Evaluation of Carbonic Maceration Effect as a Pre-Treatment on the Drying Process of Strawberry

Abstract

In the last decade, organic-based food materials have taken an increasing marketing share due to consumers’ interests. Strawberry is one of the world’s most important berry crops, with growing production. This study aimed to evaluate the drying process of organic strawberries and to determine the influences of process conditions on drying characteristics. To improve drying performance, carbonic maceration (CM) was investigated as a pre-treatment. The experimental design and the optimization of the drying with pre-treatment conditions were performed using statistical modeling (response surface method (RSM), central composite design (CCD)). Using the RSM, dependent variables such as drying time, total phenolic compound, antioxidant capacity, ascorbic acid concentration, and hue value were optimized as a function of operating conditions of CM pre-treatment and following the drying process. The results indicated that drying performance varied depending on drying temperature as well as process conditions of CM. Drying time was reduced by more than 30% with CM treatment compared to control. Furthermore, ascorbic acid content, antioxidant activity, and surface color of dried samples were protected better in the case of CM-pre-treated strawberries. Results showed that the optimum drying temperature, maceration pressure, maceration temperature, and maceration time parameters were 64.8 °C, 3.54 bar, 4.2 °C, and 4 h, respectively. The high potential of CM should be considered in terms of process improvement and product quality for drying processes. Thus, this study provides key outcomes in this respect. According to the obtained results, the CM was assessed as a promising technique applied before the drying of organic strawberries.

1. Introduction

Strawberry is a red-colored, fragrant, and delicious fruit belonging to the Fragaria genus of the Rosaceae family, whose homeland is North and South America. According to 2019 data, China is the largest producer with 3.2 million tons of strawberry production, followed by the USA (1 million tons), Mexico (861 thousand tons), and Turkey (487 thousand tons) [1]. Due to consumer preferences and global market demand for organic consumption, organic fruit production has been rising rapidly in the last decade to prevent health problems caused by pesticides and chemicals. Aside from the fact that organic products are mostly preferred by customers, studies reveal that the color values, sugar content, β-carotene, and ellagic acid content, as well as antioxidant, anthocyanin, ascorbic acid, and phenolic substance content are higher in organic strawberries than in conventional strawberries [2,3,4,5,6].

The drying process, which is one of the oldest food preservation methods, provides better protection of the nutrient compounds of food products, extends the shelf life, and reduces the packaging, storage, and transportation costs and makes the products available even in the off-season. However, drying foods with existing systems may take quite a long time and affect the quality of the final product negatively [7]. To overcome this problem and/or to limit these adverse effects by reducing process time and improving the drying rate, different pre-treatments have been applied before the conventional drying of fruits and vegetables. Some of these pre-treatments are immersion in sodium hydroxide solution [8] or metabisulfite solution [9,10], hot steaming [11,12] or bleaching [13], freezing [14,15], and osmotic dehydration [16]. These pre-treatments positively influence some chemical or physical properties of dried products. Among various chemical and/or physical pre-treatments, CM treatment has come to the forefront in recent years as an alternative and low-cost technology. As the basis for CM, a sealed tank with strong pressure resistance and temperature control is required. In the system, firstly the fruits are placed in this tank, followed by a carbon dioxide gas addition. The structure of the fruit undergoes various modifications after being kept for a certain amount of time. CM was discovered by Michael Flanzy in 1934 and has been used to enhance the flavor of final products in winemaking from grapes [17]. The CM process is thought to cause some physicochemical changes in the structure and vacuole of grapes, such as cell wall collapse and cell membrane hydrolysis. The CM technique has recently been used as a pre-treatment application before drying various fruits [18,19,20,21,22,23]. When this technique is used in the drying process, it can shorten the drying time by accelerating the mass transfer and provide an improvement in some quality parameters of the dried product.

RSM is a useful approach for improving food production processes. It is a beneficial method, since it allows for the evaluation of the effects of several variables and their interactions on one or more response variables. In the study, both CM and air-drying process conditions were optimized using RSM. According to the central composite rotatable design, CM pressure, temperature, time, and drying temperature were selected as continuous factors, while the drying rate was the response. Temperature ranges of 50 and 70 °C were selected for conventional drying. The drying process ended when dry matter content reached 10% on a wet basis.

The CM application and its effects were investigated as a pre-treatment in the drying of organic strawberries. The aim of this study is to determine the drying characteristics and some quality properties of dried (10% moisture, <0.4 water activity (a_w)) final products using the traditional hot-air-drying process that can be applied in the production of dried strawberries in the industry. In addition to this, the study aimed to determine the effects of CM pre-treatment on the drying process and the product, to determine the optimum conditions of CM pre-treatment, and to determine the most suitable storage conditions for untreated (control) and pre-treated dried strawberries produced under optimum conditions.

2. Materials and Methods

2.1. Materials

The certified organic Albion (TR-OT-014-081/02-32) variety was used as material and obtained from a local certified organic producer. Fresh organic strawberries having uniform size and color and without surface damage or diseases were selected for each batch. Harvested strawberries were kept in polyethylene boxes, immediately transferred to the laboratory, and stored at 4 °C until the experiments. Drying experiments were completed to prevent the chemical and physical properties of fresh strawberries from changing, in seven days for each batch. The average weight of organic strawberries was 9.15 ± 0.48 g each with a diameter of 35.13 ± 2.7 mm in length and 26.12 ± 1.7 mm in width. The initial moisture content was 90.07 ± 1.05%.

2.2. Experimental Design and Optimization

To determine optimum drying temperature and CM pre-treatment conditions, an experimental matrix was arranged as a CCD. While drying time was selected as a response, drying temperature, CM pressure, temperature, and time were selected as independent variables. Selected ranges and the RSM design of experiments are given in

Table 1. Dependent variables of coded and uncoded values.

