Sustainable Use of Tomato Powder Derived from Food Waste in Yogurt Formulation

Abstract

The valorization of food processing by-products is a key strategy for advancing sustainability in the agri-food sector. This study developed a fermented milk product incorporating tomato powder (TP) obtained from surplus tomatoes not meeting retail appearance standards. Four yogurt formulations were prepared containing TP (2% and 4%, w/v) and two controls with skim milk powder adjusted to equivalent total solids. Samples were inoculated with a commercial starter culture and fermented at 42 °C to a final pH of 4.6. TP addition did not hinder fermentation but altered acidification kinetics, as the 4% TP yogurt exhibited a faster initiation (T_m ≈ 80 vs. 120 min in the control) yet a slower rate of pH decline (V_max = 0.009 vs. 0.019 pH units/min). TP-fortified yogurts exhibited higher water holding capacity (98% vs. 83%), increased firmness (87 g vs. 47 g), and substantially elevated viscosity (63,000–68,000 mPa·s) while lycopene enrichment enhanced color attributes. Viable counts of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus remained within typical ranges (~6.8 and ~4.9 log CFU/g, respectively, after 24 h), confirming that TP did not compromise microbial activity. Overall, incorporating TP improved structural and functional properties while simultaneously providing tomato-derived antioxidants and promoting a sustainable, circular utilization of surplus tomato streams in fermented dairy products.

1. Introduction

Yogurt is the second most widely consumed fermented milk product on a global scale, after cheese [1], valued for its high nutritional quality, probiotic potential, and recognized therapeutic properties that contribute to its strong consumer acceptance [2]. Depending on production practices and regional preferences, yogurt can vary considerably in composition, texture, and sensory profile, with additional modifications achieved through the incorporation of stabilizers or fortifying agents [2,3,4]. Among the most common formulation strategies is the addition of skim milk powder (SMP), which increases non-fat solids, improves texture, and enhances water holding capacity [5,6,7,8]. While effective, this approach reflects a growing trend in yogurt innovation: the development of products enriched with functional ingredients [9,10,11,12,13,14]. At the same time, reliance on dairy-based fortifiers raises questions regarding sustainability and cost, particularly as consumer demand shifts toward environmentally conscious food innovations [15,16,17]. Recently, significant research has focused on the use of natural plant-based powders as functional additives to enhance the nutritional, physicochemical, and sensory properties of yogurt [18].

In parallel, the agri-food sector faces the urgent challenge of food waste reduction, with fruits and vegetables accounting for the highest proportion of wasted biomass globally [19]. Reducing losses in these categories has been shown to lower greenhouse gas emissions and ease pressure on land and water resources, reinforcing the sustainability rationale for valorization strategies [20,21]. Recent estimates indicate that nearly one-third of all food produced globally for human consumption, approximately 1.3 billion tons annually, is lost or wasted, representing significant environmental, social, and economic challenges [22].

Tomato production plays a central role in the global agri-food sector, reaching 189 million metric tons in 2021 with an estimated market value of USD 130 billion [23]. Despite this economic significance, large quantities of tomato by-products, including peels, seeds, pulp residues, and unsold fruits, are generated across the supply chain, from agricultural production and post-harvest sorting to industrial processing and retail distribution [20,24]. Critical loss points such as sorting, grading, storage, and transportation, contribute to approximately 30.3% of total losses [25] and global post-harvest losses are estimated to be between 25% and 42%, reflecting significant inefficiencies in production and distribution systems. Survey-based evidence further shows that storage beyond six days can result in losses exceeding 20%, prompting retailers to impose strict maximum residency times to maintain shelf-life standards [20]. Additionally, tomato processing commonly generates about 20% residue consisting of skins, seeds, and cores [24].

In the European Union, overall tomato post-harvest wastage exceeds 3 million metric tons annually, and at the retail level, an additional 5% of fresh market tomatoes are lost due to surplus and unsold products. These losses are also environmentally significant: research related to the UK supply chain indicates that tomato food loss and waste from production to retail accounts for approximately 0.43 Mt CO₂e annually, representing nearly 26% of the emissions associated with horticultural produce [20]. Collectively, these figures highlight the substantial volume of surplus tomatoes available for upcycling into value-added ingredients, offering a significant opportunity to reduce waste while promoting sustainability [26].

