Variations in the Qualitative Characteristics of Pork at Various Post-slaughter Combinations of Times and Temperatures Mimicked as in the Indian Retail Market

Global pork production was estimated about 110.09 million tonnes and out of which mere 0.36 million tonnes was the India’s contribution in the year 2019 FAO (2020). As per Basic Animal Husbandry Statistics BAHS and FS (2020) pig meat was only 0.4% of the total meat production in Bareilly city, India. Per capita availability of meat is 4.54 kg as against 11 kg recommended by Indian Council of Medical Research. All this data shows tremendous potential for growth of pig meat industry in India.
       
But currently pig farming is highly unorganized with the highest proportion of the animals being reared by small farmers and tribal population in North Eastern region of India (Kadirvel et al., 2020) with very low-input/output ratio. It is a common practice to buy pig meat from the local shop, instead of standard packaged meat and to cook it immediately. Both consumers as well as retailers are not aware about quality and safety of pig meat in Indian conditions (Ojha et al., 2019).
       
Meat quality is a combination of subjective and objective measurements which vary across markets, particularly international markets. Production of high-quality pig meat determines producers’ economic gain because meat quality and consumer preference are interrelated (Lindahl et al., 2001; Hunt et al., 2012). Some of the most common parameters used in determining pork quality are post-slaughter pH, colour, tenderness, water holding capacity, marbling and chemical composition. All these parameters are influenced by genetics as well as non-genetic/ environmental factors. The environmental factors may be either on-farm as well as processes involved in breeding, slaughtering, meat processing and storage (Pearce et al., 2011; Xu et al., 2012; Tomovic et al., 2014). Freshly cut muscle when converted into meat, it leads to an accumulation of lactic acid in muscle tissue due to lack of oxygen supply, which leads to acidification and drop in pH (PIC, 2003; Pearce et al., 2011) suggested that the preferred ranges for ultimate pH were 5.7-6.1. Meat color and water holding capacity are determined by the rate at which the postmortem pH declines (Tomovic et al., 2014). The water-holding capacity (WHC) or drip loss is the amount of moisture in pig meat that is lost when it is cut, which determines cooking loss during cooking procedures (Pearce et al., 2011). WHC is important visual cues to assess meat quality as well as cooking losses produce an economic cost to meat retailers (Watanabe et al., 2018).
       
According to FAO (2019) fourteen percent of the food produced globally is lost during the post-harvest production stage before reaching the retail stage of the food system. For all types of food, the most critical point of loss is at the farm level, just after harvest, where inadequate storage facilities and poor handling practices play a significant role. Therefore, study was conducted to understand post-slaughter changes in various quality parameters of pig meat exposed to different storage conditions as expected in the Indian retail market.

Sample collection
 
Semitendinosus muscle samples were collected immediately after slaughter from twelve pig (7±1 month age). The study was designed in such a way that each sample was kept at four different storage temperatures i.e. 4±1°C, 18±2°C, 25±2°C and 37±2°C. Therefore, each sample was cut in to four parts and packed in low density polyethylene (LDPE) pouches. Further these four pouches of per sample were stored at four different storage temperatures.
 
Pork quality parameters studied
 
Eight physico-chemical parameters were studied to determine the quality of pork at various storage conditions. Core temperature (°C), pH, Warner-Bratzler Shear force (kg/cm²), Expressible water (%), cooking loss (%) and colour profile i.e. a* (redness) value, b* (yellowness) value and L* (lightness) value were measured for each pork sample.
 
Core temperature
 
Core temperature was measured by digital food probe thermometer (MCP digital multi stem thermometer). The probe was inserted up to 4 to 5cm deep inside each sample at three different places to record three core temperature readings. The average of these three readings was considered for further analysis.
 
pH
 
pH was measured via the method given by Trout et al., (1992) by using digital pH meter (Hanna Instruments, HI200201, Woonsocket, RI-USA).
 