Dependent Variable	Minimum Value	Maximum Value
X1: Drying temperature (°C)	50	70
X2: CM pressure (bar)	1	4
X3: CM temperature (°C)	4	40
X4: CM time (hour)	4	24

and

Table 2. Response surface methodology design of experiments.

Runs	Maceration			Drying Temperature (°C)
	Temperature (°C)	Time (h)	Pressure (bar)
1	4	14	2.50	60
2	22	24	2.50	60
3	22	14	2.50	50
4	40	14	2.50	60
5	22	4	2.50	60
6	22	14	2.50	70
7	22	14	1.00	60
8	22	14	2.50	60
9	22	14	4.00	60
10	22	14	2.50	60
11	31	19	3.25	65
12	13	9	3.25	65
13	22	14	2.50	60
14	13	19	3.25	55
15	31	9	1.75	65
16	13	9	1.75	55
17	13	19	1.75	65
18	31	9	3.25	55
19	22	14	2.50	60
20	31	19	1.75	55
21	13	19	1.75	55
22	22	14	2.50	60
23	31	19	1.75	65
24	31	9	3.25	65
25	13	19	3.25	65
26	31	19	3.25	55
27	31	9	1.75	55
28	13	9	1.75	65
29	22	14	2.50	60
30	13	9	3.25	55

, respectively

2.3. Carbonic Maceration Pre-Treatment

A laboratory-scale chamber was used to expose the fresh strawberries to high-pressure carbon dioxide (CO₂). This chamber was a horizontal cylindrical-shaped tank and could hold around 10 kg of fresh fruit. The maximum pressure level was 4 bar, according to construction limits. The chamber had also heating and cooling assemblies. After loading fresh strawberries into trays, the lid of the chamber was tightly closed, and a vacuum was applied for 5 min to evacuate the air in the tank. Meanwhile, the heating or cooling system was operated to ensure the desired maceration temperature. After vacuum application, a valve of the CO₂ line was opened, and the tank was filled to attain the target pressure level. Following the CO₂ pressure being attained, the valve was closed, and the recording application time was started. Fresh organic strawberries were cut into slices of 7.34 ± 0.46 (mm) and dried under the drying conditions indicated in Table 2 with CM pre-treatment. For each drying trial, approximately 2500 g of fresh organic strawberries were placed in the CM tank. At the end of the maceration process, the exhaust valve of the chamber was opened, and CM pre-treatment was ended. Following the CM pre-treatment, the samples were immediately placed in a conventional oven for drying.

2.4. Drying Experiments

The drying process was achieved in a conventional oven having a forced air circulation system. The oven was also equipped with an automated scale system that could take weighing and data saving in desired time intervals. The airspeed was set to 1.5 m/s in the oven. For each drying experiment, 2500 ± 10 g of CM-treated strawberries were placed in the drying oven immediately after CM treatment. The drying process was ended when the moisture content of the product was reduced to under 10% on a wet basis (10% moisture will indicate the water activity is approximately below 0.4).

2.5. Drying Rate

The drying rate (Dr) (g water/(g dry matter ∗ h)) was calculated according to the following equation (Equation (1)) [24].

$$\mathrm{Dr} =\left(M_{t+dt}-M_t\right)/dt$$

(1)

where $M_t$ and $M_{t+dt}$ are the moisture contents (db) at measuring time $d$ and $t$ + $dt$.

2.6. Rehydration Rate

The rehydration rate of dried organic strawberries was carried out with distilled water at 20 ± 1 °C, then 5 g of the dry samples were weighed, treated with 400 mL of distilled water for 4 h, and the weights were taken at regular intervals (5 min, 10 min, 15 min, 30 min, 1 h, 2 h, and 4 h). The rehydration rate was calculated using the following equation (Equation (2)) [25].

$$\mathrm{RR}={\mathrm{W}}_{\mathrm{r}}/{\mathrm{W}}_{\mathrm{d}}$$

(2)

W_r: Mass after rehydration (g), W_d: Mass of the dried sample (g).

2.7. Water Activity

The water activity (a_w) measurement for the sample was made with an electronic a_w measuring device, the Thermoconsanter TH 200 (Novasina, Axair Ltd., Pfäffikon, Switzerland), at 25 °C temperature. The sample was carefully homogenized in a crusher prior to measurement (for 5–10 s). The values of a_w represent the arithmetic means of at least three independent determinations, and the temperature was also automatically corrected by the device.

2.8. Acidity and pH Value

The acidity value (%) of the sample was determined as citric acid equivalents. The pH value was read potentiometrically with a pH meter (Sevencompact pH meter, Mettler-Toledo, Greifensee, Switzerland) after the preparation stage.

2.9. Soluble Dry Matter (ᵒBx)

Measurements were made with a refractometer (Hanna Instruments, HI96801, Cluj-Napoca, Romania) at room temperature. Values were expressed in degrees of Brix (ᵒBx).

2.10. Color Measurement

Color parameters (L*, a*, b*, C*, hue) of the samples were measured by the colorimeter (NH310 High-Quality Portable Colorimeter, Shenzhen 3NH Technology CO., LTD., China). Measurements were made in five replicates per trial. Chroma values and hue angle of samples were calculated according to the following equations and Equation (4)). The browning index (BI) was calculated using Equation (5) [26].

$${\mathrm{C}}^{*}=\sqrt{{\left({\mathrm{a}}^{*}\right)}^2+{\left({\mathrm{b}}^{*}\right)}^2}$$

(3)

$${\mathrm{H}}^{*}=\arctan \frac{{\mathrm{b}}^{*}}{{\mathrm{a}}^{*}}$$

(4)

$$\mathrm{BI}=\frac{\left[100\left(\mathrm{x}-0.31\right)\right]}{0.17}\hspace{1em}\hspace{1em}\mathrm{x}=\frac{\left({\mathrm{a}}^{*}+1.75{\mathrm{L}}^{*}\right)}{\left(5.645{\mathrm{L}}^{*}+{\mathrm{a}}^{*}-3.012{\mathrm{b}}^{*}\right)}$$

