Development of Modified Gas Composition for Atmosphere Packaging of Sliced Apple Chips

Abstract

In this study, we report the development of mixed gas composition for packaging dried apple slices in a modified atmosphere to extend their shelf life and maintain their quality. We used the response surface methodology to optimize oxygen and carbon dioxide concentrations in the mixture for packaging in a modified atmosphere based on the changes in mass, hardness, browning index, polyphenols, and vitamin C content during apple chip storage. Studies have shown that the optimal concentration of oxygen should be 2.663% and carbon dioxide 3.785% when packaging dried apple slices in a modified atmosphere to obtain minimal changes in the measured quality attributes. These findings can be applied in food processing and storage, providing a useful guideline for improving the preservation and nutritional value of dried fruits.

1. Introduction

One of the most popular and widely processed fruits is apples. A leading European country in global apple production and export is Poland. In 2023 alone, almost 3.9 million tons of apples were harvested [1]. The average monthly fruit consumption in Polish households increased from 3.46 kg/per capita in 2010 to 3.71 kg/per capita in 2023 [2]. Apples are a source of dietary fiber (2–2.4 g/100 g), vitamin C (3–23 mg/100 g), and microelements such as calcium (6 mg/100 g), potassium (107 mg/100 g), and zinc (0.04 mg/100 g [3,4,5]. Apples are especially rich in polyphenols (66.2–211.9 mg/100 g) [6,7]. Moreover, these compounds exhibit antioxidant properties. Levels of particular elements differ between apple varieties and depend on cultivation conditions [8,9,10]. Considering the sensory and nutritional values of apples, these fruits are good materials for further processing.

The shelf-life of seasonal fruits and vegetables should be prolonged using appropriate processing techniques that preserve their natural values [11,12]. Moreover, growing consumer consciousness and nutritional knowledge drive them to search for high-quality products rich in beneficial substances. As a result, convenient and functional food is more desirable [13]. Technologies aimed at minimizing food processing while preserving its sensory, nutritional, and textural properties—ensuring both efficiency and safety—are being used. Emerging green methods are expected to be cost-effective, energy-efficient, and adaptable to different types of food and processing methods. However, many current green technologies do not fully meet these expectations, prompting the food industry to seek alternatives free from synthetic chemicals [14].

Convection drying is a basic and commonly used food shelf-life prolongation process aiming to reduce water activity. It extends the shelf-life of the product and maintains its nutritional value. However, high temperatures can damage thermolabile bioactive compounds, cause discoloration, influence the texture, appearance, etc. [15]. Most food products are heat sensitive and drying procedures should be carefully conducted. Research has shown that drying at lower temperatures contributes to lower losses of nutritional compounds [16]. Fruits are one of the most commonly dried products. An example of a dehydrated food is dried sliced apple chips, which have become a popular snack recently [17]. This form of apples is very convenient and ready to eat, as the fruit has already been washed and/or peeled before further processing. Likewise, dried apple chips are lighter than fresh fruits and can be easily transported without bruising. Due to the water evaporation during drying, the change of matrix from apples to apple chips makes the compounds share higher because of its concentration [18].

Considering product quality, one of the most important attributes is color and shrinkage [16]. These parameters are connected with food quality evaluation and decide consumer satisfaction. The product’s appearance gives information about its freshness, taste, aroma, moisture content, and nutritional value [19,20]. Apples and their processed forms, such as dried or apple chips, are particularly vulnerable to quality deterioration due to oxidation and microbial activity. The slicing process damages the cellular structure, leading to accelerated enzymatic browning and changes in texture and flavor [21,22]. The enzymatic browning causes color changes due to the oxidase reaction of phenolic compounds with atmospheric oxygen which diffuses into fruit tissue. To prevent this negative impact, anti-browning agents such as citric acid are used [23]. During drying, either enzymatic and non-enzymatic color changes occur. These could enhance the negative effect and affect the acceptability of dried and semi-dried products [24]. Another fundamental quality determinant is texture. The drying process intensively influences the fruit’s microstructure so there is a risk of receiving products of undesirable texture. The most important textural parameter of dried apples is hardness which is associated with moisture content [18]. Optimizing process parameters is crucial for achieving a satisfactory final product quality.

