RESEARCH ARTICLE

The Influence of Different Packaging Materials and Atmospheric Conditions on the Properties of Rinds PREFACE API 2015

Kristi Kõrge* Katrin Laos Tallinn University of Technology Tallinn University of Technology

ABSTRACT

Rancidity development in high content products is a common off flavor flaw in foods. Packaging is often used to avoid spoilage and extend shelf-life. The properties of pork rinds packaged in four different packaging materials with and without nitrogen were studied during 120 days of storage (22 °C, RH 60%, absence of light). The influence of different packaging materials and atmospheric conditions on pork rinds’ water activity, hardness, crispness and rancidity development was determined. The PET/ PE packaging material had lower barrier properties for the product in both atmospheric conditions compared with PP/metPP (40 and 50 μm) and PET/PETmet/PE. PP/metPP 50 and PET/PETmet/PE with nitrogen atmospheres were demonstrated to be the most suitable packaging materials for pork rinds.

KEYWORDS

Pork rinds, shelf-life, flow packaging materials, rancidity

*Kristi Kõrge Corresponding Author [email protected]

Journal of Applied Packaging Research 1 INTRODUCTION packaging and, among others, the following mate- rials are used: PET – polyethylene terephthalate, Pork rinds are a nutritious snack food made out PETmet - polyethylene terephthalate metallized, of pork skin. They are also referred to as scratch- PE – polyethylene and PP – polypropylene [10]. ings (United Kingdom), scrunchions (Canada), Modified atmosphere packaging (MAP) is a popular chicharrones (South America) or pork way to extend product shelf-life [11]. MAP is based (New Zealand and Australia) [1] - [4]. The pro- on altering the composition of gases in contact with duction of pork rinds is a two-step process. First, a food by replacing the air in a sealed food package pork carcasses are typically chilled and de-skinned with strictly controlled gaseous mixtures, contain- to produce a gelatinous, tender substance. Stripes ing carbon dioxide, nitrogen or other gases [12]. of about 1 to 1.5 cm in thickness and 2 to 5 cm in Different packaging materials and techniques are length are cut. Next, skin stripes are dehydrated available on the market and therefore the question with pressure and deep-fried under high tem- remains as to which is the best material in terms of peratures (200-220 °C) [5]. The finished product cost-effectiveness and the food itself. acquires a crisp, puffy feeling in the mouth and, The aim of this study was to evaluate the ran- after the addition of salt, a delicate flavor similar cidity and texture of pork rinds during a 120-day to is produced. Pork skin is usually stripped shelf-life period, packaged under different atmo- away very precisely, but leaving extra meat on it spheres (ambient air and MAP (N2)) in four differ- during carving increases its consumption value. ent commercial packaging materials. Consumers see pork rinds as a good meat source. Pork rinds are an excellent source of proteins MATERIALS AND METHODS (approx. 70% w/w) and (30% w/w) and contain zero carbohydrates [5]. The fat in the product mainly consists of unsaturated fatty acids that are suscep- Materials tible to autoxidation. Lipid oxidation is a complex Pork rinds were provided by a local producer. process in which unsaturated fatty acids react with After production the rinds were immediately frozen molecular oxygen via a free radical mechanism or at -20 °C and stored for 30 days until they were in a photo-sensitized oxidation process [6]. Hydrop- packaged in the studied materials. eroxides decompose to an array of volatile com- Four different packaging materials were pounds, including aldehydes, ketones, esters and provided by a local packaging company. The mate- acids, which influence food flavor, contributing to rials varied in thickness, oxygen transmission rate rancid, soapy, oily and fishy tastes [7]. It has been (OTR) and vapor transmission rate (VTR) parame- shown that lipid oxidation is one of the few reactions ters; three were metal laminated (PET/PETmet/PE, that accelerate below the freezing point of water PP/metPP 50 and PP/metPP 40) and one was trans- and is related to the water content of fatty tissue parent (PET/PE) (Table 1). The packaging materi- [8]. In order to maintain the physical, chemical and als were chosen taking into consideration cost and sensory quality of the product, proper packaging the parameters for dried foods. The packaging bags material must be chosen. This can be achieved by were handmade in the laboratory with dimensions choosing the most suitable packaging material and of 20 x 26 cm, using a heated sealer ME-500HI through the manipulation of the gaseous environ- (Mercier Corporation Impulse, Netherlands), with ment [9]. The general packaging for the rinds is flow a sealing time of 3 seconds and a temperature of

The Influence of Different Packaging Materials and Atmospheric Conditions 2 Table 1: Parameters of the packaging materials

