THE EFFECT OF MODIFIED ATMOSPHERE PACKAGING ON THE QUALITY OF MINIMALLY PROCESSED APPLES

Róża Biegańska-Marecik, Janusz Czapski, Katarzyna Czaczyk

ABSTRACT

The study compares different conditions of packaging apple slices subjected to vacuum impregnation with ascorbic acid, 4-hexylresorcinol, calcium chloride and sucrose. Packaging in modified atmosphere (MA) containing: 2, 4, 6, 8 and 10% CO₂, 2% O₂ (nitrogen to 100%), packaging in nitrogen atmosphere with 2% O₂, vacuum packaging and air packaging were applied. Vacuum packaging of apple slices subjected to processing made it possible to obtain the product with best sensory quality among all the tested packaging methods. The most effective method in terms of microflora growth inhibition on apple slices turned out to be modified atmosphere containing 4% CO₂ and nitrogen atmosphere packaging. In case of vacuum packaging and air packaging counts of some groups of microorganisms during the storage of apple slices increased slightly, which however did not have a decisive effect on the microbiological quality of the product.

Key words: modified atmosphere packaging, minimal processing, vacuum impregnation, apples..

INTRODUCTION

The production of minimally processed food has become feasible thanks to the development of modified atmosphere packaging of food products. This method consists in the replacement of air in the package with a mixture of gases, with the composition selected depending on the packaged product [4]. Modified atmosphere in which oxygen content was reduced and carbon dioxide content – increased, was applied for the first time in 1927 to extend the shelf life of apples [5].

The effectiveness of modified atmosphere storage depends on the decrease in the product metabolism level. Modified atmosphere with an increased CO₂ content and a lowered O₂ content may reduce the level of respiration, the production of ethylene and sensitivity to its concentration, decay and physiological changes [8,23]. Fresh fruit and vegetables exhibit metabolic activity for a long time after harvest; it is caused both by the tissue activity and external factors, such as physical damage, bacterial flora, the loss of water, and the temperature of storage. During anaerobic respiration in the product the accumulation of ethanol, aldehydes and organic acids may occur, which results in a deterioration of the product sensory attributes [20].

The initial concentration of gas in products packaged in modified atmosphere considerably affects the degree of respiration, e.g. oxygen concentration lowered to 2% and carbon dioxide concentration increased to approx. 5% resulted in an over 10 times decrease in the intensity of respiration in broccoli [24]. Numerous studies have been conducted on the selection of modified atmosphere packaging conditions for various minimally processed vegetables and fruits. Most authors suggest the application of oxygen at the level of 1-3%, whereas the concentration of carbon dioxide at 3-10% [1, 16, 22]. In contrast, Hong and Gross [13] used modified atmosphere with the content of 4, 8 and 12% CO₂ with the addition of 1% and 20% O₂ in the packaging of fresh-cut tomatoes. Pesis et al. [18] showed that modified atmosphere with the content of 10% O₂ and 5% CO₂ reduced chilling injury in mango fruits. Amanatidon et al. [2] showed that the quality of minimally proce ssed carrots packaged in modified atmosphere containing 50% O₂ and 30% CO₂ was comparable or better than that of carrots packaged in the atmosphere containing 1% O₂ and 10% CO₂.

The aim of this study was to determine the effect of modified atmosphere packaging conditions on the physicochemical indexes, sensory attributes and microbiological quality of minimally processed apples stored at the temperature of 4°C.

MATERIALS AND METHODS

Apples of the Jonagold variety, originating from the pomiculture experimental station In Przybroda near Poznań, belonging to the Agricultural University of Poznań, were used as experimental material. Fruits were peeled, cored and sliced into 1-cm thick slices, which were deaerated and vacuum impregnated with a solution containing enzymatic browning inhibitors and sucrose, with the following composition: 1% ascorbic acid, 0.005% 4-hexylresorcinol, 0.3% calcium chloride and 20% sucrose. Deaeration and vacuum impregnation were conducted at the pressure of 70 mbar. Apart from minimally processed slices, control slices – not subjected to minimal processing – were also prepared. All slices were packaged in batches of 100 g each in bags by Multivac, made from laminate (oriented polyamide/polyethylene) with the dimensions of 15 x 21 cm and gas permeability in cm³ / m² / 24 h: O₂ – 45, CO₂ – 200 and N₂ – 8, respectively. Bags were sealed using an AG 900 closing machine by Multivac.

