. The highest values were observed in the non-deodorized control (17.83%) and the activated carbon-treated sample (17.89%). The ethanol-treated (E) and fermented (F) samples exhibited lower lipid contents, 10.29% and 10.61%, respectively. Notably, the high lipid value obtained for the control sample resulted from subjecting the biomass to the same soaking, drying, and grinding steps used in the activated carbon treatment. When this pre-treatment was not applied, the lipid content in non-deodorized A. platensis dropped to 5.66 ± 0.74%, consistent with the range reported in the literature from 3.63% to 11.55% [38]. This finding suggests that the additional processing steps—particularly mechanical grinding after biomass dehydration—may facilitate greater lipid release and recovery during the modified Bligh and Dyer extraction, especially in microalgae with rigid cell walls [39,40]. In contrast, the reduction observed in ethanol-treated A. platensis can be attributed to the documented capacity of ethanol to dissolve lipids and dehydrate cells, a strategy previously reported to reduce lipid content in algae effectively [24]. Moreover, in the case of fermented samples, the freeze–thaw steps employed during processing likely caused cell rupture, releasing intracellular lipids that, due to their lower density, were partially lost during the centrifugation step.

The protein content of the studied powders in dry base ranged from 64.66% to 68.78%, with the highest value observed in A. platensis treated with ethanol. Previous studies have reported protein values ranging from 60.51% to 75.6% in processed spirulina [36,41].

The moisture content ranged between 4.94% and 7.76%, with the lowest levels observed in A. platensis treated with ethanol. The dehydration induced by this solvent can explain this effect [42]. In contrast, fermented A. platensis and A. platensis treated with activated carbon exhibited the highest moisture content, which could be associated with water retention due to modifications in biomass structure. This observation may be attributed to the milling process applied to these powders, which increased their water retention capacity by enlarging the water-binding surface area [43].

3.1.2. Yogurt Enriched with A. platensis

The proximal content of the studied yogurts significantly varied depending on the amount of added algae and the method employed for deodorization (Figure 1).

A significant effect of A. platensis addition was observed on the ash content of yogurt based on the deodorization process employed (Figure 1a). In general, an increase in ash content was observed as the amount of A. platensis powder increased. Yogurts containing activated carbon-treated and fermented A. platensis exhibited the lowest ash content compared to other treated variants. On the other hand, yogurts made with untreated and ethanol-treated A. platensis powder had higher ash content, exceeding that of the control yogurt, which is consistent with previous studies [14,15]. This increase in ash content can be attributed to the mineral composition of A. platensis, which contains calcium, iron, magnesium, and other essential minerals.

As expected, the protein content in yogurt was affected by the addition level of A. platensis (Figure 1b). As the concentration of A. platensis in the formulation increased, the protein content increased, ranging from 19.4% in the control yogurt to an average of 24.8% in yogurts containing 2.5% A. platensis. This observation is consistent with previous studies [44], which highlight the high protein content of A. platensis (50–70% of its dry weight). As shown in the previous section, deodorization treatments did not significantly affect protein content, resulting in a similar effect in yogurt. Variations in protein content were only observed as a function of the concentration of A. platensis powder added.

Overall, no significant variations in fat content were observed among the different formulations (Figure 1c). However, the observed trend suggests that untreated A. platensis and A. platensis treated with activated carbon may slightly increase lipid content compared to other treated variants. It is also evident that the small amount of A. platensis used in the formulation may not be sufficient to modify the final lipid concentration in the yogurt significantly.

The moisture content in the yogurt was similar among the different yogurts enriched with A. platensis powder, ranging from 81.56% in yogurt without algae to 79.64% in yogurt enriched at a concentration of 2.5% (Figure 1d). As the concentration of A. platensis increased, yogurt’s moisture content decreased. This observation is consistent with Bchir et al. [16], who reported that adding A. platensis increases the solid concentration, thereby reducing moisture content. Despite the observed differences, moisture values are falling within the established range for commercial yogurts (<84%), according to Igbabul et al. [45], ensuring appropriate texture and mouthfeel.

Barkallah et al. [15] reported that the total solid, protein, fat, and ash contents were higher in yogurts enriched with A. platensis compared to the controls. Similarly, da Silva et al. [41] observed that adding dry A. platensis increases the solids of yogurt, explaining the lower moisture content in formulations with higher A. platensis concentrations. Therefore, incorporating A. platensis in yogurt significantly influences the product’s proximal composition. The choice of A. platensis deodorizing treatment and the added amount allow for the modulation of moisture, protein, and ash content, making it a key factor in developing functional products with an improved nutritional profile.

3.2. Analysis of pH, Acidity, and Color

The pH decreased significantly (p < 0.05, ANOVA) over storage time (

Table 2. Color, pH, and acidity in yogurts enriched with A. platensis during storage.