(5)

2.11. Total Phenolic Content (TPC)

The method modified by [27] was used to determine the total amount of phenolic substances. After the extracts were diluted with methanol, 40 µL of the sample was taken and vortexed by adding 2.4 mL of distilled water, and 0.2 mL of Folin–Ciocalteu solution was added and vortexed once more. Then, 0.6 mL of saturated sodium carbonate (Na₂ CO₃) was added. Then, 0.76 mL of distilled water was added to the mixture and left to stand in the dark for 2 h. Samples were read at 765 nm in a spectrophotometer (T70 + UV/VIS spectrophotometer, PG Instruments, Leicestershire, UK). The total amount of phenolic substance in the extract was expressed as mg gallic acid equivalent (GAE) per 1 g of dry matter [27].

2.12. Ascorbic Acid Content (AAC)

To determine the ascorbic acid content, samples were cut into small pieces and mixed with 4.5% meta-phosphoric acid at a ratio of 1:10 (g/mL) and homogenized. Subsequently, it was centrifuged at 4000× g rpm for 20 min at 4 °C. The supernatant was filtered through a 0.45 µm Millipore filter and 20 µL of the filtrate was injected into HPLC (Agilent, 1260 infinity, Santa Clara, CA, USA). A C18 column (250∗4.6 mm. ID: 5 µm ACE, Aberdeen, Scotland) and diode array detector (DAD, Agilent, 1260 MWD VL, Santa Clara, CA, USA) were used. The mobile phase was ultrapure water: phosphoric acid (pH 2.2), the column temperature was 30 °C, the flow rate was 0.8 mL/min, and the total run time was 30 min [28].

2.13. Hydroxymethylfurfural Content (HMF)

To determine the HMF content of the dried material, the method published by [29] was used.

2.14. Determination of Antioxidant Activity by ABTS Method

In the analysis, a 7 mM ABTS solution containing 2.45 mM potassium persulfate was prepared, and this solution was kept for 12–16 h, resulting in the formation of a ABTS•+ radical; 10 µL of the extract was added (diluted appropriately) to 1 mL of ABTS•+ solution and vortexed for 10 s. The color change was followed by reading the absorbance in the spectrophotometer at a wavelength of 734 nm. Considering the absorbance values which were determined at the end of 6 min, the percent decrease (inhibition) rate compared to the initial value was calculated. Analyses were performed in three parallels for each extract. Results were given as TROLOX^® equivalent (T.E.)/g d.m. The following equation (Equation (6)) was used to calculate the inhibition rate (%) [30].

$$\% \mathrm{Inhibition}=\left[\frac{\mathrm{IAV}-\mathrm{AV}}{\mathrm{IAV}}\right]\times 100$$

(6)

$\mathrm{IAV}$: initial absorbance value, $\mathrm{AV}$: absorbance value at the end of 6 min.

2.15. Texture Analysis

The textural properties of the samples were determined with a texture analyzer (TA-XT Plus, Texture Stable Micro Systems, Godalming, UK). A shell resistance (puncture) test and light knife blade (LKB) test were applied to the samples. The trials were carried out in three parallels [31].

2.16. Scanning Electron Microscopy (SEM)

Samples that were dried under optimum conditions (carbonic maceration and subsequent convection drying) and subjected to convection drying without pre-treatment were visualized, and the structural changes caused by carbonic maceration pre-treatment were revealed in microscopic dimensions. This analysis was made as a service procurement in the SEM laboratory located at Süleyman Demirel University YETEM (Innovative Technologies Center).

2.17. Energy Utilization

The energy utilization of the hot-air-drying oven was calculated using the following equation (Equation (7)) [3,32].

$$\mathrm{EU}={ \mathrm{A}}_{\mathrm{d}}\times {\mathrm{V}}_{\mathrm{a}}\times {\mathsf{\rho }}_{\mathrm{a}}\times {\mathrm{c}}_{\mathrm{P}}\times \mathsf{\Delta }\mathrm{T}\times \mathrm{t}$$

(7)

$\mathrm{EU}$: Energy utilization (kcal/h), ${\mathrm{A}}_{\mathrm{d}}$: drying area (m²), ${\mathrm{V}}_{\mathrm{a}}$: air velocity (m/s), ${\mathsf{\rho }}_{\mathrm{a}}$: air density (kg/m³), ${\mathrm{c}}_{\mathrm{P}}$: specific heat of air (kcal/kg °C), $\mathsf{\Delta }\mathrm{T}$: temperature difference (°C), t: time (s).

2.18. Statistical Analysis

Statistical analysis of the optimization study was made using the Minitab statistical package program in which the trial design was created. Each analysis result was loaded into the program, and the coefficient of determination (R²), adjusted coefficient of determination (R² adj), and predicted coefficient of determination (R² pred.) were examined. In addition, the variance tables and the Pareto cards giving information about the importance of the parameters were also evaluated. Apart from the optimization study, the comparison of the relevant analysis results in the samples was carried out statistically, and the significance levels of the possible differences were determined. Fresh samples, convectional dried samples, and samples dried under optimum CM conditions were also compared. For this purpose, analysis of variance (ANOVA) with the Tukey test was applied using the Minitab statistical package program (p ≤ 0.05). Results are expressed as mean ± standard deviation of the mean. The figures were created with the sigma plot software.