The modern food market imposes increasingly stringent requirements regarding food products’ quality, shelf life, and safety. In response to these demands, innovative packaging technologies are being developed to extend product shelf life while maintaining high sensory and nutritional values. One effective method for extending the shelf life of food is modified atmosphere packaging (MAP), which involves replacing air with a specifically designed gas mixture suited to the product [25,26,27]. Modified atmosphere packaging helps maintain product quality and additionally extends its shelf life. The usage of MAP packaging to control changes in fruits and vegetables during storage (ripening, browning, etc.) [28]. It also reduces the respiration rate. The following gases are currently in use: O₂, CO₂ and N₂. Creating a suitable gas mixture for a specific product is essential [27,29,30]. There is little research taking into consideration the MAP gas composition used for apple storage [31].

Therefore, our research aimed to develop and optimize the gas composition in modified atmosphere packaging of apple dry slices to extend their shelf life without deterioration of quality. To our knowledge, there is little research investigating the physicochemical changes during storage in terms of dried apple slices in modified atmosphere packaging where different concentrations of oxygen and carbon dioxide have been used. Since this study aims to determine the optimal gas composition for storing dried apple slices, the results could be beneficial for scientific and industrial entities, as well as consumers who seek high-quality products with an extended shelf life, without the need for artificial preservatives.

2. Materials and Methods

2.1. Apple Dried Slice Preparation

The experimental material comprised apples Malus domestica cultivar Egremont Russet. Apples, harvested during the same period (October) and originating from a local orchard (Grójec, Mazovia, Poland) stored at 4 °C until the moment of the experiment. The apples were selected based on their appearance and stage maturity, which was evaluated using a dry basis, reflective index, and acidity.

Table 1. Mean and standard deviation of chosen parameters of fresh apples.

	Mean ± SD
Dry Basis (%)	16.03 ± 1.72
Reflective index (°Brix)	14.23 ± 1.16
Acidity (mEq NaOH/100 g)	10.54 ± 0.93
Vitamin C content (mg/100 g dry basis)	64.26 ± 1.45
Total phenols content in water extract (mg/g dry basis)	71.12 ± 2.02

shows the chosen parameters of fresh apples. Apples were washed, drained, and cored using a cork borer (25 mm diameter) and then mechanically cut perpendicular to the fruit axis into 4 mm-thick slices using a slicer machine (model 210p CE, Ma-Ga, Bydgoszcz, Poland). Only the eight central slices of each apple were used. To avoid enzymatic browning reactions, the slices were immersed in a 0.1% citric acid solution immediately after cutting until drying (approx. 1 min) and were blotted with filter paper.

2.2. Convective Drying

Air-drying of apple slices was performed in a drying oven (model FED 115, Binder, Tuttlingen, Germany) at 70 °C with forced convection (air velocity 1.5 m/s, 35–40% RH). Samples were placed on the perforated tray positioned perpendicular to airflow. The drying process was carried on until the equilibrium weight state (about 11% water content in dried samples).

2.3. Modified Atmosphere Packaging

After cooling at ambient temperature for 1 h in the desiccator with silica gel, dried apple sliced chips were packed in transparent polypropylene bags (PP package, thickness 550 µm) and covered with PET/CPP/AF laminate (thickness 44 µm).

The apple chips were packed at 1050 mbar pressure and supplied with following an initial gas mixture: 100% N₂, 2% O₂/2% CO₂ in N₂, 2% O₂/4% CO₂ in N₂, 4% O₂/2% CO₂ in N₂, 4% O₂/4% CO₂ in N₂ (prepared by Air Liquide, Cracow, Poland) with using packing machine (Sealpac M3, Oldenburg, Germany) and stored in the protected from light, at ambient temperature. Sampling was performed on 0, 15, 30 days.