Parameter Measured	Storage (Days)	Yogurt Enriched with A. platensis
		YT	YA					YE					YC					YF
		0%	0.5%	1.0%	1.5%	2.0%	2.5%	0.5%	1.0%	1.5%	2.0%	2.5%	0.5%	1.0%	1.5%	2.0%	2.5%	0.5%	1.0%	1.5%	2.0%	2.5%
pH	1	4.57	4.46	4.51	4.50	4.47	4.46	4.47	4.49	4.52	4.50	4.52	4.51	4.52	4.56	4.61	4.53	4.53	4.54	4.57	4.62	4.53
	8	4.46	4.35	4.41	4.40	4.40	4.41	4.30	4.34	4.35	4.34	4.36	4.34	4.36	4.38	4.37	4.38	4.40	4.40	4.47	4.50	4.43
	15	4.44	4.32	4.38	4.37	4.37	4.39	4.28	4.31	4.34	4.33	4.37	4.35	4.36	4.38	4.35	4.37	4.34	4.50	4.43	4.47	4.38
	22	4.42	4.33	4.38	4.38	4.38	4.43	4.25	4.29	4.31	4.32	4.34	4.31	4.40	4.33	4.39	4.32	4.30	4.46	4.44	4.46	4.35
	29	4.41	4.30	4.33	4.35	4.36	4.39	4.25	4.30	4.32	4.34	4.35	4.34	4.41	4.35	4.33	4.35	4.27	4.38	4.39	4.41	4.32
Acidity (%)	1	0.87	1.03	1.01	1.07	1.07	1.13	0.91	0.93	0.96	1.00	0.93	0.99	0.94	0.85	0.81	0.82	0.95	0.82	0.90	0.84	1.06
	8	0.92	1.04	1.07	1.09	1.13	1.16	0.98	1.01	1.00	1.05	1.03	1.04	1.02	0.98	1.02	1.00	0.96	0.88	0.92	0.86	1.05
	15	0.93	1.05	1.08	1.10	1.13	1.16	1.07	1.08	1.08	1.12	1.10	1.06	1.07	1.09	1.15	1.06	1.00	0.92	1.01	0.91	1.12
	22	0.96	1.04	1.04	1.11	1.11	1.14	1.08	1.09	1.07	1.12	1.13	1.03	1.00	1.01	1.11	1.10	1.01	0.97	1.01	0.97	1.12
	29	0.96	1.07	1.07	1.11	1.11	1.14	1.09	1.09	1.09	1.11	1.10	1.03	1.00	1.01	1.11	0.99	1.00	0.97	1.01	0.97	1.15
L*	1	90.35	73.06	67.42	61.17	56.27	51.22	69.90	64.41	61.55	56.43	51.99	73.47	63.46	57.03	52.10	49.10	74.61	67.71	64.16	61.16	56.65
	8	88.82	74.14	66.06	61.83	56.09	51.53	71.99	65.86	61.49	54.65	51.81	73.69	63.40	57.87	53.37	50.37	75.89	68.89	65.17	61.32	58.21
	15	91.15	74.26	66.26	62.16	56.73	52.03	73.25	66.93	62.31	58.23	52.45	73.71	64.03	57.58	53.33	50.33	75.46	69.38	65.09	61.59	58.82
	22	90.21	74.39	67.56	62.70	56.57	52.28	72.90	66.68	62.63	55.61	52.33	73.57	63.70	57.72	54.38	51.38	75.66	68.97	64.99	62.15	58.96
	29	90.80	74.38	66.28	62.10	56.10	51.01	73.02	66.90	62.52	56.04	52.66	74.26	64.55	58.32	54.01	51.01	75.08	68.23	64.48	62.61	57.03
a*	1	−2.65	−5.87	−5.86	−7.58	−8.07	−7.68	−4.99	−5.81	−5.5	−6.73	−7.06	−3.06	−3.05	−3.04	−2.66	−2.40	−2.75	−2.63	−2.50	−2.33	−2.02
	8	−2.59	−5.46	−5.95	−6.85	−7.07	−7.11	−4.80	−5.62	−5.01	−6.11	−6.52	−2.83	−2.79	−2.91	−2.71	−2.45	−2.92	−2.79	−2.74	−2.17	−2.28
	15	−2.65	−5.09	−5.61	−6.30	−6.44	−6.61	−4.68	−5.49	−4.96	−6.07	−6.52	−2.78	−2.82	−2.77	−2.57	−2.31	−2.87	−2.94	−2.69	−2.41	−2.37
	22	−2.62	−4.81	−5.4	−6.21	−6.13	−5.66	−4.53	−5.25	−4.88	−6.01	−6.14	−2.72	−2.64	−2.58	−2.51	−2.25	−2.89	−2.83	−2.56	−2.52	−2.40
	29	−2.54	−4.6	−4.94	−5.67	−5.76	−5.66	−4.40	−5.21	−4.91	−5.90	−6.05	−2.85	−2.89	−2.80	−2.52	−2.32	−2.75	−2.68	−2.54	−2.55	−2.08
b*	1	7.24	6.06	6.65	8.31	9.29	10.10	4.43	5.21	4.72	4.86	5.25	5.45	6.64	6.47	6.78	6.96	4.89	5.29	5.97	6.38	7.01
	8	6.92	6.44	6.66	8.18	9.16	10.28	4.75	5.55	4.71	5.19	5.47	5.57	6.48	6.68	7.32	7.50	5.24	5.46	6.15	5.70	6.73
	15	7.00	6.71	7.55	8.59	9.55	10.51	4.64	5.51	4.77	5.35	5.39	5.39	6.15	6.49	6.99	7.17	5.26	5.80	6.66	6.24	7.31
	22	7.16	6.48	7.45	8.73	9.27	9.44	4.93	5.59	4.94	5.37	5.48	6.14	6.28	6.93	8.04	8.22	5.52	5.57	6.16	6.23	7.35
	29	6.94	6.55	7.31	8.83	9.91	10.26	4.84	5.58	5.17	5.20	5.46	5.88	6.96	7.30	7.90	8.15	5.34	5.62	6.29	6.51	7.13
Calculated color	1
	29