The first stage of the investigations aimed at a comparison of sensory quality and physicochemical properties of apple slices, which were vacuum impregnated and then packaged under different conditions. Packaging in modified atmosphere containing 2%, 4%, 6%, 8% and 10% CO₂ (the other components: 2% O₂ and N₂ up to 100%) was applied, and then – after the optimum carbon dioxide content in the used atmosphere was selected – it was compared with nitrogen packaging, packaging in modified atmosphere (containing 4% CO₂), vacuum packaging (the value of pressure during packaging – 80 mbar) and air packaging.

The packaged product was stored at the temperature of 4°C for 15 days. The appraisal of the product conducted after 1, 10 and 15 days of storage consisted in the measurement of color parameters of homogenized apple slices in the CIE L*a*b* system in reflected light (U-3000 Hitachi spectrophotometer), measurements of pH values and refractometric measurements of the extract content. Sensory appraisal was conducted in a 5-point hedonic scale. Color (tone, uniformity, desirability), texture (type, tenderness), taste (typicality, intensity) and aroma (typicality) were evaluated. Moreover, oxygen and carbon dioxide contents were also measured in the bags with the product. These measurements were taken for vacuum impregnated apple slices and control slices packed in modified atmosphere, in the nitrogen atmosphere with some oxygen content and in the air. In this experiment the initial oxygen content in the bags with the product was increased up to 5%, both in case of modified atmosphere packagi ng (4% CO₂, 5% O₂ and 91% N₂), and in case of nitrogen packaging. Samples after packaging were stored at the temperature of 4°C and for the purpose of comparison at the temperature of 15°C. Measurements were taken using a PBI Dansensor Check Mate 9900 apparatus.

In the second stage of the investigations microbiological analysis was performed for apple slices subjected to vacuum impregnation with the above mentioned solution and then packaged under various conditions and stored at the temperature of 4°C. Packaging in nitrogen, in modified atmosphere containing 4% CO₂, 2% O₂ and 94% N₂, under vacuum conditions and air packaging were applied. For comparison control slices packed in the same way as samples subjected to processing were also analyzed. Microbiological analysis conducted after 1, 7 and 15 days of storage included the determination of the total mesophilous bacteria count, the total psychrophilous bacteria count, the count of mesophilous moulds, the count of psychrophilous moulds, the yeast count, the acetic acid bacteria count, the lactic acid bacteria count, the bacterial count for genus Pseudomonas, the general coli titer, and the presence of anaerobic bacteria.

A two-way analysis of variance was used for the statistical analysis of the results.

RESULTS AND DISCUSSION

Apple slices impregnated with a solution inhibiting enzymatic browning, packaged in modified atmosphere containing 2-10% CO_2, exhibited a light yellow color, uniform throughout the slice, with not traces of browning. No significant effect of the applied CO₂ concentrations was found on either color parameters measured instrumentally or color quality appraised in sensory examination. The values of color parameters and the notes in color sensory examination were similar. However, a slight increase was observed in the notes of sensory color examination of apple slices during storage, which was connected mainly with the lightening of color caused by the action of the impregnation solution. Significant differences for individual CO₂ concentrations were found in the sensory examination of taste, aroma and texture of the product (Table 2). The highest notes in sensory examination were given to apple slices packaged at the 2 and 4% concentration s of CO₂ in modified atmosphere (Table 1). The taste and aroma of these samples were evaluated as typical and the texture as firm. At the 6, 8 and 10% CO₂ concentrations in the atmosphere used in the packaging process the taste and aroma of the product deteriorated, and – to a lesser degree – also texture evaluated in sensory examination was considered worse. The growing carbon dioxide concentration in modified atmosphere resulted in the growing sensation of the product saturation with carbon dioxide, which had a negative effect on the taste and was described by the assessment panel as a sensation similar to that felt when consuming a “carbonated” product. It caused a deterioration of notes given for taste.

On the basis of the obtained results modified atmosphere used in further investigations contained 4% CO₂. Such packaging conditions of apple slices resulted in a product exhibiting good physicochemical properties and at the same time did not cause a deterioration of sensory examination notes connected with the dissolution of large amounts of CO₂ in the cell sap.