3. Results

This study aimed to evaluate the drying process of organic strawberries and to determine the influences of CM pre-treatment and drying process conditions on drying characteristics. Fresh, organic, Albion variety strawberries were used in the study, and color, a_w, humidity, °Bx, pH, and titration acidity analyses of the fresh fruit before the drying process were carried out. The color values of fresh fruits were determined as L* 42.25 ± 2.86, a* 23.11 ± 5.46, b* 13.64 ± 5.18, C* 26.96 ± 4.99, and hue° 27.61 ± 1.82. Humidity (%), a_w, °Bx, pH, titration acidity (%), and degree of ripening (°Bx/T.A) were also 90.07 ± 1.05, 0.98 ± 0.01, 12.27 ± 0.51, 3.88 ± 0.13, 1.23 ± 0.02, and 9.98 ± 0.12, respectively. In addition to these, ascorbic acid content, total phenolic content, antioxidant capacity, and anthocyanin content were found as 673.3 ± 32.8 mg A.A./100 g DM, 47.41 ± 2.01 mg GAE/g DM, 89.51 ± 2.09 µmol T.E./g DM, and 1.6 ± 0.11 g P3G/100 g DM, respectively.

The results of moisture, a_w, °Bx, pH, % titration acidity, AAC, TPC, ABTS, and ACC of the strawberries used in the study were found to be quite similar to other strawberries in the literature [2,4,5,6,33,34,35]. While the L* value and hue angle of fruits were higher, the a* value was lower compared to the value of strawberries in the literature [2,36,37]. The Bx/T.A ratio, which shows the ripening stage of fruit, corresponded to a sweet and intense taste according to the defined ratio (°Bx/T.A: 9.25–12.00) for these characteristics [31].

3.1. Changes in Physicochemical and Functional Properties of Strawberries with Drying Process

In this study, sliced organic strawberries were dried according to the RSM experimental design with a combination of CM pre-treatment and following a hot-air-drying process. In addition, several physical and chemical properties of untreated (control) and pre-treated dried strawberries were compared. CO₂ pressure, time of maceration, and the temperature values of CM and hot-air-drying were selected as independent variables. Although dependent variables were determined as drying time, color parameters (L*, a*, b*, C* h° values), textural properties, rehydration ratio, TPC, AAC, ACC, antioxidant capacity (ABTS), and HMF, only the following ones were used in the optimization: drying time, total phenols, antioxidant capacity (ABTS), ascorbic acid, and hue angle, since the models developed for them were statistically significant (R² > 0.85, predicted R² > 0.70, and lack-of-fit values >0.05) (

Table 3. Dependent variables used in optimization; drying time, total phenolics, antioxidant capacity (ABTS), ascorbic acid, and h° of dried strawberries with CM pre-treatment.

Run a	A b	B	C	D	E
1	270	21.29	40.75	401.41	23.51
2	225	16.85	27.60	121.87	32.42
3	500	19.16	34.24	218.98	29.74
4	195	19.74	38.10	64.61	22.19
5	290	17.91	34.53	390.88	29.07
6	225	15.64	30.23	162.42	29.64
7	247	18.41	33.51	386.68	33.51
8	320	18.50	31.74	357.11	28.96
9	265	19.92	36.19	101.14	35.61
10	260	19.55	34.24	213.21	29.25
11	175	16.20	24.74	112.80	29.76
12	195	21.08	27.28	367.87	29.44
13	250	17.62	24.59	308.86	28.59
14	255	20.13	27.84	231.48	28.46
15	210	17.90	25.71	233.07	25.27
16	330	18.73	27.65	698.53	28.56
17	205	18.08	24.24	302.13	31.29
18	340	18.88	26.30	227.14	29.18
19	250	17.41	21.15	313.88	27.65
20	295	18.38	27.06	423.34	28.18
21	325	17.42	36.57	431.45	28.64
22	210	18.76	36.66	297.54	28.45
23	200	18.10	36.08	292.01	32.48
24	210	16.02	41.68	305.90	26.03
25	185	16.58	33.61	410.59	32.55
26	260	18.85	38.94	80.67	27.68
27	310	17.81	41.52	387.71	26.91
28	190	17.86	43.89	495.10	25.81
29	235	18.65	38.06	394.80	28.15
30	285	24.03	45.19	388.91	31.07
Mean	257.06 ± 64.71	18.53 ± 1.82	33.00 ± 6.40	304.07 ± 138.63	28.94 ± 2.83

^a Randomly distributed. ^b Each capital letter represents the following corresponding name; A: drying time (h), B: total phenolic content (mg GAE/g DM), C: antioxidant capacity (µmol T.E./g DM), D: ascorbic acid content (mg A.A./100 g DM), E: hue angle value.

).

Color (L*, a*, b*, C*, h° values), a_w, moisture, pH, °Bx, titration acidity, texture, and rehydration ratio were determined as physicochemical analyses. The C*, h°, and a_w values of dried strawberries were determined to be 31.42 ± 3.09, 28.94 ± 2.83, and 0.43 ± 0.05, respectively. Humidity (%), °Bx, pH, and titratable acid (%) were also 8.98 ± 1.90, 67.97 ± 3.23, 3.68 ± 0.08, and 6.32 ± 0.81, respectively. The moisture, a_w, pH, and titration acidity (%) results of the strawberries used in the study were found to be quite similar to the literature [2,4,5,6].

In this study, the textural analysis of dried strawberries was also examined. Without CM treatment of dried samples (control), hardness values (g force), shell resistance (g force), and elasticity (mm) values ranged from 1751.92 to 3577.11, 521.23 to 1703.35, and 2.01 to 3.14, respectively. CM-treated dried samples’ hardness values (g force), shell resistance (g force), and elasticity (mm) values ranged from 1406.27 to 3385.39, 390.32 to 1463.77, and 2.16 to 3.92, respectively. These indicated that strawberries dried with CM treatment were softer and more elastic than control. The rehydration analysis results in the study ranged from 3.19 to 3.86 g/g. The texture and rehydration rate analysis results were consistent with previous research [37,38,39,40,41].