2.4. Measurement of Dry Basis, Reflation Index, and Acidity

The dry basis of raw apples was measured gravimetrically by drying the samples at 105 °C. The reflective index, as sugar, was determined in apple juice using a hand-held optical refractometer (model WS-B80, ATAGO Co., Ltd., Tokyo, Japan) and expressed as °Brix. Titratable acidity was determined by titration against a 0.1 N NaOH solution to pH 8.1 and the acidity was expressed as mEq NaOH/100 g.

2.5. Color Measurement and Browning Index

The color of dried apple sliced chips was instrumentally measured using a Minolta CR-400 colorimeter (Konica Minolta Inc., Osaka, Japan). The following parameters were determined L* (lightness, L* = 0 black, L* = 100 white), a* (−a* = greenness; +a* = redness) and b* (−b* = blueness; +b* = yellowness). To achieve accurate results the calibration was done with a white reference standard (L* = 98.45, a* = 0.10, b* = 0.13). The colorimeter was set to a measurement area of 8 mm, illuminant D65, and standard observers 2°. Every sample (0, 15, and 30 days of storage in different atmospheres) was measured in three different locations of ten chips.

The color changes during storage were determined using the color difference coefficient (ΔE), which was calculated from the following Equation (1):

$$∆E=\sqrt{{\left(L_x^{\mathrm{*}}-L_0^{\mathrm{*}}\right)}^2+{\left(a_x^{\mathrm{*}}-a_0^{\mathrm{*}}\right)}^2{+\left(b_x^{\mathrm{*}}-b_0^{\mathrm{*}}\right)}^2}$$

(1)

where L*₀, a*₀, and b*₀ are the initial color coordinates of dried apple sliced chips (day 0) and L*_x, a*_x, and b*_x were the color coordinates of dried apple sliced chips after storage 15 or 30 d.

Browning index (BI) was also reported. This indicator shows changes in brown color in food products containing sugar. Results were calculated according to the equations below (2 and 3, respectively) [23].

$$BI={\displaystyle \frac{\left[100\left(x-0.31\right)\right]}{0.17}}$$

(2)

where

$$x={\displaystyle \frac{(a^{\mathrm{*}}+1.75L^{\mathrm{*}})}{(5.645L^{\mathrm{*}}+a^{\mathrm{*}}-3.012b^{\mathrm{*}})}}$$

(3)

2.6. Textural Properties

The maximum force required to puncture the dried apple slice chips was used as a parameter to determine the textural properties. The force at fracture (hardness) was measured in apple slices chips by a puncture test using a universal testing machine Instron (Model 5965, Norwood, MA, USA). For the puncture test, apple chips were placed on a platform and a cylindrical 8 mm diameter probe was used to puncture the samples in the center at a crosshead speed of 60 mm/min (load cell 500 N) until the sample was penetrated through the whole height of the tissue (displacement of the probe 5 mm).

2.7. Total Phenol Content

Dried apple slices were analyzed for total phenol content according to the Folin–Ciocalteu assay [32]. Samples were prepared as described by Sudha et al. [33] with authors modifications. Three grams of homogenized material was defatted with 40 mL of ether/chloroform mixture (1:1 v/v). After drying, 0.5 g of sample was weighed and 5 mL of water (for aqueous extraction) was added. Then, test tubes were mixed for 3 min and centrifuged for 10 min. For each sample 0.1 mL of its water extracted supernatants was diluted with 0,9 mL of water followed by the addition of 1 mL Folin–Ciocalteau’s reagent and 2 mL of 10% sodium carbonate solution. After 1 h of mixing in a tube rotator and incubation at room temperature, absorbance was measured at 765 nm with a UV-VIS spectrophotometer (Shimadzu UV-1800, Kyoto, Japan). The results of the total phenol content examination were expressed as milligrams of gallic acid equivalent (GAE) per gram of dry weight.