While comparing packaging in modified atmosphere (MA) containing 4% CO₂, 2% O₂ and 94% N₂, in nitrogen atmosphere with 2% oxygen content, as well as vacuum and air packaging, it was found that the method of packaging did not have an effect on the product color, measured both instrumentally and in sensory examination (Table 3). Significant differences in color were observed only between saturated samples and control samples, i.e. apple slices not subjected to processing and packaged as proper samples (Fig. 1). In control samples, without the application of saturation, tissue browning was observed irrespective of the adopted packaging method. Also Gorny et al. [10], while investigating the quality of fresh-cut pears stored in modified atmosphere, did not find the effect of the investigated atmosphere composition (0.25 and 0.5 kPa O₂, 5, 10 and 20 kPa CO₂) on enzymatic browning inhibition of the fr uit tissue. Similarly to this study, the addition of inhibitors was necessary to inhibit browning.

The highest notes of the sensory examination after 1, 10 and 15 days of storage were given to vacuum packaged and air packaged samples (Table 3). These samples received higher notes especially for taste (Fig. 2) and texture assessment. A statistically significant effect of the applied packaging method was found on the notes in the sensory examination of the taste of apple slices during storage; however, no significant differences were observed in the sensory examination of their aroma (Table 4). The taste of nitrogen atmosphere packaged samples was medium typical, with a low concentration. Apples packaged in modified atmosphere containing CO₂ received lower notes for their taste due to the release of gas bubbles, which could be sensed in the mouth, and which was evaluated by the assessment panel as disadvantageous.

In terms of the texture in sensory examination the most advantageous method turned out to be vacuum- and air packaging. Samples packaged in modified atmosphere containing CO₂ and nitrogen packaged samples showed less compact texture as early as after 1 day of storage. Moreover, accelerated tissue softening was also observed in comparison to vacuum and air packaged samples.

According to numerous authors, the selection of an appropriate atmosphere composition used for the packaging of minimally processed products retaining live tissue is an essential factor in extending their shelf life, as an inappropriate gas concentration may result in the acceleration of adverse physiological processes, tissue browning and changes in their sensory attributes: taste, aroma and texture [13, 19, 21, 24]. Most authors suggest applying the lowest possible (1-2%) CO₂ content in the modified atmosphere used in apple packaging or the application of vacuum packaging [6, 17].

On the basis of the measured oxygen ad carbon dioxide contents in packages with the processed product stored both at the temperature of 4°C and 15°C, it was found that oxygen content decreased very fast during storage, irrespective of the adopted packaging method (Fig. 3). The amount of oxygen decreased at the slowest rate in case of samples subjected to vacuum impregnation. It was approx. 2% in these samples, packaged both in MA and in nitrogen, after 1 day of storage. Also in these packages the lowest amounts of released CO₂ were observed. The lowest oxygen consumption and the lowest amounts of released carbon dioxide, observed in the packages with vacuum impregnated apple slices, indicate a slower metabolism of these samples and most probably result from the changes in the fruit tissue caused by the process of deaeration and vacuum impregnation. Filling the intracellular spaces with the saturation solution could have resulted in the slowing down of oxygen uptake and the release of carbon dioxide by the tissue.

At the temperature of 15°C the rates of oxygen consumption and the production of carbon dioxide were faster than at the temperature of 4°C. The level of CO₂ content at the temperature of 15°C after 15 days of storage was higher in the air-packaged samples by approx. 15% than in the samples packaged in the nitrogen atmosphere and in the MA packaged samples.

The selection of modified atmosphere packaging conditions for minimally processed vegetables and fruits depends to a large extent on the rate of respiration of a given product. Minimally processed fruits and vegetables may be characterized by very diverse levels of oxygen uptake and the release of carbon dioxide, depending not only on the species-specific properties of the raw material, but also on the method of processing, the degree of comminution and the duration of product storage [8]. Factors affecting the level of oxygen uptake and the release of carbon dioxide should be determined for each product separately [8, 12]. Appropriate modified atmosphere packaging conditions may be determined by defining the mathematical model of the process of respiration at a given temperature and at a given oxygen content in combination with the application of a packaging material with appropriate permeability [15].

The evaluation of the microbiological quality of air-packaged control samples showed that after 1 day of storage the count of mesophilous bacteria and moulds in the air was 10² units/g, whereas after 7 and 15 days of storage it increased to 10³ units/g (Figs. 4 and 6). The number of psychrophilous bacteria increased from the level of 10³ units/g (after 1 day of storage) to 10⁴ units/g (after 7 and 15 days) (Fig. 5). The presence of yeast was detected only after 15 days of storage, and it amounted to 10² units/g. The lactic acid bacteria counts increased after 7 days (10² units/g) and 15 days of storage (10³ units/g).

