Department of Food Engineering, Pamukale University, TürkiyeEngin DemirayTeamIn《A research paper titled 'The Effect of Drying Temperature and Thickness on the Drying Kinetic, Antioxidant Activity, Phenolic Compounds, and Color Values of Apple Slices' was published in the Journal of Food Quality. This study investigated the effects of different drying temperatures (45, 55, 65 ° C) and apple slice thicknesses (1.5 mm and 5 mm) on drying kinetics, antioxidant activity, total phenolic compounds, and color changes during hot air drying.

Abstract

Dried fruit slices are important, healthy, and popular snacks that are increasingly valued due to their high nutritional content. In this context, this study mainly focuses on the production of healthy apple chip snacks and measures them at three different temperatures（Degradation kinetics of antioxidant activity, total phenolic compounds, and color values of apple crisp snacks during convective hot air drying at 45, 55, and 65 ° C and sample thickness (1.5 and 5 ± 0.5 mm). Simultaneously calculated the drying kinetics, desorption isotherms, activation energy, and half-life of apple crispy snacks. The Page model and GAB model are the best models for determining the drying behavior (R ²>0.992) and desorption behavior (R ²>0.9979) of apple snacks, respectively, with the highest R ² values. All samples were dried during the deceleration stage. The effective moisture diffusion coefficient (D_eff) value increases with the increase of air temperature and slice thickness. The antioxidant activity, total phenolic compounds, and total color change of the sample with a thickness of 5 mm follow first-order reaction kinetics. Under conditions of low thickness (1.5 mm) and temperature (45 ° C), higher antioxidant activity, phenolic compounds, L value, and lower half-life value were observed. The calculated activation energy value of total phenolic compounds is higher than the activation energy value of antioxidant activity. The results indicate that choosing low temperature and low slice thickness can produce apple crispy snacks with high nutritional value.

Experimental materials and instruments

Experimental materials

Apple（Storage Conditions:Store in a refrigerator at 4 ° C until the experiment begins to dry）

Reagent:70% ethanol, methanol, DPPH (2,2-diphenyl-1-picrylhydrazine), Trolox, Folin Ciocalteu reagent, sodium carbonate, gallic acid.

Experimentinstrument

Drying cabinet、vernier caliperThe GBXeFAst Lab water activity meter、Ultrasonic water bath、Track shaker、Centrifuge、spectrophotometer、Colorimeter、analytical balance

GBXeFAstLab water activity meter

Experimental Procedure

Sample preparation: Wash, peel, and remove inedible parts from apples, then cut them with a knifeThin films with thicknesses of 1.5 mm and 5 ± 0.5 mm.

Hot air drying: Place the apple slices in a drying cabinet andDry at three temperatures of 45, 55, and 65 ° C with a constant air flow rate of 0.2 m/s. Take out the tray and weigh it every 30 minutes, and record the data. After drying, the sample is placed in a dryer at room temperature for 30 minutes, then at 4 ° C for 1 hour, and finally cooled and frozen at -20 ° C. Measure antioxidant activity, total phenolic compounds, and color values every 30 minutes during the drying process.

GBXeFAst Lab water activity meter

Changes in moisture content during the drying process of apple slices（（a) =1.5 0.5 mm thickness, (b)=5 0.5 mm thickness)

The relationship between drying rate and moisture content of apple slices（（a) =1.5 soil 0.5 mm thickness, (b)=5 0.5mm thickness)

Desorption isotherm modeling: selectionGAB, BET, Oswin, Henderson, Halsey models.

Preparation of extraction solution: Take approximatelyGrind 2 g of apple sample, add 10 mL of 70% ethanol, sonicate for 10 minutes, shake on a shaker for 15 minutes, centrifuge (10 ° C, 7450 rpm, 10 minutes), take the supernatant, repeat extraction once, merge and dilute to 25 mL with 70% ethanol, store at -20 ° C.

Determination of antioxidant activity（DPPH method: Adjust the absorbance of DPPH methanol working solution to 1.20 ± 0.02, and prepare a standard curve using Trolox. Mix 150 μ L of sample or standard with 2850 μ L of DPPH working solution, react in the dark for 60 minutes, and read the absorbance at 515 nm.