Utilizing these waste streams by converting them into powder offers a promising strategy to develop functional food products, minimize food waste, and support circular bioeconomy principles, with the valorization of fruit and vegetable residues in dairy formulations gaining increasing attention in recent years [27,28,29,30]. Examples of successful incorporation of such powders in yogurt production include apple pomace, carrot pomace, mango and potato peel, passion fruit and carrot fiber, which have been shown to enhance nutritional value, texture, antioxidant activity, and probiotic viability in yogurt and other fermented milk products [28,29,30,31,32,33,34,35,36]. Similarly, vegetable powders such as carrot, beetroot, onion, and champignon have been successfully used in fat-free dairy desserts to improve rheological behavior and antioxidant properties without compromising consumer acceptance [36]. Collectively, these studies demonstrate the potential of agro-industrial by-products as functional ingredients in dairy systems.

Within this broader context, recent research has also investigated tomato-based fortification strategies in yogurt. Tomato powders are particularly attractive due to the fact that they are rich in lycopene, phenolic compounds, organic acids, and dietary fiber [23,37]. Some studies on tomato-enriched yogurts have focused on the incorporation of tomato juice, which has been shown to enhance antioxidant capacity and bioactive compound content but often leads to notable changes in acidity, viscosity, and sensory attributes [38,39]. A smaller number of studies have also examined the use of tomato-derived powders, such as pomace, peel, or hot- and cold-break powders, reporting improvements in nutritional value, fiber enrichment, and antioxidant properties [40,41,42]. Among these, only the work of Alqahtani et al. [40] investigated the use of tomato pomace, an industrial by-product, in yogurt fortification.

Thus, despite growing interest in functional fortification, literature specifically addressing the use of tomato powder (TP), particularly derived from retail-level surplus fruits, as a partial substitute for milk solids in yogurt formulations remains limited, with key questions persisting regarding its effects on fermentation kinetics, microbial viability, structural attributes, and consumer perception. Given the chemical composition of tomato powder, its incorporation was considered likely to influence fermentation behavior and contribute to modifications in physicochemical properties through its fiber, pectin, and organic acid content, while also affecting color through its natural pigments. To address this gap, the present study investigates the incorporation of TP obtained from surplus supermarket tomatoes, specifically overripe fruits that remain unsold, into yogurt formulation. Two experimental samples (containing 2% and 4% TP) were compared with controls prepared with the addition of SMP adjusted to equivalent total solids. Physicochemical, microbiological, and rheological analyses were performed to evaluate the impact of TP addition on final product quality.

2. Materials and Methods

2.1. Materials

Fresh cow’s milk (3.5% fat) and SMP were purchased from a local market in Lemnos, Greece. A commercial freeze-dried yogurt starter culture, Yoflex™ Express 1.0 (Chr. Hansen, Hørsholm, Denmark), containing Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus, was used for yogurt manufacture. M17 Agar Medium and de Man, Rogosa and Sharpe (MRS) Agar Medium (Biolab, Budapest, Hungary) were employed for the selective enumeration of lactic acid bacteria. All chemicals and reagents were of analytical grade.

2.2. Tomato Powder (TP) Preparation

The TP was prepared from tomato surplus obtained by local markets as follows: overripe tomatoes were sorted to remove the spoiled ones and then were thoroughly washed with potable water. The cleaned tomatoes were then chopped, blended, and homogenized using a stainless-steel double-blade SilverCrest Mini Chopper (Model SMZ 260 D4, Lidl Stiftung & Co. KG, Bochum, Germany) at room temperature. The homogenized tomato pulp (dry matter 4.6 ± 0.1%) was frozen at −18 °C and subsequently freeze-dried at −30 °C under vacuum (0.001 Pa) for 24 h, using a BIOBASE Freeze Dryer BK-FD10S (Jinan, China). The lyophilized powder was finely ground and sieved through mesh screens with openings of up to 0.8 mm to produce a uniform TP. The powder was stored in airtight containers in a dry cabinet at room temperature until being used in yogurt formulations.

TP was characterized for its bioactive and physicochemical properties. Lycopene content was quantified spectrophotometrically according to the method described by Fish et al. [43] and expressed as milligrams of lycopene/100 mg of dry weight. Moisture content was determined gravimetrically by oven drying at 105 °C to a constant weight, following the official AOAC method [44].