150 °C. The N2 gas (E941) for packaging was food the time of analysis, the packs were retrieved from grade. storage and measured one by one. An apparatus injection needle was used to penetrate the packag- Packaging of the pork rinds ing material. Before injection, the specific piercing Frozen pork rind samples were thawed over- area was covered with a special patch material to night at 22 °C, RH 60%. 40 g of the sample was prevent excess leaking from/into the pack envi- repacked into four different packaging materi- ronment. After a two-minute test, the patches were als and two different atmospheres: 2N (100%) and removed and the holes were sealed with strong atmospheric air with 21% O2. The packages were sealer tape. The samples were analyzed in triplicate sealed with a vacuum packer Lynx 32 (Henkel- at the beginning of the experiment and at the end of man, Netherlands) and a heated sealer ME-500HI, a 120-day shelf-life period. To prevent any changes respectively. The packaged samples were stored in in the pork rinds due to the destructive method, a closed container at 22 °C and an RH of 60% in the samples were tested for water activity and texture absence of light until analysis. on the same day.

Head-space analysis Water activity The oxygen concentration in the packag- The water activity (aw) of the pork rinds was ing was analyzed with an Agilent 490 Micro GC determined using an Aqualab water activity Biogas Analyzer, equipped with a CP-Molsieve analyzer (Meter Group, USA). The samples were 5A channel (pressure 200 KPa, injector tempera- pulverized for 30 seconds in a mortar and imme- ture 110 °C and column temperature 80 °C; the diately inserted into the analyzer to prevent water carrier gas was argon), a CP-PoraPLOT U channel uptake. The analysis was run in triplicate, analyzing (pressure 150 KPa, injector temperature 110 °C a single rind from each pack, one at a time (n=3x2). and column temperature 80 °C; the carrier gas was helium), and a thermal conductivity detector. At

Journal of Applied Packaging Research 3 Hardness and crispness differences (p ≤ 0.05) between the samples were Hardness and crispness were analyzed with a found. Texture analyzer TA-XT2i (Stable Micro Systems, UK), using a 3-mm diameter lance probe. The RESULTS AND DISCUSSION samples were compressed 25% at a speed of 1.0 mm/s. Compatible software for the texture analyzer made it possible to determine the hardness and Head-space analysis crispness of the samples at the same time during Oxygen contents in the head-space were compression. The graph generated revealed peaks measured to find the changes in the oxygen level by when the probe pierced the samples. Every peak packaging materials. The mean oxygen content in represented the resistance force to the probe by the headspace of packages without pork rinds was the measured sample. The highest peak was inter- 21.53% (Table 2) that is 0.58% more than theoreti- preted as the hardness of the product, expressed in cal value of atmospheric air (20.95%). The oxygen Newtons (N). Crispness was interpreted as the total content in the head-space of the pork rinds packed peak count. The analysis was run in triplicate, ana- without N2 after 30 days of storage was less than lyzing a single rind at a time (n=10) from each pack. in empty packages and decreased with 120 days of storage in all packaging materials. It can be Sensory analysis assumed that the oxygen was consumed in oxida- Sensory analysis was conducted with a trained tion reactions as the level of oxygen in the empty panel of eight assessors using descriptive sensory packages did not change. analysis. The analysis was comprised of a group The pork rinds packed with N2 showed low of test methods (a flavor profile [13] and a texture oxygen content after 30 days of storage in all pack- profile [14]) to quantify the perceived sensory inten- aging materials. After 120 days of storage, the sities of the product [15]. oxygen content of the PET/PETmet/PE, PET/PE The analysis was run in triplicate, with a total and PP/metPP 40 packaging materials remained of five sessions in 120 days for sample evalua- the same, while the oxygen content of PP/metPP 50 tion. During these sessions, the sample packs were increased to 2.55±0.20%. It is clear (Table 1) that opened one by one and each assessor had access to the OTR of PP/metPP 50 was high (90 cm3m-2/24h the food samples. The samples were described in 23°C 0% RH), which explains the small increase in terms of rancidity and texture attributes using a 0 = the oxygen content. none/chewy to 15 = very strong/too hard scale with increment points of 1. Purified filtered water and Water activity reference materials (heated rapeseed oil for rancid- Pork rinds are a heterogeneous product with ity, TUC crackers for crunchiness and puffed low water activity (aw < 0.36) (Table 3). Jensen & cakes for crispness) were available at all times. The Risbo [16] found pork rinds had an even lower aw assessors were told to clean their palates in between value of 0.17. This is probably due to the variations the samples. in drying of the product. With this low value, pork rinds are microbiologically stable but chemical and Statistical analysis enzymatic reactions can occur which result in dete- The results from each analysis were subjected rioration [17]. Products with low water activity are to analysis of variance (ANOVA), and significant susceptible to rancidity, with the O2 atmosphere

The Influence of Different Packaging Materials and Atmospheric Conditions 4 Table 2: Effect of storage on oxygen content in the head-space of different packaging systems, with and without pork rinds

Table 3: Effect of storage time on the water activity of pork rinds packaged in different packaging systems