2.8. Vitamin C Content

The vitamin C content was determined using the HPLC method, with results calculated on a dry basis [34]. The vitamin C content was determined using the high-performance liquid chromatography (HPLC) method with a Shimadzu Prominence system equipped with a diode array detector (DAD). The chromatographic separation was performed on a Restek C18 column (4.6 mm × 250 mm, 5 μm; Restek Corporation, Bellefonte, PA, USA). The mobile phase consisted of a mixture of 0.1% metaphosphoric acid in water and methanol (95:5, v/v) with an isocratic flow rate of 1.0 mL/min. The column temperature was maintained at 25 °C, and the detection wavelength was set at 245 nm. The injection volume was 20 μL. For method validation, the limit of detection (LOD) and the limit of quantification (LOQ) were determined. The LOD was established at 0.05 mg/L, while the LOQ was 0.15 mg/L. The method’s precision was assessed by repeatability tests, showing relative standard deviations (RSD) below 2%. L-ascorbic acid (Sigma-Aldrich, St. Louis, MO, USA) was used as a standard for the calibration curve. Standard solutions were prepared in 0.1% metaphosphoric acid, and calibration curves were constructed within the concentration range of 0.5–50 mg/L, yielding a correlation coefficient (R²) above 0.999. The results were calculated on a dry basis and expressed in mg of vitamin C per 100 g of sample.

2.9. Consumer Acceptance

The consumer acceptance test of apple chips was carried out according to Velickova et al. [35] with several modifications. The consumer acceptance test of apple chips was conducted with 42 consumers (28 females and 14 males). These consumers were aged between 21 and 43 years old. All subjects declared themselves as consumers of chips and other crispy products. The chip samples were evaluated at room temperature 4 h after drying (control sample). A sample consisted of three intact chips of each type of packaging. The presented samples, placed in separate disposable closed dishes, were labeled with a three-digit code. Consumers were asked to evaluate their overall acceptance using a hedonic 10 cm unstructured scale with defined marginal values (0—dislike significantly and 10—like extremely) according to European Norm PN-EN ISO 11136:2017-08 [36] and PN-EN ISO 5492:2009 [37].

2.10. Statistics

The statistics included employing response surface methodology with the design of the experiment based on the historical data. Two factors (O₂ and CO₂ concentrations) influenced eight responses (mass loss, hardness, lightness, browning index, ΔE, total phenol content, vitamin C content, and overall quality). The results were calculated using Design Expert 10 (Stat-Ease Inc., Minneapolis, MN, USA). The responses obtained for the assays were subject to the central composite rotational design (CCRD), which was used to study the effects of the variables—to model the optimal concentration of used gases. The behavior of responses was presented as contour plots. After the successful analysis of statistical models, the point-prediction calculations were performed to compare the predicted and experimental data, which were compared with the ANOVA statistical test with a significance level of α = 0.05.

3. Results and Discussion

Currently, consumer expectations are focused on high-quality food with a positive impact on health and extending shelf-life [30]. Therefore, the food industry should be looking for preservation methods that minimally process raw food. Drying is the most common form of food preservation [38,39]. During the drying process, many physicochemical changes occur [40] such as water loss and shrinkage of the material, which affect the hardness and mechanical properties of tissue [41]. However, drying improves food stability by considerably reducing the water content and microbiological activity of the material, while minimizing physical and chemical changes during storage [42]. The selection of appropriate storage conditions and packaging methods is the final step in producing dried fruit [43]. The application of modified atmosphere packaging may help reduce further losses of bioactive components, as well as maintain the adequate texture of dried apples.

3.1. Mass Loss

The mass loss (ML) during the storage process of dried apple slices is presented in Figure 1. There is a tendency to show that there were insignificant differences between the sample of apple slices stored for 15 and 30 days. However, there were significant differences in ML when different concentrations of O₂ and CO₂ were taken into consideration. The highest ML was observed when the highest concentration of O₂ was used independently of storage duration (6.8% and 8.2%, respectively; p = 0.001). The concentration of CO₂ was the factor influencing significantly the ML of dried apple slices but the significance was lower (p = 0.017). The plot in Figure 1 proves that the interaction factor between the two used gases was visible as well.