Additionally, in this study, three different anthocyanin compounds were used as standards, including pelargonidin-3-glucoside, cyanidin-3-glucoside, and cyanidin-3-rutinoside. As an anthocyanin, only pelargonidin-3-glycoside was found. The HMF analyses indicated that the amount of this compound in dried strawberry samples was under the detection limits of HPLC. All data, including ascorbic acid, pelargonidin-3-glucoside, and HMF, agreed with previous studies [2,3,4,5,6,7,33,42].

The drying times were found to range from 175 to 500 min, and that was greatly affected by the different drying and CM conditions, and a wide range of drying time was obtained in the drying processes (

Table 4. Equation coefficients and lack-of-fit values from models of the studied responses for drying time, TPC, AAC, ABTS, and h° of dried strawberries with CM pre-treatment.

Variables a	A b	B	C	D	E
β0	4761 ***	−47.9 ***	3348 ***	59.34 ***	69.77 ***
β1 (X1)	−138.1 ***	1.748 ***	−40.8 ns	−0.1821 *	−0.7490 ns
β2 (X2)	-	16.50 **	−983 ***	-	−7.51 **
β3 (X3)	-	−0.256 ***	−8.97 ***	−1.02 ns	0.9515 **
β4 (X4)	−2.583 *	0.260 **	−11.31 ***	−0.3667 ***	−3.155 ***
β11 (X1X1)	1.054 ***	0.01188 *	-	-	-
β22 (X2X2)	-	-	-	-	2.455 ***
β33 (X3X3)	-	0.00596 **	-	0.02221 ***	−0.01910 ***
β44 (X4X4)	-	0.01203 *	-	-	0.01706 **
β12 (X1X2)	-	−0.1938 **	14.79 **	-	-
β13 (X1X3)	-	-	-	-	-
β14 (X1X4)	-	-	-	-	-
β23 (X2X3)	-	−0.1110 ***	-	-	−0.0685 **
β24 (X2X4)	-	−0.1320 *	-	-	−0.1883 ***
β34 (X3X4)	-	0.01446 **	-	-	0.05573 ***
Model	***	***	***	***	***
R2	89.64	89.65	85.16	92.39	97.04
Pred. R2	80.67	72.69	70.87	87.56	89.73
Lack of fit	0.109	0.213	0.731	0.492	0.235

^a Variables: β₀ is the constant coefficient; β_i is the linear coefficient (main effect); β_ii is the quadratic coefficient; β_ij is the two factors’ interaction coefficient. ^b Capital letter represents the following corresponding name; A: drying time (h), B: total phenolic content (mg GAE/g DM), C: ascorbic acid content (mg AAC/100 g DM), D: antioxidant capacity (µmol T.E./g DM), E: hue angle value. ^ns, not significant (p > 0.05); *, significant at p ≤ 0.05; **, significant at p ≤ 0.01; ***, significant at p ≤ 0.001—the values with a significance level less than 0.05 were removed from the model using backward elimination. Subscripts 1, 2, 3, and 4 represent drying temperature, CM pressure, CM temperature, and CM time, respectively.

). To the best of our knowledge, no study was in the literature in which CM pre-treatment was performed before drying strawberries. However, it was reported that pre-treatments applied before drying, such as sonication, osmotic drying, immersion in sodium metabisulfite and ethyl oleate solutions, pulsed electric fields, and holding under hydrostatic pressure, shortened the drying time of strawberries [37,43,44,45,46,47,48]. Similar to our results, 30–72% reduction in the drying time was reported for fruits and vegetables such as red grapes, tomatoes, potatoes, plums, capia peppers, and ginger that were pre-treated with CM before drying. The shortening of the drying time with the CM pre-treatment was thought to be due to the deformation of the cell structure and vacuoles of fruits due to the application of the carbon dioxide gas [18,19,20,22,23,49].

The TPC and antioxidant capacity (ABTS) of the dried samples were determined. They were in the range of 15.18–24.03 GAE/g DM and 21.15–45.19 μmol T.E./g DM, respectively (Table 4). TPC ranged from 11 to 31 mg GAE/g DM in strawberry drying studies, and antioxidant capacities ranged from 2.7 to 382 mol TE/g DM. These results were remarkably close to the values obtained in our research [7,37,42,50,51].

Values of ascorbic acid in the dried samples ranged from 64.61 to 698.53 mg A.A./100 g DM, and the changes were influenced by the process conditions of CM pre-treatment and drying (Table 4). The amount of ascorbic acid in the literature ranged from 86 to 710 mg A.A./100 g DM, which is consistent with previously reported findings [2,4,5,6,7,33].

Hue angle represents the dominant color wavelength and is angularly defined from 0° to 360° (Table 4). This dimension is defined by Munsell as “the quality that distinguishes one color from another, such as yellow, green, blue, or purple from the red” [52]. According to [53], the color angle has the strongest correlation with visual assessment. The research found that the lowest hue angle value was 22.19 and the highest was 35.61. Many studies in the literature reported a hue angle of 28.50 for strawberries, which is similar to our results [33,47,54].

3.2. Modeling of Drying Time, Total Phenolic Content, Antioxidant Capacity, Ascorbic Acid Content, and Hue Angle

In this study, the goal was to optimize the CM-pre-treated hot-air-drying process conditions in terms of drying time, color parameters, moisture, a_w, pH, Bx, titration acidity, rehydration ratio, texture analyses, total phenolic content, antioxidant capacity as ABTS method, ascorbic acid content, HMF content, and anthocyanin content using RSM. The drying time, TPC, AAC, ABTS, and hue angle were found to be significant results for model optimization. The model parameters and regression coefficiency of developed models for CM-treated dry strawberries are presented in Table 3. For modeling, the alpha value was chosen as 0.05, and variables were progressively excluded from the regression model at each step, using backward elimination regression to find the best-explained data. The results revealed that the models produced for drying time, TPC, ABTS, AAC, and hue value of dried strawberry samples performed well in explaining data variations based on process parameters. As can be seen in Table 3, these models could explain more than 90% of the variation in hue value and ABTS and 85% for drying time, TPC, and AAC, and the models were significant for these five responses (p ≤ 0.001).