2.3. Yogurt Preparation

Two yogurt formulations were prepared by supplementing whole cow’s milk with 2% (TY-2) and 4% (TY-4) TP. These TP levels were chosen to match the solids normally contributed by SMP in yogurt formulations [8]. Control samples (C-2 and C-4) were prepared by adding SMP to achieve equivalent total solids content. The mixtures were homogenized at 1300 rpm for 10 min using a Witeg Homogenizer HG-15A (Witeg Labortechnik GmbH, Wertheim, Germany) to ensure uniform dispersion of ingredients and complete powder hydration. Subsequently, the homogenized mixtures were pasteurized at 90 °C for 10 min in a PRECISTERM Selecta thermostatic water bath (J.P. Selecta, Barcelona, Spain). After cooling to approximately 40 °C, the mixtures were inoculated with a commercial starter culture following the manufacturer’s instructions. Aliquots of 100 mL from each formulation were transferred into sterile glass bottles with screw caps, and fermentation was carried out in a static incubator at 42 °C until the pH reached approximately 4.6. At the end of the incubation, the fermented samples were immediately cooled to 10 °C in an ice bath and then stored at 4 °C for 24 h prior to subsequent analyses. All experiments were performed in triplicate to ensure reproducibility.

2.4. Physicochemical Analyses

2.4.1. Determination of Acidification Kinetics Parameters

During fermentation, pH measurements were recorded at 15 min intervals using a pH meter (Mettler Toledo MP 220, Giesen, Germany). The maximum acidification rate (V_max) was determined from the pH–time curves according to the following equation:

$$V_{\mathrm{m}\mathrm{a}\mathrm{x}}={\left({\displaystyle \frac{\mathsf{\Delta }\mathrm{p}\mathrm{H}}{\mathsf{\Delta }t}}\right)}_{\mathrm{m}\mathrm{a}\mathrm{x}}$$

and expressed as an absolute value. In addition, the time of maximum acidification rate (T_m) and the time required to reach pH 4.6 (T_e) were extracted as characteristic kinetic parameters of the fermentation process [45]. After 24 h of fermentation, the pH of the yogurt samples was measured.

2.4.2. Determination of Titratable Acidity

During fermentation, titratable acidity (TA) was measured at 15 min intervals following the official methods of analysis [45]. After 24 h of fermentation, the TA of the yogurt samples was measured. Results were expressed as g of lactic acid/100 g of fermented sample.

2.4.3. Determination of Water Holding Capacity and Syneresis

The water holding capacity (WHC) of the samples was determined as follows: specific sample quantity (10 g) was centrifuged (1000 rpm, 10 min) in pre-weighted centrifuged tubes. After liquid removal, the remaining residue was determined. WHC was expressed as the percentage of the solid residue after liquid removal (% WHC). Susceptibility to syneresis was determined as described by Wu et al. [46] and expressed as percentage of the liquid weight filtered (syneresis, %).

2.4.4. Viscosity Measurement

Viscosity of the samples was determined at 4 °C with the aid of a viscometer (Rotational viscometer Myr V1/V2, Conbest Sp. Zo., Kraków, Poland). All of the measurements were conducted at 4 ± 0.1 °C, ensured by means of a controlled bath unit and an external thermostatic bath. The flow curves of the fermented samples were obtained by varying shear rates between 0.3 and 60 rpm.

2.4.5. Texture Profile Analysis (TPA)

A TPA of Y-B and Y-C was carried out at 4 °C using a Texture Analyzer (XT.TA Plus Texture Analyzer, Stable Micro Systems Ltd., Godalming, Surrey, UK). The texture profiles were measured directly in the fermentation bottles of both milks to avoid modifications of the structure. The samples were subjected to a double compression test (50% compression) at a crosshead speed of 1 mm/s. Data obtained from the force-time curves were used for the calculation of hardness, adhesiveness, springiness, cohesiveness and gumminess. TPA was performed after 24 h (4 °C).

2.4.6. Color Measurement

Color values (L*, a*, b*) were determined using a Minolta CR-400 Chroma Meter (Konica Minolta, Tokyo, Japan), calibrated with a white tile. Measurements were in triplicate.

2.4.7. Lycopene Determination

Lycopene was extracted from 0.5 g of freeze-dried yogurt powder using a solvent mixture of hexane:acetone:ethanol (2:1:1, v/v/v). The mixture was shaken for 20 min and then centrifuged at 4000 rpm for 10 min at room temperature. The absorbance of the supernatant was measured at 472 nm using a UV–Vis spectrophotometer (Shimadzu UV-1800, Kyoto, Japan). Lycopene concentration was calculated using the molar extinction coefficient of lycopene in hexane at 472 nm (ε = 172,000 L·mol⁻¹·cm⁻¹) and expressed as mg/100 g dry weight. All measurements were performed in triplicate.