YT: Yogurt without A. platensis powder, YA: Yogurt with untreated A. platensis powder, YE: Yogurt with A. platensis powder deodorized with ethanol, YC: Yogurt with A. platensis powder deodorized with activated carbon, YF: Yogurt with A. platensis powder fermented. The data shown are the average of three measurements.

). Yogurt samples containing non-deodorized A. platensis (YA) exhibited a lower initial pH (day 1) (4.46–4.51) compared to the control (YT: 4.57), correlating with a higher initial acidity (1.03–1.13% vs. 0.87%). This trend aligns with previous studies indicating that A. platensis can act as a natural fermentation enhancer [46,47]. This property has been attributed to organic compounds in microalgae, such as chlorophyll, carotenoids, exopolysaccharides, fatty acids, and bioactive peptides [48,49].

In contrast, deodorized formulations (YE, YC, and YF) exhibited initial pH and acidity values closer to the control. This observation suggests that deodorization might mitigate primary acidification or reduce the fermentation-promoting effect observed in previous studies. This effect could be related to a potential reduction in bioactive compounds that stimulate the growth of lactic acid bacteria (LAB). However, throughout storage, YE, YC, and YF showed a more significant pH decline (0.19, 0.19, and 0.20 units, respectively) than YA (0.13 units). By day 29, all A. platensis-enriched formulations reached an average pH of 4.35, significantly lower than the pH observed in control samples of 4.41 (p < 0.05).

The progressive pH reduction (ΔpH ≈ 0.18 units) and acidity increase (Δ ≈ 0.11%) in all formulations suggest residual metabolic activity of lactic cultures or interactions between the yogurt matrix and A. platensis powders. Patel et al., [47] reported a 20% reduction in fermentation time and a pH decrease from 4.43 to 4.17, with acidity increasing from 0.84% to 1.22% over an 18-day storage period in A. platensis-enriched yogurts, which is comparable to the results found in the present study.

The addition of A. platensis drastically reduced luminosity (L*) in yogurts, following a quadratic concentration-dependent relationship (R² > 0.95). The YC 2.5% formulation exhibited the lowest L* value (49.10), while the highest was observed in YE 0.5% (69.90) (Table 2). A progressive decrease in luminosity was noted as A. platensis concentration increased (ΔL* ≈ 10.06). The YC formulation showed the highest L* reduction rate (−12.02 units per concentration unit). In contrast, the lowest effects were observed in YF and YE (−8.49 and −8.76 units per concentration unit, respectively), resulting in lighter yogurts, likely due to pigment loss during deodorization. These findings are consistent with previous studies indicating that A. platensis, in free or microencapsulated form, significantly affects yogurt colorimetric values, particularly luminosity (L*), impacting the product’s visual perception [41,50].

Overall, greenness (negative a*) was more pronounced in YA. The YC and YF formulations exhibited a lower green hue, suggesting that deodorization treatments led to pigment loss, particularly chlorophyll. A gradual loss of green hue was observed during storage, especially at high concentrations of YA and YE. Conversely, an increase in yellow tonality was recorded in YA, YC, and YF as a function of concentration (R² > 0.99), with no visible changes in YE. All formulations showed increased b* coordinate values over storage time (R < 0.75). The observed reduction in greenness (a*) and the increase in yellowness (b*) may be attributed to chlorophyll oxidation and the formation of degradation products such as pheophytins [51]. This trend has also been reported by Nourmohammadi, Soleimanian-Zad and Shekarchizadeh [17], who observed a decline in greenness and an increase in yellowness over storage time in A. platensis-enriched yogurts.