Warner-bratzler shear force
 
Warner-Bratzler shear force value of pork sample was determined by the method of Berry et al., (1981). Warner-Bratzler apparatus (Model: 81031307, G.R. Elect MFGCo., USA) was used to measure the shear force value at a crosshead speed of 2 mm/s.
 
Per cent expressible water
 
Percent expressible water was determined by modified method of Gnanasambandam et al., (1992). 500 mg pork sample (B) was sandwiched between the two pre-weighed filter papers (A). This sandwich was again sandwiched between two polyethylene sheets. Further pressure was applied on this double sandwiched pork sample by keeping weight of 18.5 kg for 2 minutes. Meat flakes were weighed (C) after removing the weight. Filter papers were dried at room temperature and weighed again after drying (D). The calculation was as follows
Weight of filter paper =A
Weight of meat sample=B
Weight of meat flakes=C
Weight of dried filter paper= D
Weight of meat residues adhered to the filter paper: E=D-A
Actual weight of meat sample: F=E+C
 

 

 
Cooking loss
 
Cooking loss was measured by the method of Babiker​ et al. (1981).  100 gm pork sample was placed in polythene bag and cooked in a thermostatically controlled water bath at 90±0.5°C for 90 minutes. Subsequently, it was cooled under tap water for about 20 minutes and further dried with paper towel.

 

 
Instrumental colour
 
Colour profile of pork was measured using Hunter Colour Lab (Mini XE, Portable type). The instrument was calibrated using light trap/black glass and white tile provided with the instrument. Hunter L* (brightness-100 or lightness-0), a* (+redness/- greenness), b* (+yellowness/-blueness) values were recorded at three different points on a uniform layer of pork sample.
 
Frequency of measurement of pork quality parameter
 
As described earlier each sample was cut into four equal parts and further these parts were kept at four different storage temperatures i.e. 4±1°C in refrigerator whereas 18±2°C, 25±2°C and 37±2°C in BOD incubator (ACME lab instruments private limited). Core temperature (°C) and pH of pork was measured on 1-, 2-, 4-, 6-, 9- and 24- hours of storage duration. Whereas, rest of the pork quality parameters were measured on 1-, 3-, 6- 9- and 24-hours of storage duration. This difference in storage duration was kept to avoid error in measurement of all seven parameters at once. A total of two-thousand-sixteen records of all the pork quality parameters were collected in this study.

Statistical analysis
 
Data was tested for sphericity and further analyzed by within-subject design. Significant interaction effects and pie chart were plotted using ggplot2 package (Wickham et al., 2016) whereas correlation analysis was performed using Performance Analytics package (Peterson, 2020) for the statistical programming language R.

Twelve Semitendinosus muscle samples of pig meat collected from local market were matched for their quality in this within-subject design. The independent variables storage temperature and storage duration accounted for ³90% of the variability in the quality of pig meat in terms of core temperature, pH, shear force value, cooking loss and expressible water of pork (Fig 1). It indicates that post-slaughter storage temperature and storage duration also influence the quality of pig meat available to consumer.


       
Storage temperature of pig meat met the assumption of sphericity in all the studied quality parameters, except in a* (redness) value, as tested by Mauchly’s procedure (Table 1, 2, 3). Mauchly’s test indicated a violation of the sphericity assumption for storage duration in pig meat core temperature, pH, cooking loss and colour profile parameters. Similarly, the interaction effect was found to be highly significant in all the quality parameters of samples, except in a* (redness) value and reported as the Greenhouse-Geisser corrected results (Table 1, 2, 3). The significant interactions were plotted in Fig 2,3,4,5,6,7,8 and 9.






