2.5. Microbiological Analysis

Aliquots of the fermented samples were withdrawn under aseptic conditions and, after appropriate dilutions, were placed onto plates containing specific media. Counts of S. thermophilus and L. delbrueckii subsp. bulgaricus were enumerated on M17 and MRS agar, respectively. M17 plates were incubated aerobically at 37 °C for 72 h, while MRS plates were incubated anaerobically at 37 °C for 72 h in the presence of CO₂ (Oxoid™ CO₂ Gen™ Sachet, Thermo Scientific, Basingstoke, Hampshire, UK). Plates containing 30–300 colonies were enumerated and recorded as log CFU/g of yogurt sample.

2.6. Statistical Analysis

All experiments were performed in triplicate. Results are expressed as mean ± standard deviation. Statistical analysis was carried out using one-way ANOVA followed by Tukey’s HSD test at a significance level of p ≤ 0.05 using IBM SPSS Statistics v25 (IBM Corp., Armonk, NY, USA).

3. Results and Discussion

3.1. Acidification Kinetics and Titratable Acidity

The acidification kinetics and TA of fermented milk samples supplemented with SMP or TP are summarized in

Table 1. Acidification kinetics and titratable acidity parameters of fermented milk samples supplemented with 2% and 4% skim milk powder (C-2, C-4) and 2% and 4% tomato powder (TY-2, TY-4).

Sample 1	Vmax 2 (pH Units/min)	Tm 2 (min)	pHTm 2	pHe 2	pHt24 2	Titratable Acidity (%Lactic Acid) t0 2	Titratable Acidity (%Lactic Acid) t24 2
C-2	0.017 ± 0.002 a	130 ± 17 a	5.43 ± 0.09 a	4.68 ± 0.03 c	4.50 ± 0.14 a	0.89 ± 0.02 a	1.04 ± 0.04 a
C-4	0.019 ± 0.002 a	120 ± 0 a	5.52 ± 0.03 a	4.69 ± 0.02 bc	4.52 ± 0.14 a	0.76 ± 0.04 b	0.95 ± 0.04 a
TΥ-2	0.019 ± 0.005 ab	120 ± 0 a	4.98 ± 0.03 b	4.65 ± 0.04 a	4.55 ± 0.05 a	0.89 ± 0.02 a	1.03 ± 0.15 a
TΥ-4	0.009 ± 0.001 b	80 ± 35 a	5.20 ± 0.28 ab	4.62 ± 0.03 ab	4.52 ± 0.01 a	0.89 ± 0.04 a	0.95 ± 0.04 a

¹ Mean value of three independent experiments ± SD; Different superscript letters within the same column indicate statistically significant differences (p < 0.05, Tukey’s HSD). ² Parameters include maximum acidification rate (V_max), time to reach maximum acidification rate (T_m), pH at T_m (pHT_m), time to reach pH 4.6 (T_e), pH at T_e (pH_e), pH after 24 h fermentation (pH_t24), and titratable acidity (% lactic acid) after fermentation (t₀) and after 24 h (t₂₄).

. The maximum acidification rate (V_max) ranged between 0.009 and 0.019 pH units/min. A significantly lower V_max (p < 0.05) was observed for the TY-4 sample (0.009 ± 0.001 pH units/min) compared with all other formulations, whereas C-2, C-4, and TY-2 exhibited comparable rates (0.017–0.019 pH units/min). Despite this lower rate, TY-4 reached its maximum acidification significantly earlier (T_m = 80 ± 35 min) than the control samples (120–130 min, p < 0.05). The fact that the sample containing 4% TP exhibited a shorter Tm but, at the same time, exhibited a lower V_max indicates that the early phase of bacterial metabolism was stimulated, while the overall rate of pH decline was simultaneously moderated. The earlier initiation of acidification is likely due to the presence of readily fermentable sugars (glucose and fructose) and micronutrients in TP, which can promote the early growth and metabolic activity of LAB. However, as fermentation progresses, the higher buffering capacity, viscosity, and fiber-rich matrix of the 4% TP formulation limit the rate of pH decrease. This effect can be attributed to the presence of organic acids, minerals, and polysaccharides in TP, which interact with milk proteins and enhance the overall buffering capacity of the system. Specifically, these components can temporarily bind hydrogen ions and thereby resist rapid changes in pH [47,48]. During fermentation, hydrogen ions progressively protonate phosphate and citrate groups naturally present in milk, leading to the dissolution of micellar calcium phosphate and the release of calcium and casein molecules into the serum phase [48]. The additional organic acids and minerals supplied by TP likely altered this equilibrium, reinforcing the buffering effect and contributing to the slower rate of pH decline observed in TP-fortified samples. Furthermore, pectin and other polysaccharides present in TP can interact with casein micelles through electrostatic and steric associations, increasing the viscosity of the continuous phase and reducing the effective diffusion of hydrogen ions. This restricted diffusivity, together with the enhanced water-binding and thickening effects of soluble dietary fibers, contributes to slower propagation of acidification within the matrix, as previously reported in dairy systems [49,50].