3.3. Syneresis

Syneresis is a fundamental parameter for evaluating the structural stability of yogurt. Figure 2 shows the syneresis observed in the studied yogurt formulations during four weeks of refrigerated storage. Plain yogurt (YT) formulated without adding A. platensis showed a decreasing tendency to release whey during storage, varying from 30.65% on day one to 25.68% on day 29. This behavior has been previously reported by Nourmohammadi, Soleimanian-Zad and Shekarchizadeh [17] who observed that yogurt syneresis decreases during the first week, probably due to the increase in hydrogen bonds between the caseins and water, which increases the water retention capacity within the protein network and therefore decreases syneresis. This decrease in syneresis could also be favored by employing a formulation in which starch and powdered milk are added [52,53].

Conversely, the syneresis observed in yogurts formulated with A. platensis remained stable, with some treatments showing a slight tendency to increase their syneresis during refrigerated storage. Average syneresis values across storage time were 34.51%, 32.09%, 28.26%, and 26.93% for treatments formulated with carbon-activated-deodorized A. platensis (YC), fermented A. platensis (YF), ethanol-deodorized A. platensis (YE), and non-deodorized A. platensis (YA), respectively. All these average values remained close to the syneresis range observed for plain yogurt (YT, 28.36%).

Yogurt formulated with A. platensis deodorized with activated carbon (YC, Figure 2b) showed a positive correlation between syneresis and storage time (R² > 0.75). The syneresis observed in YC at the 0.5% addition level (25.93 ± 0.04%) did not significantly differ from that observed in plain yogurt (YT) (28.34 ± 0.03%). However, at concentrations of 1.5%, 2.0%, and 2.5%, syneresis was higher (35.27 ± 0.02%, 35.44 ± 0.03%, and 34.29 ± 0.03%, respectively), with the highest syneresis recorded at an addition level of 1.0% (41.64 ± 0.03%). Previous studies suggest that activated carbon can alter the electrostatic charge of A. platensis surface proteins, leading to peptide absorption, limiting their interaction with dairy proteins, and promoting syneresis depending on pH and ionic strength [54,55].

In yogurt formulated with non-deodorized A. platensis (YA, Figure 2a), syneresis was lower at a 0.5% concentration (22.38 ± 0.02%), with no significant differences (p > 0.05) found among higher concentrations and plain yogurt (YT). Similar results were described by Nourmohammadi, Soleimanian-Zad and Shekarchizadeh [17], who found a stabilizing effect at concentrations of 0.5%. Yogurt formulated with fermented A. platensis at an addition level of 0.5% (YF, Figure 3) also showed syneresis comparable to that observed in plain yogurt (YT) (28.20 ± 0.02%). However, adding higher concentrations of fermented A. platensis caused higher syneresis levels (up to 35.37 ± 0.01%). Finally, in yogurt formulated with A. platensis deodorized with ethanol (YE, Figure 3), only the 2.5% treatment exhibited significantly higher syneresis (31.08 ± 0.05%) than the rest of the studied concentrations.

Previous studies indicate that adding A. platensis to yogurt can reduce syneresis, although the effect depends on the concentration and storage span. Barkallah, Dammak, Louati, Hentati, Hadrich, Mechichi, Ayadi, Fendri, Attia, and Abdelkafi [15] reported that A. platensis can act as a stabilizer at a concentration of 0.25%, which is significantly lower than the addition levels considered in the present study. In contrast, Aurora et al. [56] observed that concentrations above 5% increased syneresis in goat yogurt, attributing this behavior to pH reduction, which decreases casein solubility and promotes water expulsion [56,57]. Other studies have confirmed that low concentrations (<1%) of A. platensis increase the water-holding capacity (WHC) of yogurt [14,15], attributing this effect to a higher content of soluble fiber and protein [58]. However, Pan-Utai et al. [59] observed that at high concentrations of A. platensis (>5% fresh), the WHC of yogurt is reduced, due to the presence of insoluble fibers that alter the protein network, since A. platensis is known to contain between 11.2% and 19.6% of insoluble fiber [60].

3.4. Sensory Evaluation of Yogurts Enriched with A. platensis

Incorporating A. platensis into fermented dairy products can significantly alter their sensory attributes, affecting consumer acceptance [61]. In this study, the sensory attributes spoon texture, odor, flavor, mouthfeel, and overall acceptability (Figure 3), as well as purchase intention (Figure 4), were evaluated in yogurts enriched with A. platensis powders subjected to different deodorization treatments: untreated (YA), deodorized with activated carbon (YC), deodorized with ethanol (YE), and fermentation with S. cerevisiae (YF).

The texture on the spoon was negatively affected by increasing the concentration of A. platensis, particularly in the YC formulations, where 2.0% and 2.5% treatments received the lowest scores (~6.3). YA and YF at 0.5% achieved the highest ratings (~9.5) among the studied treatments. The reduced scores observed in YC may be due to non-polar particles resulting from the treatment, which disrupt the gel protein network [62].

Odor differences among formulations were less pronounced. However, YC at 2.0% and 2.5% was rated less favorably (7.5–8.1), potentially due to the removal of desirable aromatic compounds during activated carbon adsorption, as reported in other food systems [63].