3.2. Hardness

Hardness was measured as the value widely conceived as the most vital parameter regarding dried products. The examination revealed that hardness was highest in the samples with the highest O₂ concentration (13.6 N after 15 days of storage and 15.1 N after 30 days) (Figure 2). Nevertheless, the influence of CO₂ was more significant (p = 0.001) than the influence of oxygen (p = 0.007). Here, the day of storage affected the hardness values as well, but on the lowest level of significance (p = 0.021). The case of fresh apples stored in a modified atmosphere demonstrated a positive effect on texture properties (hardness) and the beneficial role of this packaging system in extending its shelf life [44,45]. The lower O₂ content and higher CO₂ content in MA composition resulted in better product quality and shelf life [44]. For dried fruit, a crunchy sensation/feeling is important due to the increasing popularity of this group of food as snacks [46,47]. One way to maintain acceptable textural properties of food products is the use a packaging system with a modified atmosphere [48]. Different gases which fill the package are used depending on the type of product. In the case of fruits and vegetables, the natural interaction between the respiration rate of the product and the transfer of gases through the packaging material is most commonly used [30].

3.3. Color Attributes

The color attributes regarding the L*—lightness, browning index (BI), and ΔE are presented in Figure 3. The L* of the dried apple slices is regarded as one of the most important visual parameters for the customers. While using the modified atmosphere packaging, it is most valuable to counteract the changes during the storage of the product and approximate the attributes at the end of storage to ones from the beginning.

The color and lightness of the product depend on the presence of the compounds of brown solid, which can be determined by examining the potential of browning. During the drying of the apple tissue, enzymatic and non-enzymatic browning reactions occur, significantly affecting the content of brown pigments in dried apples, regardless of the method and drying parameters [49]. The smallest changes in the L* color coordinate of measured dried apple slices were observed when a concentration of 2% of oxygen and 4% of carbon dioxide and either concentration on the 4% level were used (Figure 3a,b). The concentration of oxygen was the key point in this observation because the influence of oxygen was estimated on the level p = 0.48 and the concentration of carbon dioxide was insignificant (p = 0.384).

The enzymes and chemical structure of apples determine this product as less durable to enzymatic browning. Thus, the BI of the dried apple slices was calculated to examine whether the used gases could prevent the dried apple slices from browning. The results provided us with the information that the least significant change of this parameter was observed while storing the dried apples in the concentration of 2% oxygen and 4% carbon dioxide (Figure 3c,d).

Due to the long storage of dried apple slices, the change between the samples collected on the first day of storage before packaging was examined considering the a* and b* color coordinates. Due to better comparison of results, the ΔE value was calculated, and the results are presented in Figure 3e,f. As is visible from the L* parameters, here, the most significant factor influencing the ΔE was the day of storage (p = 0.13). The presented plot shows that the concentration of used gases was an insignificant factor (O₂—p = 0.510, CO₂—p = 0.157). The concentration of carbon dioxide was the most important factor shaping the color of storage dried apple slices.

3.4. Chemical Composition

The chemical composition of the dried apple slices is presented in Figure 4. The total phenolic content (TPC) was estimated using water as the extract solution. The TPC in water was at least affected by the two extreme concentrations (0 and 4%) (Figure 4a,b). Considering the entire matrix of results, it was statistically proven that the concentration of CO₂ was a significant factor influencing the TPC (p = 0.053). The degree of change in storage of dried products mainly depends on the water content of the dried product, temperature, and storage time. With a long duration of storage, nutrient content decreases [50,51]. Studies carried out on dried fruit [52] indicate that a significant loss of TPC is followed by storage. Accordingly, the loss of vitamins and changes of polyphenolic compounds during storage considerably reduced antioxidant activity. The antioxidant activity is one of the most important characteristics of plant material [52,53]. The factor determining the effectiveness of antioxidants is the composition of fruits, as well as the technological process conditions [52,54]. However, Lavelli and Vantaggi [51] found that dried apples were relatively stable during storage and had optimal conservation of antioxidants when the right packaging methods were used. Also, Udomkun et al. [43] showed that ascorbic acid concentration was significantly affected by both storage time and packaging system. Very promising results were observed in terms of vitamin C content, as insignificant differences during storage were found in three out of five examined concentrations (Figure 4c,d). The factor that influenced the vitamin C content was CO₂ concentration (p = 0.0104).