The pH evolution throughout fermentation followed a consistent trend across treatments. At the time of maximum acidification rate, pH values ranged from 4.98 ± 0.03 (TY-2) to 5.52 ± 0.03 (C-4). Significant differences (p < 0.05) were observed only for TY-4 when compared to C-2 and C-4, whereas TY-2 did not differ significantly from any of the other formulations. The higher pHT_m values observed in C-2 and C-4 confirm a slower initial acidification in SMP formulations compared to those containing TP, likely due to the absence of additional fermentable substrates. At the end of fermentation (pH_te), all samples reached values close to the target pH of 4.6, with minor yet statistically significant differences (p < 0.05) that likely reflect matrix buffering effects rather than true variations in fermentation endpoint. After 24 h, no statistically significant post-acidification differences (p > 0.05) were detected, indicating that TP addition did not promote excessive acid production during short-term storage, a desirable feature for maintaining product stability.

Titratable acidity results supported the observed pH trends. After fermentation, TA values ranged from 0.76 ± 0.04% to 0.89 ± 0.02%. The significantly lower TA observed in C-4 (p < 0.05) compared with the other samples indicates that increasing SMP concentration enhanced the buffering capacity of the medium. As a result, a greater portion of the acid produced during fermentation was neutralized by milk proteins and phosphate salts, leading to a lower measured acidity [47]. After 24 h, TA increased moderately in all formulations (0.95–1.04%), reflecting continued LAB activity, but no significant differences were observed among samples (p > 0.05). The comparable TA and pH values at the end of fermentation and after 24 h confirm that TP incorporation did not inhibit lactic acid production and that LAB remained metabolically active across treatments.

Overall, the acidification profiles indicate that TP exerts two distinct yet complementary effects on fermentation. At 2% concentration, TP provides fermentable carbohydrates and micronutrients that enhance microbial activity, resulting in acidification behavior similar to the control. At 4%, although the fermentation initiates faster, the higher solids content, fiber, and buffering capacity slow down the acidification rate, yielding a more gradual pH decrease. This dual effect may explain why TY-4 exhibited an earlier Tm but a lower Vmax. Importantly, the final pH and titratable acidity values were similar across all samples, indicating that TP concentration primarily affected acidification kinetics rather than the fermentation endpoint. These findings confirm that TP can be successfully integrated into fermented milk matrices without impairing the acidification process.

3.2. Microbiological Characteristics

The viable counts of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus at the end of fermentation (T0) and after 24 h of storage (T24) are presented in

Table 2. Monitoring of lactic acid bacteria after fermentation (T0) and after 24 h of storage (T24) at 4 °C of fermented milk samples supplemented with 2% and 4% skim milk powder (C-2, C-4) and 2% and 4% tomato powder (TY-2, TY-4).

Samples	L. delbrueckii subsp. bulgaricus		S. thermophilus
	Storage Time 1
	T0	T24	T0	T24
	Log CFU/g
C-2	5.32 ± 0.01 b,c	6.45 ± 0.24 a,b	3.5 ± 0.13 a	4.9 ± 0.39 a
C-4	4.81 ± 0.42 c	6.85 ± 0.13 a	3.17 ± 0.20 a,b	4.75 ± 0.42 a
TΥ-2	6.69 ± 0.01 a	6.61 ± 0.13 a,b	3.11 ± 0.48 a,b	4.56 ± 0.22 a
TΥ-4	5.94 ± 0.34 b	6.21 ± 0.19 b	2.61 ± 0.44 b	4.32 ± 0.28 a

¹ Mean value of three independent experiments ± SD; Different superscript letters within the same column indicate statistically significant differences (p < 0.05, Tukey’s HSD).