Flavor was the most sensitive attribute, affected by the deodorization method and the A. platensis concentration. At 0.5%, all formulations were well accepted (7.9–9.2). As concentrations increased, flavor acceptability decreased, starting from 1.5% in YA and YE and from 1.0% in YC. These results are consistent with previous findings indicating that high concentrations of A. platensis tend to reduce sensory acceptability due to the development of metallic or unpleasant flavors, often attributed to lipid oxidation and pro-oxidant minerals in yogurt [14,15,16,44]. In contrast, YF consistently scored above 8.1 for all concentrations, demonstrating the potential of fermentation to mitigate the presence of undesirable compounds such as pyrazines, which are known to cause off-flavors [27,64].

For the attribute texture in the mouth, formulations containing 0.5% A. platensis (YF, YA, and YE) once again received the highest scores (≥9.3), while YC at 2.0% and 2.5% received the lowest scores (6.2–6.7). Panelists described these samples as gritty or lumpy, a phenomenon also reported in other studies that linked granularity to insoluble particles in spirulina [65].

Overall acceptability reflected a clear preference for formulations containing 0.5% A. platensis, particularly YE, YA, and YF, all of which scored above 9.2. In contrast, YC at 2.0% and 2.5% was the least accepted (<6.9), described as bitter, with herbal notes, and a lumpy, watery texture. Conversely, YF was described as pleasant, with a fermented flavor and sweet notes reminiscent of “candied pumpkin,” a creamy texture, and mild acidity, even at high concentrations.

Purchase intention was strongly influenced by overall acceptability, flavor, and mouthfeel. As shown in Figure 4, yogurts enriched with fermented A. platensis (YF), particularly at 0.5%, 1.0%, and 1.5%, achieved the highest purchase intentions, outperforming all other treatments. Even at 2.0% and 2.5%, YF maintained high purchase intention (~80%), a trend not observed in any other formulation.

In contrast, yogurts with A. platensis untreated (YA), treated with ethanol (YE), or treated with activated carbon (YC), showed a marked decline in purchase intention above 1.5%, reaching levels as low as 0% in YC-2%. These findings underscore the strong relationship between the deodorization method, A. platensis concentration, and consumer acceptance.

These findings confirm that A. platensis concentrations above 0.5% negatively affect yogurt’s sensory acceptability [15,66], especially when using untreated or activated carbon–treated A. platensis. In contrast, fermentation improves the sensory profile of enriched yogurt even at higher concentrations, positioning it as the most effective strategy to increase the concentration of A. platensis in functional foods.

3.5. Yogurt Texture Profile

Yogurt texture is a key sensory attribute that significantly influences consumer acceptance. This study evaluated the parameters of hardness, adhesiveness, cohesiveness, gumminess, and springiness in control yogurts and yogurts enriched with different concentrations of deodorized A. platensis. The results showed that adding A. platensis significantly affected texture, depending on the deodorization method and concentration. The most notable changes were observed in the attributes of hardness, adhesiveness, and gumminess, while cohesiveness and springiness were less affected (Figure 5).

Yogurt with fermented A. platensis (YF) showed the highest values of hardness (0.71 ± 0.04 N), adhesiveness (−2.45 ± 0.23 N·s), and gumminess (0.59 ± 0.03 N), even surpassing the control (YT). This behavior suggests that fermentation could enhance the yogurt matrix, possibly by adding exopolysaccharides [67]. In contrast, YC displayed a progressive decrease in these parameters with increasing concentrations of A. platensis (R² > 0.95), indicating that activated carbon treatment may weaken the protein network. YA exhibited low values without a clear trend, whereas in YE, adhesiveness increased with concentration, though gumminess remained largely unaffected. Cohesiveness was slightly higher in YC (0.90 ± 0.04), and no significant differences were detected in springiness between formulations (average ≈ 0.96, p > 0.05).

Incorporating plant-based compounds can modify yogurt texture depending on the type and amount added. Moderate doses typically improve firmness and viscosity, while higher amounts may destabilize the gel structure [67,68]. In this study, fermentation emerged as the most promising treatment to improve yogurt texture, whereas activated carbon caused adverse effects. These results align with previous findings by Bchir et al. [16], who reported no textural changes with 0.5% A. platensis, and Patel, Jethani, Radha, Vijayendra, Mudliar, Sarada and Chauhan [47] who observed a reduction in firmness with increasing concentrations of fresh biomass of A. platensis, deeming products unfit for consumption beyond 8% biomass. Similarly, Barkallah, Dammak, Louati, Hentati, Hadrich, Mechichi, Ayadi, Fendri, Attia and Abdelkafi [15] attributed firmness loss to gel network disruption. Furthermore, the presence of lipids in the microalgae and their interaction with dairy proteins may influence texture and mouthfeel [61].