3.5. Overall Quality

The overall quality was assessed in the hedonic scale and is presented in Figure 5a,b. The scale is enlarged in the plots to make them more visible. The general trend was that the overall quality of the dried apple slices was less acceptable to the consumers on the last day of storage. The second factor that influenced the quality was the concentration of CO₂ (p = 0.041). The overall acceptance of apple chips may have been influenced by changes in their lightness L* and BI, as well as their hardness, depending on the gas mixture composition and storage time.

3.6. The Optimization of the Gas Concentration

Due to the different data achieved from this research, we decided to statistically model the concentration of gases that could be used in modified atmosphere packaging. As the response variables, we decided to minimize the mass loss, hardness, and browning index while maximizing the nutritional value and quality of dried apple slices. The results from the model gave the information that the optimal concentration of oxygen should be 2.663% and carbon dioxide—3.785%.

3.7. Verification of the Models

The verification of the model was completed in the following way. A gas mixture used in the final product contained O₂—2.663% and CO₂—3.785% in N₂. Then, the ML, hardness, L*, BI, ΔE, TPC, and vitamin C content and overall quality of apple chips stored for 15 and 30 days were measured and applied to the model based on regression and contour plots. The calculated and measured parameters were then compared, as listed in

Table 2. The verification of the model with the measured values of mass loss (ML), hardness, L*, browning index (BI), ΔE, total phenols content (TPC), vitamin C content, and overall quality of apple chips stored 15 and 30 days.

O2 (%)	CO2 (%)	Storage Period (d)		ML (%)	Hardness (N)	L* (%)	BI	ΔE	TPC (mg/g Dry Basis)	Vitamin C Content (mg/100 g Dry Basis)	Overall Quality
2.663	3.785	15	C *	2.08	5.18	75.49	40.46	5.18	41.22	53.78	7.64 a
			M **	2.02	5.12	75.88	40.90	5.24	41.67	53.67	8.26 b
		30	C	3.59	6.32	74.68	42.88	4.46	41.02	53.42	5.86 a
			M	3.68	6.47	75.02	43.16	4.58	41.55	53.25	6.25 b

*—calculated, **—measured; (a, b)—different letters show a significant difference (p < 0.05).

. No significant differences were found between the predicted and measured values, confirming positive verification of the fitted models. Therefore, the bias factors for almost all responses would suggest a strong association between the predicted and experimental factors. The exception was overall quality, where significantly higher measured values were recorded than predicted values (p ≤ 0.05). This could be attributed to the fact that consumer evaluations tend to be highly subjective and have lower repeatability than instrumental measurement methods.

4. Conclusions

Dried apple slices could be used as an alternative source of vitamins, polyphenols, and other nutritional elements. In our study, we conducted a series of measurements to prevent the apple slices from decreasing in quality over time by using modified atmosphere packaging. We used different gas (oxygen and carbon dioxide) concentrations. By employing the response surface methodology, we were able to optimize these parameters and provide products that were insignificantly different from the control sample. We demonstrated that the optimal concentration of oxygen should be 2.663% and carbon dioxide—3.785%. With this knowledge, the optimized gas mixture could be used by researchers and food technologists to maintain the desired physicochemical properties of dried apple slices. The results of this study could lead to better control of packaging parameters, enhancing both shelf life and consumer acceptance of dried apple products.