Drying time is the most important independent variable in the drying process, since the goal is to reduce energy consumption and improve dried product quality [32]. The first model was developed for drying time. When the mathematical equation was examined, it was observed that only two first-order terms belonging to drying temperature and CM time as well as the second-order term of drying temperature were found as statistically significant ones. All remaining terms of the second-order polynomial were omitted from the equation. The lack-of-fit test result showed that there was not any significant fitting problem for the relevant model (p > 0.05). To represent the change in drying time with drying temperature and CM time, Figure 1 was drawn. As significant terms, the curvature effect of drying temperature was observed in Figure 1, where drying time only linearly changed with CM time. Thus, it can be said that the temperature increase caused a decrease in drying time up to 65 °C, but a further increase in temperature did not create any significant change. On the other hand, process time was prolonged with a decrease in CM time. It is well-known that the drying process is accelerated with a temperature increase, since more energy is thermally transferred to the food material and causes more water evaporation per unit of time [55]. In our study, a similar effect of temperature increase was seen, but it was not limitless. This may be related to the antagonistic impact of high temperature at raised levels being a cause of hardening and shrinkage. The other important factor was CM time. A prolonged pre-treatment time caused a reduction in the drying process. The reason for this acceleration in the drying process may be related to the structural effects of carbon dioxide on fruit materials, since it is known that carbon dioxide is converted to carbonic acid in fruit structure after it is dissolved and causes some cracks, especially on the fruit surface, and softening in fruit tissue, and it facilitates water transfer throughout the fruit matrix. With an increase in pre-treatment, the effect of dissolved carbon dioxide in fruit tissue progressed more and made the fruit environment suitable for molecular water transfer to the surface. There were no studies on CM-treated strawberries. However, parallel to our findings, drying time was reduced by 30–72% in samples dried using different drying techniques after CM pre-treatment for red grape, capia pepper, plum, ginger, tomato, and potato [18,19,23,49]. Modeling studies on hot-air-drying strawberries have found that a temperature range of 60–65 °C is the optimum drying temperature value, which is consistent with our findings [3,43].

Phenolic compounds are important food components because they participate in a variety of processes such as enzyme inhibition, taste and odor production, color development and change, antibacterial and antioxidant effects, and participation in color formation and change, all of which have a direct impact on the final product [56]. The developed model explained nearly 90% of the variability in TPC as a function of CM pre-treatment and drying conditions. First-order drying and CM temperatures, as well as the interaction terms of CM pressure and time, were found to be significant in TPC models at the level of 0.001 (Table 3). First-order CM pressure and time, second-order CM pressure, and the interactions between drying temperature/CM pressure and CM temperature/CM time were significant at the level of 0.01 (Table 3). Furthermore, the interaction of second-order drying temperature with time of CM and pressure of CM with time were both significant at p ≤ 0.05 for TPC. The result of the lack-of-fit test indicated that there was no significant fitting problem for the relevant model (p > 0.05).

The variation of TPC depending on temperature, pressure and time of CM, and drying temperature are given in Figure 2a–c and Figure 3a–c. At low CM pressure levels, the amount of TPC first decreased and then increased as the CM temperature rose, but with increasing CM pressure, only a decrease in TPC was observed with CM temperature elevation. The highest TPC content was seen to be available at the highest CM pressure and lowest CM temperature levels (Figure 2a). Another parameter, CM time, showed a curvature effect on the TPC of strawberries (Figure 2b,c), and its lower levels were suitable for achieving the higher TPC values. In the experimental design, drying temperature was also examined, and its interaction with pre-treatment conditions was also evaluated. All these relations were illustrated in Figure 3a–c. An increase in drying temperature from 50 °C to 70 °C resulted in a decrease in TPC, while with high CM pressure values, an increase in drying temperature resulted in better-preserved TPC. Thus, it is concluded that the temperature level between 55 and 65 °C was suitable for the drying of strawberries, and the lowest temperature and shortest time of CM pre-treatment at the highest pressure level were better to accompany drying to obtain TPC-rich dried strawberries. To the best of our knowledge, no CM-pre-treated drying study for strawberries was found in the literature. However, in line with our findings, phenolic compounds were reported to be preserved 11–60% better during drying after CM application for red grape, capia pepper, plum, ginger, tomato, and potato [18,19,20,23,49]. This variation can be attributed to changes in the low pH values of the CM-treated samples, which can improve TPC extraction [57].

The strawberry fruit is high in ascorbic acid, a naturally water-soluble vitamin. It is an effective reducing and antioxidant agent that promotes collagen formation in fibrous tissue, bones, teeth, capillaries, and skin. The developed model, as a function of CM pre-treatment and drying conditions, was able to explain 85% of the variability in AAC. According to the mathematical model, it was discovered that only the first-order term related to drying temperature was statistically insignificant (p > 0.05). All remaining first-order terms were found to be significant p ≤ 0.001. None of the second-order terms were in the developed model (p > 0.05). Only one interaction term (drying temperature–CM pressure) was found to be statistically significant (p ≤ 0.01) (Table 3). The result of the lack-of-fit test showed that there was no problem with model fitting (p > 0.05). The effects of drying temperature and CM conditions on AAC are shown in Figure 4a–c. AAC was negatively affected by CM pressure, time, and temperature, as illustrated in Figure 4a,b, and AAC dropped as all three factors increased. Figure 4c depicts the interaction impact of drying temperature and CM pressure on AAC. AAC decreased as CM pressure increased; however, increasing drying temperature at high pressure levels provided better AAC protection. This might be owing to the drying time being reduced at high pressure and high temperatures. Since no result was published to identify the effect of CM on the AAC of strawberries for a drying process, our results were compared in terms of trends. According to the literature, AAC was preserved better when CM was used as a pre-treatment and its value was 121–582% higher than that obtained in drying processes of different vegetables and fruits without any pre-treatment application [19,20].