3.6. Volatile Compounds

3.6.1. Volatile Compounds in A. platensis Powders

Microalgae, including Arthrospira platensis, produce a wide range of volatile organic compounds (VOCs), such as aldehydes, ketones, alcohols, esters, terpenes, furans, pyrazines, hydrocarbons, and sulfur- or nitrogen-containing compounds. The profile of these compounds can vary significantly depending on cultivation conditions, processing, and storage [5]. In this study, deodorization treatments substantially altered the volatile composition of A. platensis. A total of 46 compounds were identified in the non-deodorized sample (A), 39 in the activated carbon-treated sample (C), 33 in the ethanol-treated sample (E), and 39 in the fermented sample (F) (Supplementary Table S1).

The chemical classes detected include hydrocarbons, alcohols, ketones, aldehydes, acids, furans, pyrazines, and aromatic compounds.

Table 3. Classification and relative content of volatile organic compounds (VOCs) in deodorized and undeodorized A. platensis powders.

Chemical Class	A. platensis Treatment				Relative Content *
	A	C	E	F
Hydrocarbons	8	4	3	3
Alcohols	8	8	7	9
Ketones	12	12	10	12	0%
Aldehydes	6	6	4	5	<25%
Acids and esters	3	3	3	4	25–50%
Furans	6	5	5	4	50–75%
Pyrazines	2	1	1	1	75–100%
Aromatics compounds	1	0	0	1	>100
Total	46	39	33	39

A: Non-deodorized; C: Activated Carbon; E: Ethanol; F: Fermented. * The relative concentration was calculated based on the area of the chromatographic peaks relative to that of peaks found in undeodorized A. platensis (A). Greater color saturation represents a higher relative content in that group. The numbers represent the number of VOCs detected within each chemical group.

shows the number of compounds identified in each class and their relative content compared to the non-deodorized sample.

Activated carbon treatment (C) eliminated seven compounds, reducing the total volatile content to 66% compared to the sum of the total area of VOCs found in undeodorized A. platensis (A). This reduction was mainly due to the removal of hydrocarbons, although increases were observed in the relative abundance of some chemical classes such as acids and aldehydes. Ethanol treatment (E) was the most effective at reducing the number and relative concentration of volatiles (17% compared to sum of the total area of VOCs in A), eliminating 13 compounds. It was particularly efficient at removing hydrocarbons and aldehydes. Fermentation (F) removed seven compounds, mainly hydrocarbons, and generated three new ones (2-methyl-5-hexene-3-ol, 3-methylbutanoic acid, and 2-nonanone). The total volatile content relative to the sum of the total area of VOCs in A remained at 73%, with an increase observed in the proportion of alcohols and acids.

Eight major hydrocarbons were identified in the volatile fraction of A. platensis. The non-deodorized sample (A) displayed the most complex profile, indicating that this chemical class constitutes a significant portion of the volatile profile in A. platensis [5,69]. All three deodorization treatments eliminated Pentadecane, likely enhancing sensory acceptability, as it is considered a key compound in the native aroma profile. It originates from the decarboxylation of saturated fatty acids such as palmitic and stearic acids and is associated with unpleasant odors such as “seaweed,” “cardboard,” and “cereal” [69,70].

Nine alcohols were identified, with concentrations varying depending on the treatment. Fermentation led to a marked increase in compounds such as 1-pentanol (539%) and phenyl ethyl alcohol (946%), both typical products of yeast and lactic acid bacteria metabolism via the Ehrlich pathway from amino acids such as leucine and phenylalanine [27,69]. The increase in the concentration of these compounds may enhance the aromatic profile by contributing floral and fruity notes, although excessive levels can become overpowering [71]. Conversely, alcohols such as hexanol, cyclohexanol (1,3-dimethyl-, cis-), and 2,4-pentadien-1-ol (3-pentyl-) decreased after deodorization, especially with ethanol, where their concentrations dropped below 10%. This highlights ethanol’s effectiveness as a solvent for moderately polar compounds responsible for herbaceous, musty, or earthy odors, such as hexanol [69,72].

Twelve ketones were detected, their relative abundance dependent on the deodorization treatment employed. Compounds such as 2-octanone, 2-heptanone, and 6-methyl-2-heptanone showed substantial reductions under all treatments. 2-Butanone, associated with solvent-like odors, was virtually eliminated (>99%) in all cases. 6-Methyl-2-heptanone, with a low odor threshold (0.008 mg/kg), was reduced to 35%, 8%, and 5% in C, E, and F, respectively. This reduction is desirable as these compounds are linked to unpleasant notes such as “rancid butter,” “earthy,” or “moldy” aromas [69,72]. 2-Nonanone was detected exclusively in ethanol and fermented treatments, suggesting its formation during fermentative processes or released from the cellular matrix. This aligns with reports in other fermented microalgae, where volatile compounds arise from lipid catabolism or microbial transformation [27,73]. Cyclic ketones and carotenoid-derived compounds are generated through the oxidative cleavage of β-carotene and have been reported as key contributors to the aromatic profile of A. platensis and other microalgae [70,74]. Compounds such as Isophorone, α-Ionone, β-Ionone, and their epoxides were significantly reduced following deodorization. β-Ionone, which has an extremely low odor threshold (0.0001 mg/kg), was reduced to 46%, 10%, and 35% in treatments C, E, and F, respectively. Its residual presence may contribute to the final aroma with violet- and woody-like notes [69,72].