. For L. delbrueckii subsp. bulgaricus, the highest initial counts (p < 0.05) were observed in TY-2 (6.69 ± 0.01 log CFU/g), followed by TY-4 (5.94 ± 0.34 log CFU/g), while the lowest were recorded in C-4 (4.81 ± 0.42 log CFU/g). This suggests that the addition of TP promoted the early growth of L. delbrueckii subsp. bulgaricus. After 24 h of storage, viable counts of L. delbrueckii subsp. bulgaricus increased across all formulations, reaching 6.21–6.85 log CFU/g. No significant differences (p > 0.05) were detected between tomato-enriched and control yogurts at this stage, indicating that TP supplementation had no adverse effect on bacterial persistence during 24 h storage.

Regarding S. thermophilus, the initial counts at T0 ranged between 2.61 and 3.50 log CFU/g, with C-2 showing the highest value. After 24 h, all samples exhibited a significant increase (p < 0.05), achieving counts between 4.32 and 4.90 log CFU/g. Similar to L. delbrueckii subsp. bulgaricus, no significant differences (p > 0.05) were observed among formulations, suggesting that the presence of TP did not negatively influence the growth or survival of S. thermophilus.

Overall, all yogurt samples maintained viable counts of both starter cultures within the typical range reported for fermented milk products (approximately 10⁶–10⁷ CFU/g), although values remained slightly below the recommended minimum of 10⁷ CFU/g for probiotic functionality. The tomato-supplemented samples, particularly TY-2, exhibited comparatively higher initial counts of L. delbrueckii subsp. bulgaricus, which may be attributed to the presence of fermentable sugars, organic acids, and essential micronutrients (e.g., potassium and magnesium) in TP that can stimulate lactic acid bacterial metabolism. These findings are consistent with Demirci and collaborators [42], who observed that yogurts enriched with hot- or cold-break tomato powders showed higher survival of L. delbrueckii subsp. bulgaricus and S. thermophilus throughout refrigerated storage compared with controls. Additionally, the results of the present study also align with reports related to the survival of LAB in tomato-based matrices, although performance depends on the substrate’s acidity and phenolic content [51].

3.3. Susceptibility to Syneresis, Water Holding Capacity and Texture Profile Analysis

The water holding capacity and susceptibility to syneresis of the yogurt samples are shown in

Table 3. Susceptibility to syneresis and water holding capacity of fermented milk samples supplemented with 2% and 4% skim milk powder (C-2, C-4) and 2% and 4% tomato powder (TY-2, TY-4).

Samples 1	Syneresis (%) 2	WHC (%) 2
C-2	37 ± 1.73 a	92 ± 1.22 b
C-4	41 ± 4.36 a	83 ± 3.46 a
TΥ-2	40 ± 2.55 a	89 ± 1.00 b
TΥ-4	41 ± 1.00 a	98 ± 0.00 c

¹ Mean value of three independent experiments ± SD; ² Different superscript letters within the same column indicate statistically significant differences (p < 0.05, Tukey’s HSD).

. Significant differences (p < 0.05) were observed in WHC among formulations, whereas syneresis did not differ significantly (p > 0.05). The addition of TP markedly improved WHC, with the highest value recorded for TY-4 (98 ± 0.0%). Notably, although both 4% TP and 4% SMP increased the total solids content of the formulation, the improvement in WHC was observed only in TP-enriched yogurts. This suggests that the nature of the added solids, i.e., polysaccharide-rich tomato powder versus protein-dominant skim milk powder, plays a crucial role in water retention. Specifically, the polysaccharides present in TP, particularly pectin, behave similarly to hydrocolloid stabilizers by binding water molecules and forming associative linkages with milk proteins that restrict water mobility and enhance gel hydration. Such interactions have been shown to strengthen the casein network and reduce serum separation in yogurt systems [52]. Thus, the enhanced WHC in tomato-enriched yogurts can be attributed to the high dietary fiber content of TP, particularly pectin, which interacts with milk proteins and strengthens the gel network’s ability to retain water [18,49]. In contrast, increasing SMP primarily raises protein and mineral concentrations, which may enhance buffering capacity but does not necessarily promote water binding within the gel structure. Similar improvements in water retention have been reported in yogurts fortified with fruit or vegetable by-products, where pectin–casein interactions lead to a more compact gel structure and reduced whey separation [29,30,31]. Syneresis, expressed as the percentage of whey expelled, showed no significant differences among treatments (p > 0.05), with values ranging from 37.0 ± 1.73% (C-2) to 41 ± 1.00% (TY-4). The absence of significant variation in syneresis despite the increase in WHC suggests that the additional tomato solids contributed mainly to internal water retention rather than altering serum release at the macroscopic level. Overall, the improved WHC combined with low syneresis is desirable for maintaining gel integrity, product appearance, and the overall shelf-life of yogurt.