The application of vacuum impregnation of apple slices which were then air-packaged resulted in a slower growth of mesophilous bacteria and moulds, and coli bacteria in comparison to samples not subjected to processing. At the same time a faster growth of yeast and bacteria from genus Pseudomonas were observed.

The application of vacuum packaging did not have a decisive effect on the counts of microorganisms in comparison to air-packaged samples. Both in vacuum packaged and air-packaged samples only a slight growth was observed of some groups of microorganisms (psychrophilous bacteria, mesophilous moulds and yeast).

Nitrogen atmosphere packaging with the 2% oxygen content resulted in a decrease in the counts of mesophilous bacteria and moulds after 1 day of storage in comparison to air-packaged samples. Moreover, a decrease was observed in the counts of these groups of microorganisms during a 15-day storage of the product. In vacuum impregnated samples, which were nitrogen atmosphere packaged, also a decrease was observed in the counts of psychrophilous bacteria and in comparison to air-packaged samples. On the other hand, in control samples, which were nitrogen atmosphere packaged, the presence of yeast was not detected throughout the whole duration of product storage.

Packaging of apple slices – both vacuum impregnated and control – in modified atmosphere containing 4% CO₂, 2% O₂ and 94% N₂ caused a lowering in the counts of mesophilous bacteria and moulds after 1 day of storage in comparison to air-packaged samples. In modified atmosphere packaged samples, similarly to nitrogen atmosphere packaged samples, a decrease in the counts of mesophilous bacteria and moulds was observed during the 15 days of product storage. The counts of psychrophilous bacteria and of yeast were also lower than in air-packaged samples; however, no significant effect of modified atmosphere was found on the counts of.

The counts of coli group bacteria and of psychrophilous moulds were similar in all the samples, irrespective of the packaging method applied.

Most of the investigated groups of microorganisms exhibited slower growth rates in samples subjected to processing with a solution inhibiting enzymatic browning than in control samples, not subjected to processing. On the other hand, a higher growth of yeast was observed in vacuum impregnated samples than in control samples. It may be assumed that it is connected with an increase in the sucrose concentration in apple slices.

Advantages resulting from the application of modified atmosphere with a lowered oxygen content and an increased carbon dioxide content to extend the shelf life of minimally processed fruits and vegetables were confirmed by numerous authors [2, 3, 9, 13, 14]. Hao and Bracket [11] suggested that the extension of product shelf life by the application of modified atmosphere with an increased carbon dioxide content and a lowered oxygen content is probably caused by the slowing down of life processes, and not the inhibition of microorganism growth. However, on the basis of the results of microbiological analyses presented in this study a decrease was found in the counts of selected groups of microorganisms. Finn and Upton [7] reported a decrease in the counts of Bacillus cereus and Staphylococcus aureus bacteria in fresh-cut carrots and cabbage at the application of modified atmosphere. The same authors found that in the fresh-cut cabbage subjected to storage Escherichia coli developed faster when it was packed in modified atmosphere. The results presented in this study concerning modified atmosphere packaged samples also indicate a slight growth of bacteria of the coli group during product storage.

CONCLUSIONS

On the basis of the conducted experiments it was found that the physicochemical properties and sensory attributes of apple slices are best preserved by vacuum packaging. The application of air packaging resulted in a slight deterioration of the quality of apple slices. An additional advantage in favor of vacuum packaging is the appearance of the product in the package, which was appraised the highest in that variant. Among the investigated packaging methods the least advantageous in terms of the effect on the quality of apple slices were packaging in modified atmosphere containing 4% CO₂, 2% O₂ and 94% N₂, and nitrogen atmosphere packaging. In spite of that, the product packaged under such conditions exhibited good quality.

The most effective-in terms of microflora growth inhibition-in case of apple slices turned out to be modified atmosphere containing 4% CO₂, 2% O₂ and 94% N₂, and the nitrogen atmosphere with 2% oxygen, mainly due to the slower growth of mesophilous bacteria, moulds and lactic acid bacteria. In vacuum packages and air-packaged samples a slight growth of some groups of microorganisms was observed; however, it did not result in a deteriorated microbiological quality of apple slices. The results presented in this study indicate that the application of vacuum packaging did not result in considerable differences in the counts of the investigated microorganisms; however, on the basis of sensory examination results vacuum packaging was decided to be the packaging method giving a product with the highest sensory quality.

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