Aldehydes are key compounds in sensory perception due to their low odor threshold [70,73]. The six detected aldehydes showed high sensitivity to the treatments. Hexanal, one of the most frequent aldehydes with high sensory intensity (green, grassy, or fishy notes) [70,75], increased significantly after activated carbon treatment (C: 218%) but decreased markedly after ethanol treatment (E: 18%) and fermentation (F: 60%). This difference may be explained by lipid oxidation induced by C, while fermentation likely reduced its concentration due to microbial metabolism, in agreement with Sahin, Hosoglu, Guneser and Karagul-Yuceer [27]. β-Cyclocitral and safranal, carotenoid-derived aldehydes, were reduced in all three treatments, particularly in E and F. This improves the sensory profile due to their low threshold (0.003 mg/kg) and their association with herbal and minty notes [69,70]. Aldehydes were generally sensitive to deodorization treatments, reducing undesirable compounds such as hexanal and butanals after ethanol extraction and fermentation. The effectiveness of these treatments may be attributed to yeast enzymatic activity, which transforms aldehydes into less volatile and more neutral alcohols or acids [27,72].

Four compounds classified as carboxylic acids and esters were identified. All of them are considered of significant sensory impact, primarily associated with strong odors [72]. Butanoic acid was the most prominent, increasing to 355% (C) and 226% (F). This compound is associated with “cheesy” notes and is characteristic of fermentation with S. cerevisiae [27,72]. On the other hand, 3-methylbutanoic acid, which imparts lactic-acid-like notes, was detected exclusively after fermentation, suggesting its origin from amino acid metabolism, particularly leucine [27]. Additionally, hexanoic acid increased in C (204%) and F (121%). This acid is linked to fruity and cheesy notes [72]. Its increase in fermented samples has been previously reported as a product from incomplete β-oxidation of fatty acids by yeast strains, contributing to more complex and persistent sensory profiles [27].

Two pyrazines were detected, with trimethylpyrazine being eliminated by all treatments. Meanwhile, 2,5-dimethylpyrazine was significantly reduced (down to 2% in F), in agreement with previous studies reporting its metabolism by yeasts, as these compounds can be used as carbon and nitrogen sources under aerobic conditions [27]. In products such as A. platensis, 2,5-dimethylpyrazine is associated with bitter and nutty flavors [70,74].

Finally, the only aromatic compound identified—benzene,1,3-bis(1,1-dimethylethyl)—was detected in A and F but was eliminated in C and E. This compound, which may originate from contamination, is undesirable, and its reduction is considered beneficial [76].

Deodorization treatments reduced the presence of compounds associated with unpleasant odors and, in some cases, promoted the formation of more pleasant sensory notes. Ethanol extraction proved to be the most effective treatment for removing undesirable aldehydes and ketones. At the same time, fermentation favored the formation of volatile compounds characteristic of fermented products, such as specific acids and aromatic alcohols.

3.6.2. Volatile Compounds in Yogurts Enriched with A. platensis Powders

The incorporation of A. platensis powder into yogurt, up to a concentration of 2.5% (w/v), resulted in the elimination of 24 volatile organic compounds (VOCs) previously identified in the microalga. These included hydrocarbons (e.g., pentadecane, nonadecane), ketones (e.g., 2-octanone, 5-hepten-2-one, 6-methyl-), aldehydes (e.g., β-cyclocitral, safranal), furans, pyrazines, and alcohols (e.g., cyclohexanol, 2,4-dimethyl-). This phenomenon can be attributed to various physicochemical and biochemical mechanisms derived from interactions with the yogurt matrix. Yogurt presents a complex network of proteins, lipids, polysaccharides, and structured water, supporting multiple types of molecular interactions. Whey proteins, such as β-lactoglobulin (βLG) and bovine serum albumin (BSA), show affinity for ketones, aldehydes, ionones, and esters through hydrophobic interactions and hydrogen bonding, forming protein-aroma complexes that reduce the availability of volatiles in the gas phase [77]. Likewise, the dispersed fat phase in the dairy matrix can retain lipophilic compounds such as hydrocarbons and ketones, thereby limiting their release into the headspace, as reported in systems with varying fat content [78].

Fermentation, together with yogurt’s viscosity, water activity, and protein structure, also influences the mobility and transformation of volatile compounds. Starter cultures may metabolize compounds such as pyrazines and aldehydes, reducing their concentrations or transforming them into less volatile and more neutral molecules [27]. Moreover, high water activity can accelerate the oxidation of volatiles, thereby diminishing their aromatic intensity [79]. Even when some compounds were not eliminated, their detection may have been masked by more abundant volatiles from the yogurt, such as short-chain fatty acids and alcohols.