Regarding the texture profile analysis (TPA) (

Table 4. Texture profile analysis of fermented milk samples supplemented with 2% and 4% skim milk powder (C-2, C-4) and 2% and 4% tomato powder (TY-2, TY-4).

Samples 1	Hardness (g)	Adhesiveness	Springiness	Cohesiveness	Gumminess
C-2	30.22 ± 2.64 a	−74.20 ± 3.35 c	3.50 ± 0.56 b	0.33 ± 0.01 a	9.97 ± 0.66 a
C-4	47.10 ± 7.63 b	−156.43 ± 60.75 bc	2.87 ± 0.74 ab	0.33 ± 0.01 a	15.57 ± 1.84 b
TΥ-2	72.10 ± 7.10 c	−430.57 ± 61.71 a	1.53 ± 0.07 a	0.33 ± 0.01 a	25.10 ± 2.92 c
TΥ-4	87.03 ± 3.67 d	−302.97 ± 139.56 ab	1.90 ± 0.70 a	0.43 ± 0.07 a	28.70 ± 1.00 c

¹ Mean value of three independent experiments ± SD; Different superscript letters within the same column indicate statistically significant differences (p < 0.05, Tukey’s HSD).

), among the measured parameters, hardness, which represents the perceived firmness of the yogurt gel, and gumminess increased significantly (p < 0.05) with TP supplementation, particularly at the 4% level (TY-4), exhibiting the highest hardness (87.03 ± 3.67 g) and gumminess (28.7 ± 1.00). The increase in these parameters indicates the formation of a denser and more rigid gel network as TP concentration increases. In contrast, springiness decreased significantly (p < 0.05) in TP-enriched yogurts compared with control samples, suggesting reduced elastic recovery after deformation, while cohesiveness remained unaffected (p > 0.05). Adhesiveness became more negative in the tomato-containing samples, particularly TY-2, reflecting stronger internal bonding and greater structural resistance during withdrawal.

The increase in hardness of tomato-enriched yogurts is consistent with their improved WHC (Table 3). The incorporation of TP, rich in dietary fibers and pectic substances, likely promoted protein–polysaccharide interactions that strengthened the gel network and limited water mobility [18,49].

Similar behavior has been observed in yogurts fortified with other fiber-rich materials, such as apple pomace [35], where insoluble fibers improved hardness but had limited effects on whey separation. Therefore, the concurrent increase in hardness and WHC with unchanged syneresis values in TP-supplemented samples reflects a microstructural rearrangement leading to a tighter, more compact gel rather than enhanced water entrapment.

3.4. Viscosity Measurements

The apparent viscosity profiles of the yogurt samples at different shear rates are presented in Figure 1. For all formulations, viscosity decreased as shear rate increased, indicating typical non-Newtonian pseudoplastic behavior, a characteristic commonly observed in yogurt and other fermented milk products. This shear-thinning property reflects the progressive breakdown of the protein–polysaccharide gel network under increasing mechanical stress, which facilitates flow. Although all samples exhibited a similar overall rheological pattern, significant differences were observed in viscosity levels among formulations. Yogurts fortified with TP, particularly TY-4, demonstrated consistently higher apparent viscosity across almost all shear rates compared to the controls (C-2 and C-4). This effect can be attributed to the presence of dietary fibers in TP, which enhanced water retention (Table 3) and strengthened the three-dimensional gel network, thereby improving the structural integrity of the yogurt matrix. Additionally, the higher viscosity observed in TY-4 could also be explained by the greater pectin content of TP and its potential interaction with calcium ions naturally present in milk. These Ca²⁺-mediated bridges between pectin chains and casein micelles contribute to the formation of a more stable, cross-linked network, enhancing the viscosity and firmness of the yogurt matrix [53].