The presence of efficient oxygen radical scavengers, such as vitamin C and phenolic compounds, is closely related to the antioxidant capacity of fruits [58]. Strawberries are nutritious fruits that contain antioxidant-rich components because they contain ellagic acid (hexahydroxydiphenic acid), polyphenolics, ascorbic acid, anthocyanins, and particularly flavonoids, which help to neutralize free radical damage [59]. In Table 3, the developed model explained the antioxidant capacity value very well as a function of CM pre-treatment and drying conditions, according to the calculated and predicted R² values. When the mathematical expression was evaluated, only two first-order terms belonging to drying temperature and CM time, as well as the second-order term of CM temperature, were found to be statistically significant (p ≤ 0.05) (Table 3). The equation was modified by removing all remaining insignificant terms from the second-order polynomial using backward elimination. In addition, there was no significant lack-of-fit problem for the relevant model (p > 0.05). Figure 5a showed the change in ABTS as a function of drying temperature and CM time. As a significant term, the curvature effect of CM temperature was also observed in Figure 5b,c, where drying temperature and CM time caused linear decreases in ABTS value. The effect of CM time on ABTS is stronger than the effect of drying temperature. ABTS values were lowest at CM temperatures ranging from 20 to 25 °C and highest at 4 °C. ABTS decreased and then increased with increasing CM temperature. (Figure 5b,c). Irrespective of conditions, ABTS values of the dried strawberries which were pre-treated with CM were higher than that value determined in the samples directly subjected to drying. A literature survey showed that CM pre-treatment helped to preserve the antioxidant potential of dried fruit or vegetable samples, and the protection level was 2–56% higher than the case in which any pre-treatment was handled before drying [19,20,23]. The higher antioxidant activity in CM-pre-treated dried strawberries may be associated with the compounds like phenolics and vitamin C. Another reason, as also found in a study, is that the stability of polyphenols can be explained by the reduction in carbon dioxide when the fruit pH changes from basic to acidic [60].

The hue value, another dependent variable examined in this study, allows the observer to distinguish one color from another. It is described using common colors such as red, green, blue, and yellow (RGB) and refers to the lightness or darkness of the color [61]. This color value has been found to be important in many drying studies, particularly on red-colored fruits like strawberries and fruits such as kiwi and banana; it is emphasized more than other color parameters in the literature [2,4,5,7,33,62]. In the current study, the hue value was discovered to be the best-explained dependent variable, with a 97% explanation percentage, approximately 90% prediction power, and no significant lack-of-fit problem (p > 0.05). Only the first-order term of drying temperature was statistically insignificant (p > 0.05), while all studied process conditions of CM were found to be statistically significant as being first- and second-order terms (p ≤ 0.01). The ANOVA table of the model for hue angle indicated that there were significant interactions between CM conditions (p ≤ 0.01), but any interaction term including drying temperature was in the model (Table 3). The drying temperature variable, either alone or in combination, had no effect on the hue value. Figure 6a–c shows the change in hue value as a function of CM temperature, CM time, and CM pressure. The curvature effects of CM temperature and CM pressure can be seen in Figure 6a, where the hue value first increased and then decreased as the CM temperature increased; on the other hand, it first decreased with an increase in CM pressure, and then the observed trend changed at the moderate pressure level. An increase was observed with a further increase in the pressure. The curvature effects of CM conditions on the hue value are shown in Figure 6a–c, where the observed trends for CM temperature and pressure were strong, but it was weak for CM time. Thus, it was understood that increasing CM time caused an increase in the hue value, but the change was not as sharp as that increase observed with CM pressure and temperature. The statistical significance of the first and second terms of the CM parameters revealed the importance of optimizing CM conditions for the dried strawberries to have the color tone similar to fresh samples. As far as we know, no studies on strawberries pre-treated with CM have been found in the literature. However, it was reported that the amount of lycopene, β-carotene, and flavonoid substances, which were highly effective on color, were better preserved in CM-treated tomato and potato samples [20,21]. According to the findings of the study, the amount of anthocyanin in the drying process was better preserved in the samples that were treated with CM. The total amount of anthocyanin substance has been found to be positively correlated with the hue value (r = 0.9) (2). In this context, it is clear that the color value of strawberries dried by the CM process was superior.

3.3. Optimization and Validation of Carbonic Maceration Pre-Treatment Process Conditions and Drying Temperature in Terms of AAC, ABTS, Drying Time, and Hue Value

The desirability function was used in this study to determine the optimum conditions. Desirability ranges from 0 to 1 for each given response, and they are calculated using the Minitab statistical software. A value of 1 reflects the ideal condition (success), whereas a value of 0 indicates that one or more replies exceed the desired limits [63]. The obtained data were statistically evaluated, and dependent variables with high predictive power were identified: drying time, TPC AAC, ABTS, and hue value. AAC, TPC, and ABTS were maximized during the optimization process, drying time was minimized, and the hue value of fresh strawberries was targeted. Under these conditions, the optimum drying temperature, CM time, CM pressure, and CM temperature were 64.8 °C, 4 h, 3.54 bar, and 4.2 °C, respectively. The desirability, accepted as a measure to achieve the desired values in the optimization of the responses, was calculated as 0.88. To evaluate whether the theoretical data predicted by the model were matched with the real data and to confirm its validity, the drying process was carried under the defined optimal conditions. The model estimated for these validation treatments was hue value, amount of AAC, ABTS, TPC, and drying time, which ranged from 25–30, 387–743 mg/100 g dry matter, 35 to 46 µmol TE/g dry matter, 21 to 28 mg GAE/g dry matter, and between 178 and 291 min, respectively. When the results of the analyses were statistically evaluated, the theoretical and experimental data remained within the confidence intervals, and the analyses were successfully validated.