A total of 45 VOCs were identified in the yogurt samples enriched with A. platensis powder. Six of these were common to the yogurt and the microalgae powder, while another six were detected only in the unenriched yogurt (Supplementary Table S2). The type and number of VOCs varied significantly depending on A. platensis powder treatment type and its concentration in the yogurt formulation.

A clear upward trend in the number of VOCs was observed as the concentration of A. platensis increased. The highest number of VOCs (39) was recorded in the yogurt enriched with untreated powder (YA 2.5%), followed by the activated carbon-deodorized powder (YC 2.5%) with 34 compounds. In contrast, ethanol-deodorized (YE 2.5%) and fermented (YF 2.5%) powders resulted in only 27 and 26 VOCs, respectively (Supplementary Table S2). These results suggest that deodorization treatments partially reduce the release of undesirable volatiles, aligning with previous findings on the sensory mitigation of A. platensis [24,25,27].

Among the hydrocarbons, decane (characterized by petroleum-like odors) was detected only in YA 2.0–2.5%, indicating a direct transfer from untreated A. platensis. This compound likely derives from lipid oxidation or degradation intermediates of unsaturated fatty acids, which may adversely affect the sensory quality of the final product [74].

A notable increase in ketone compounds such as 2-heptanone, 6-methyl, 5-hepten-2-one, 6-methyl-, Isophorone, β-ionone, and its epoxide, was observed, particularly at concentrations above 1.5% in the YA and YC treatments. These ketones are associated with olfactory descriptors including marine, woody, camphoraceous, green, fatty, citrus, fruity, and floral notes. They are most likely derived from fatty acid oxidation or carotenoid degradation and are regarded as contributors to the characteristic odor of A. platensis [69].

Hexanol, an alcohol characterized by green, fruity, and oily notes, exhibited a progressive increase with microalgae addition, especially in YA and YC. Similarly, compounds such as furan, 2-methyl- and furan, 3-methyl-, described as herbaceous, earthy, or licorice-like, exhibited a proportional increase with the A. platensis content, thereby reinforcing the distinctive aromatic contribution of this cyanobacterium [80].

Notably, 2-methyl-5-hexen-3-ol was detected exclusively in the fermented A. platensis treatment (YF), gradually increasing with the powder content. This compound has been linked to favorable sensory attributes. An increased alcohol content may positively influence the floral and sweet notes, potentially enhancing the sensory acceptance of yogurt enriched with fermented A. platensis [81].

Among the aldehydes, hexanal is one of the main volatile compounds associated with A. platensis, which was found in higher concentrations in the YA and YC treatments. Formed via lipid peroxidation induced by exposure to oxygen, light, and heat exposure, hexanal exhibits green, grassy aromas and has been extensively reported as a key contributor to the sensory profile of algae-enriched products [82].

Finally, 2,5-dimethyl pyrazine, which is associated with undesirable notes such as fishy, ammoniacal, nutty, or roasted meat flavors, was detected exclusively in samples with high microalgae concentrations (YA 2–2.5%, YC 2.5%, and YE 2.5%) and was not present in YF. The absence of this compound in fermented samples may enhance sensory acceptance by eliminating off-flavors that typically restrict the incorporation of A. platensis into food matrices [69].

Incorporating deodorized A. platensis powders into yogurt markedly diminished undesirable volatile compounds, particularly pyrazines and aldehydes. This reduction is attributed to interactions with the yogurt matrix, primarily involving proteins, fats, and polysaccharides that modulate compound retention and release. Fermentation of A. platensis proved to be the most effective treatment, as it preserved desirable compounds and enhanced the aromatic profile of the final product.

4. Conclusions

From a compositional perspective, each treatment affected the microalgae biomass differently: fermentation and activated carbon reduced mineral content, while ethanol treatment reduced lipids and moisture, increasing the relative protein concentration. These changes translated into yogurts with higher protein and ash content, albeit with moisture loss. However, sensory attributes were the most critical factor influencing consumer perception.

Yogurts formulated with untreated or activated carbon-treated A. platensis exhibited low acceptability due to herbal flavors, gritty textures, and undesirable volatiles such as hexanal, β-ionone, and pyrazines. Although all deodorization strategies reduced volatile compounds associated with off-odors, yogurts containing fermented A. platensis retained high acceptability at all tested concentrations (up to 2.5%), driven by the elimination of off-flavors and the formation of pleasant aromatic compounds such as alcohols and fermentation-derived acids. Moreover, the textural properties of these yogurts were enhanced.

These findings support fermentation as a dual-purpose strategy that not only deodorizes microalgal biomass but also improves its functional and sensory integration into dairy products, enabling the formulation of palatable yogurts containing nutritionally relevant amounts of A. platensis.