Higher viscosity in tomato-enriched samples may also contribute positively to the sensory perception of yogurt, as it is often associated with increased creaminess and improved mouthfeel, attributes known to influence consumer acceptability. Despite these differences, all formulations demonstrated stable viscosity profiles under shear, suggesting that TP incorporation did not compromise the mechanical stability of the gel network.

3.5. Color Parameters and Lycopene Content

The color characteristics (L*, a*, b*, chroma c, and hue angle h) of the yogurt samples are presented in

Table 5. Color analysis of fermented milk samples supplemented with 2% and 4% skim milk powder (C-2, C-4) and 2% and 4% tomato powder (TY-2, TY-4).

Samples 1	L*	a*	b*	c	h
C-2	91.93 ± 3.44 a	−0.33 ± 0.84 c	5.43 ± 1.62 c	5.53 ± 1.62 c	93.55 ± 9.76 a
C-4	95.40 ± 1.42 a	−1.03 ± 1.13 c	5.03 ± 1.08 c	5.25 ± 0.82 c	102.98 ± 14.94 a
TY-2	82.95 ± 1.00 b	8.15 ± 0.70 b	16.03 ± 0.90 b	18.00 ± 1.00 b	63.08 ± 1.24 b
TY-4	66.48 ± 3.33 c	18.38 ± 0.47 a	23.10 ± 1.25 a	29.53 ± 1.26 a	51.53 ± 0.96 b

¹ Mean value of three independent experiments ± SD; Different superscript letters within the same column indicate statistically significant differences (p < 0.05, Tukey’s HSD).

. Significant differences (p < 0.05) were observed between control and tomato-enriched formulations across all measured parameters. The addition of TP led to a marked decrease in lightness (L*), particularly in TY-4 (66.48 ± 3.33) compared to controls (C-2: 91.93 ± 3.44, C-4: 95.40 ± 1.42). In parallel, redness (a*) and yellowness (b*) increased significantly in fortified samples, with TY-4 showing the most intense red/orange appearance (a* = 18.38 ± 0.47, b* = 23.10 ± 1.25). These changes in color can be attributed to the presence of carotenoids, primarily lycopene, naturally present in TP [54]. The tomato powder used in this study (moisture content: 2.5 ± 0.06%) contained 20.2 ± 0.017 mg lycopene/100 g, which effectively transferred color to the yogurt matrix during fermentation (Figure S1). Lycopene concentrations measured in the fortified yogurts confirmed this effect, with values of 1.37 ± 0.002 mg/100 g for TY-2 and 2.10 ± 0.03 mg/100 g for TY-4. As expected, the control samples exhibited negligible lycopene levels (0.15 ± 0.003 mg/100 g), consistent with the absence of tomato-derived pigments. However, it should be noted that the composition of tomato powder may vary depending on factors such as variety, degree of ripeness and storage conditions (i.e., time, temperature, moisture). These parameters can influence attributes such as lycopene content, acidity, and fiber composition. Although the present study used a well-defined batch of overripe surplus tomatoes, future work should consider this natural variability when evaluating the reproducibility and broader applicability of TP as a functional ingredient in similar processes.

The chroma (c) and hue angle (h) values further support these observations. Fortified yogurts exhibited higher c values, indicating more vivid and saturated colors, and lower h values, reflecting a shift toward reddish tones compared to controls. The strong correlation between lycopene content and the increased a* and b* values confirm the role of tomato-derived pigments in modifying yogurt color properties. The enhanced redness of tomato-fortified yogurts may also serve as a visual cue of improved nutritional quality, which could positively influence consumer perception and increase market appeal.

4. Conclusions

This study has demonstrated that tomato powder obtained from surplus tomatoes not meeting retail appearance standards can be effectively incorporated into yogurt formulations as a partial substitute for skim milk powder. Specifically, its addition improved fermentation performance, enhanced water holding capacity and viscosity, increased gel firmness, and enriched lycopene content, resulting in more vivid color attributes without adversely affecting microbial viability or final pH values. These outcomes confirm that tomato powder acts as a functional ingredient that enhances the technological properties of yogurt. Beyond its functional benefits, its use contributes to sustainability by valorizing surplus tomatoes, reducing food waste, and supporting the development of clean-label, antioxidant-enriched fermented dairy products aligned with circular economy principles. Future research should focus on evaluating storage stability, consumer sensory perception, compositional attributes of tomato powder (e.g., fiber content, total phenolic compounds, antioxidant activity, particle-size distribution), and the bioavailability of tomato-derived bioactives to further optimize the application of surplus tomato powder in dairy formulations.