Determination of total phenolic compounds（Folin Ciocalteu method: Dilute FC reagent at 1:10 with a 20% sodium carbonate solution. Standard curve for gallic acid production (5-100 mg/L). Take 2 mL of sample or standard, add 10 mL of diluted FC reagent, add 8 mL of 20% sodium carbonate within 1-8 minutes, react in the dark for 2 hours, and read the absorbance at 760 nm.

Color measurement: using a colorimeter for measurementL. Calculate the values of a and b, including color phase angle, saturation, and total color change Δ E.

Degradation kinetics modeling: using zero order, first-order, and second-order reaction kinetics models (formulas)7-9) Calculate the reaction rate constant k, Q ₁₀ value, half-life t ₁/₂, and activation energy Ea.

Statistical analysis: Two parallel experiments with three replicates were conducted, usingPerform Duncan's multiple comparison test using SPSS 20.0, p ≤ 0.05。

ExperimentConclusion

Drying kinetics

The initial moisture content is6.74 kg water/kg dry matter, initial water activity 0.962. The water activity of the 1.5mm thick sample after drying is 0.348 (45 ° C), 0.278 (55 ° C), and 0.299 (65 ° C); The 5mm thick samples are 0.425 (45 ° C), 0.396 (55 ° C), and 0.353 (65 ° C).

The drying time decreases with increasing temperature and decreasing thickness:Drying time for 1.5 mm thickness is 120-180 minutes, and for 5 mm thickness it is 180-330 minutes. It takes 120 minutes for the 1.5 mm sample to decrease from 6.74 to 0.27 kg water/kg dry matter at 65 ° C.

The Page model fits the best (R ²>0.992), and the drying process is in the deceleration stage.

D_eff values: 3.37E-07 to 4.29E-07 m ²/s at a thickness of 1.5 mm, 3.07E-06 to 4.31E-06 m ²/s at a thickness of 5 mm, increasing with temperature and thickness.

Activation energy:The thickness of 1.5 mm is 10.56 kJ/mol, and the thickness of 5 mm is 14.70 kJ/mol.

Desorption isotherm

The GAB model is the best fitting model (with the highest R ²), and the k value increases with temperature. There is no consistent trend in the single-layer moisture content.

Antioxidant activity and total phenolic compounds

The antioxidant activity of fresh apple slices is289.24 μ mol Trolox equivalent/100g DM, total phenolic compounds are 957.63 mg gallic acid equivalent/100g DM.

The loss of antioxidant activity after drying is about90%, with a loss of approximately 55% in total phenolic compounds. The antioxidant activity was highest at a thickness of 1.5 mm and 55 ° C (34.44 μ mol TE/100g DM); The total phenolic compounds were highest at 45 ° C and 1.5 mm (411.90 mg GAE/100g DM).

Degradation kinetics:The antioxidant activity and total phenolic compounds of the 5mm thick sample follow a first-order reaction; The antioxidant activity of 1.5 mm thickness follows a second-order reaction, while the total phenolic compounds follow a zero order reaction.

The Q ₁₀ value shows that the temperature sensitivity is higher in the 65-55 ° C range than in the 45-55 ° C range. The half-life decreases with increasing temperature and increases with increasing thickness. The activation energy of total phenolic compounds is higher than their antioxidant activity.

Summary

When producing apple chips,The drying speed of 1.5mm flakes is faster, and the biochemical quality is better preserved. 45 ° C low temperature is more conducive to preserving antioxidant activity, phenolic compounds, and color than 65 ° C. Low temperature and low thickness conditions have advantages in preserving the quality characteristics of apple crispy snacks. The research results provide a basis for the selection of quality maintenance conditions in the drying process of apple slices.

References

1. Rasooli Sharabiani, V., Kaveh, M., Abdi, R., Szymanek, M., & Tanaś, W. (2021). Estimation of moisture ratio for apple drying by convective and microwave methods using artificial neural network modeling. Scientific Reports, 11(1), 9155.

2. Arora, B., Sethi, S., Joshi, A., Sagar, V. R., & Sharma, R. R. (2018). Antioxidant degradation kinetics in apples. Journal of Food Science and Technology, 55(4), 1306-1313.