Journal of Applied Packaging Research 5 in the aw range of 0.10 to 0.30. The reaction is the Gonzalez Martinez, Corradini and Peleg [18]. lowest at aw 0.32 and increases again in the range of Crispness was recorded through the peak 0.32 to 0.75 [17]. The water activity of the analyzed count generated by measurement by piercing the pork rinds remained in the range where the ran- sample. The more brittle the sample, the higher the cidity reaction was the slowest, supported by the peak count. The pork rinds’ mean peak count was sensory analysis, where the rancidity perception 20±5 (Figure 1 c and d). Again, different packaging results remained low. systems and the storage time presented no statis- The water activity of the pork rinds packed in tically significant differences in crispness between PET/PETmet/PE, PP/metPP 50 and PP/metPP 40 the packagings PET/PETmet/PE, PP/metPP 50 and with and without N2 slightly decreased during 120 PP/metPP 40 at the 95% confidence level (p<0.05). days of storage, probably due to the slow hydrolytic However, the crispness of the pork rinds packed in oxidation process. However, the water activity of the PET/PE material with or without N2 decreased pork rinds packed in PET/PE with and without N2 during storage, presenting the mean peak value of increased during storage. This was due to water per- 24±8 after 30 days of storage and 7±4 after 120 meation through the packaging material, since PET/ days of storage. This means that the texture of the PE has higher vapor permeability (< 10 g m-2/24h pork rinds packed in PET/PE changed from crispy 38°C 90% RH) compared with the other studied to tough/chewy. This change was due to moisture materials (Table 1). migration through the packaging material. As texture contributes to the overall enjoyment Hardness and crispness of the deep-fried [5], the mean peak count Pork rinds are a very heterogeneous food value of <10 is seen as non-consumable texture product and the mean hardness of the product value in relation to the sensory analysis. packed in different packaging systems was 44±15 N (Figure 1 a and b). As the starting point of the Sensory analysis experiment conformed to the first measurement The sensory attribute ratings, i. e. rancid off-fla- point of the storage time, the results are not shown vor, hardness and crispness during the storage time graphically. Different packaging systems and the of the pork rinds, are shown in Figure 2. On a 15 storage time presented no statistically significant point scale, a rancid attribute rate close to 15 and differences in hardness between the packagings texture attribute rates close to zero indicate growing PET/PETmet/PE, PP/metPP 50 and PP/metPP 40 negative values, i. e. unacceptably rancid was rated at the 95% confidence level (p<0.05). However, the as 15, and too hard and crispy was 0. Opposite hardness of the pork rinds packed in the PET/PE scales were chosen according to human perception: material with or without N2 increased over 50 N unacceptable taste was measured as increasing and during the storage time. This is clear evidence of good texture (crisp) in the mouth as decreasing. As “moisture toughening”, which is the loss of brittle- the starting point of the experiment conformed to ness accompanied by a corresponding increase in the first measurement point of the storage time, the the apparent hardness. This is probably a manifesta- results are not shown graphically. Different packag- tion of the progressive inability of fractures to prop- ing systems and the storage time presented signifi- agate, thus allowing the particles’ beds to resist an cant changes in rancidness, with a distinguishable increasing amount of compressive stress. This phe- change during storage at the 95% confidence level nomenon has also been observed with pork rinds by (p<0.05).

The Influence of Different Packaging Materials and Atmospheric Conditions 6 Figure 1. The influence of storage time on the hardness (a. and b.) and crispness (c. and d.) of pork rinds packed in different packaging systems

It is well known that oxygen elevates lipid oxi- in addition to auto-oxidation. dation rate in food [1], [6], [19]. It has also been N2 packaging significantly retarded the lipid proven that an N2 atmosphere prevents rancid flavor oxidation, as no rancid offflavor developed in2 N development in foods during storage [20] - [22]. packaged samples throughout the storage period, Pork rinds packed in different packaging materials except with PET/PE packaging (Figure 2 b). The without N2 (Figure 2 a) developed distinguishable rancid off-flavor development in PET/PE packaging rancid offflavor 60 days after packaging. The rancid with N2 likely resulted from excessive moisture in off-flavor can be described as a strong, easily iden- the packaging causing hydrolytic oxidation. tified, negative sensory characteristic defined as unacceptable for consumption. By 120 days, off-fla- CONCLUSIONS vor intensity was highest in rinds packed in PET/ PE and lowest in rinds packed in PET/PETmet/PE The texture changes and rancidity develop- and PET/metPP 50. As PET/PE has high VTR and ment in the packaged pork rinds during 120 days the water activity of the samples increased during of storage was studied. These results are of partic- storage, there may have been hydrolytic oxidation ular interest to food production companies in terms

Journal of Applied Packaging Research 7 Figure 2. Descriptive sensory analysis ratings of pork rinds packed in different systems during the storage time.

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