Table 5. Results of analyses with and without CM-pre-treated dried products.

Result	With Optimum CM Pre-Treated	Without CM Pre-Treatment
DT	195 ± 5 a	285 ± 10 b
DR	0.0456 ± 0.01 a	0.0313 ± 0.01 b
pH	3.85 ± 0.02 a	3.85 ± 0.01 a
Moisture %	9.86 ± 0.30 a	8.90 ± 0.25 b
aw	0.378 ± 0.03 a	0.374 ± 0.04 a
h°	27.61 ± 2.41 a	27.57 ± 4.39 a
C*	29.39 ± 6.75 a	29.59 ± 7.51 a
BI	95.86 ± 14.48 a	110.1 ± 13.15 a
TPC	45.93 ± 2.19 a	36.50 ± 2.11 b
AAC	521.70 ± 4.24 a	481.96 ± 6.71 b
ABTS	47.53 ± 0.38 a	38.95 ± 0.35 b

DT: drying time (min), DR: drying rate g water/g DM, a_w: water activity, h°: hue value, C*: chroma, BI: browning index, TPC: total phenolic compounds mg GAE/g DM, AAC: ascorbic acid amount mg AA/100 g DM, ABTS: antioxidant activity µmol TE/g DM. The ^a,b change in the same line shows the difference between the samples with and without CM pre-treatment (p ≤ 0.05).

shows the analysis results of dried samples with and without the CM pre-treatment.

When the drying times of strawberries with the same initial weight were compared, it was found that the drying time without CM was 285 min, while the drying time of the samples with CM pre-treatment at the optimum conditions was 195 min. When drying rates were compared, the drying rate of CM-pre-treated strawberries under optimum conditions was higher than that of non-CM-treated samples (Table 5). In this context, it is believed that the CM process caused structural changes which accelerated the moisture removal from the fruit matrix. When comparing the browning index values of the control and the optimal samples, the samples treated with CM had significantly lower browning index values (p < 0.05). The more intense browning reaction that occurs during heat treatment has been explained with a higher browning index in the literature [64,65]. The lower browning index of the optimal samples can be explained by the reduced drying time, increased L* value, and acidity due to the carbonic acid formed during the CM treatment [19,49]. In the literature, the low pH as an acidic environment provided by CM pre-treatment, inactivation of polyphenol oxidase and peroxidase enzymes, increased total phenolic substance, antioxidant activity, lycopene, β-carotene, β-γ-tocopherol, and flavonoids have been reported with CM applications. According to reports, CM pre-treatment protected the product well, there was no pH change in the final product, the drying time was reduced by 30–72%, and the effective diffusion value increased [18,19,20,21,23].

3.4. Scanning Electron Microscope Images

In addition to other analyses, SEM analyses for dried strawberry samples with and without CM (control) pre-treatment was performed and illustrated in Figure 7.

When the images were examined, it was seen that the cell walls of the non-CM-pre-treated samples (control) remained tight, rigid, and less porous, while the CM-pre-treated samples had collapsed zones, and the formation of holes was observed in the structure. It is believed that with these structural changes that occurred during CM treatment, the removal of water from the strawberries was facilitated, and thus the drying time was shortened. There were no studies on carbonic-maceration-treated strawberries. On the other hand, [23] reported that the pressure, temperature, and time of CM had significant effects on the microstructure of the solid matrix, and the drying rate of ginger was increased, which was similar to the results of the current study. It had been shown that as CM pressure, drying temperature, and CM time increased, the pore structures increased and expanded, and these changes were beneficial for internal moisture diffusion and moisture evaporation. These results were also consistent with the grape study published by [66]. In a study conducted by [42], SEM images were taken and examined for structural changes in frozen strawberry slices dried by pressure and hot air. The cell wall was found to be broken up, and the structure became more fragile and hollower when pressure was applied [42]. In another study by [67], strawberries were freeze-dried, vacuum-dried, and air-dried, and the products were evaluated with sensorial, textural, and structural analyses. It had been shown that the degree of drying pressure and temperature were important for the structural changes that occurred during strawberry drying, as well as for the fracture and deformation that occurred in the structure.

4. Conclusions

In this study, the potential of CM as a pre-treatment applied before convectional drying was examined for strawberry fruits, which have high economical value, especially as an organic product. To achieve the highest quality with better performance, the CM treatment and accompanying drying process were modelled and optimized. In the optimization, the models with high prediction performances were used.

As optimal conditions, the drying temperature, CM pressure, CM temperature, and CM time were evaluated, and corresponding values were found as 64.8 °C, 3.54 bar, 4.2 °C, and 4 h, respectively. The results for drying time, total phenolic content, ascorbic acid content, antioxidant activity, and hue angle were satisfied; thus, CM treatment can be classified as a promising technique applied before the drying of strawberries. Compared to the control, on almost all of the quality parameters, the positive effect of shortened drying was seen, e.g., higher TPC, AAC, antioxidant activity, and a close hue angle value to the fresh samples. CM significantly increased some color parameters (L* and hue angle) while decreasing the browning index compared to the control samples. The TPC, AAC, and ABTS of CM-treated strawberries were significantly preserved, and their textural properties were found to be softer compared to the control samples. However, the differences in pH and titration acidity were found to be insignificant.