       
The simple effects analysis indicated that core temperature, pH and shear force value of pig meat decreased significantly as a function of storage duration (hour) at the studied storage temperatures (Fig 2, 3 and 4). Subsequently, the correlation analysis also revealed that there was significant positive correlation among these pig meat quality parameters (Fig 5). These observations agree with previous report that pH of pork depends on its core temperature (Bekhit et al., 2007). Higher values of pig meat core temperature were observed at 37±2°C storage temperature. It might be due to residual metabolic activity, depletion in muscle energy reserves and cessation at blood supply leading to initiation of rigor mortis (Alvarado et al., 2002; Hopkins et al., 2014; Kim et al., 2014a). However, the rate of decline in pH of pig meat at 4±1ºC was comparatively slower and it took 24 hours to attain ultimate pH. The slower rate of glycolysis at 4±1°C might be the cause of the slower rate of decline in pH (Bekhit et al., 2007; Botha et al., 2008; Pearce et al., 2011; Kim et al., 2014b; Liu et al., 2013; Balan et al., 2020). Jose et al., (2013) studied that the total glycogen content, accessible after slaughter influences pH and the rate of pH drop. Thus, they suggested that breeding plans, feeding strategies and processing technology may all be adjusted to optimize pH and pH reductions. Holmer et al., (2009) analyzed the effect of pH on shelf-life of pork during aging and found that higher pH is associated with improved pork quality, but longer aging periods with higher pH will result in decreased shelf-life due to microbial proliferation. It is suggested that intermediate pH may provide the most desirable combination of quality and shelf-life when longer aging period is used.
       
Shear force values of meat is directly related to meat tenderness such that an increase in shear force value indicates a decrease in the tenderness of meat (Geesink et al., 2000; Kim et al., 2014a). Therefore, meat with lower shear force value is desirable. In this study shear force value of pork samples followed the similar trend as that of core temperature and pH. There was significant decline in shear force value with increase in storage duration. The lowest shear force value at 4±1°C storage temperature after 24 hours of storage was achieved because the ultimate pH and gradual decline in the core temperature of pork was observed at this storage condition. But lowest shear force value (3.92±0.09 kg/cm²) was observed at 37±2°C on 24 hours storage which might be due to early activation of calpain system in sarcoplasm and possible changes in the collagen solubility (Liu et al., 2013; Kim et al., 2014a). But present observations were not in agreement that at higher temperature exhaustion of proteolytic enzyme activities might cause higher shear force value in meat of ostrich, lamb, ovine, duck and beef (Botha et al., 2008; Rosenvold et al., 2008; Kim et al., 2014b; Choi et al., 2016; Mungure   et al., 2016).
       
Simple effect analysis indicated significant increase in percentage expressible water of pig meat as a function of storage duration (Fig 6). It was observed that the rate of increase in percentage expressible water with increase in storage duration was higher at higher storage temperature. The percent expressible water (water holding capacity) is the ability of meat to retain inherent water during storage, processing and cooking. The post-slaughter core temperature and pH are critical for the percent expressible water of meat (Olsson et al., 2005). As seen in Fig 5 the correlation analysis reveals significant negative correlation between core temperature and percent expressible water. The pig meat samples held at 37±2°C and 4±1°C temperatures showed the highest (16.50%) and lowest (11.05%) per cent expressible water, respectively. It shows that the higher temperature causes rapid decline in pH of pig meat and it is coupled with highest percent expressible water. It is consistent with earlier reports that rapid decline in pH at higher temperature adversely affects the percent expressible water of meat due to protein denaturation (Kim et al., 2014a; Kim et al., 2014b; Choi et al., 2016). Higher percent expressible water negatively affects the tenderness, appearance, yield and overall quality of meat (Kim et al., 2014a).
       
Simple effect analysis indicated significant increase in cooking loss of pig meat as a function of storage duration (Fig 7). Compared to other storage temperatures the cooking loss (%) of samples kept at 37±2°C was significantly higher whereas, it was significantly lower at 4±1°C storage. This might be due to the fact that protein denaturation in combination with high temperature and low pH causes higher cooking loss (Santos et al., 1994; Liu et al., 2013; Choi et al., 2016). Kim et al., (2017) examined the differences between meat qualities classified into five categories based on Berkshire meat quality traits and observed that post-slaughter pH after 24 hours was positively correlated with water holding capacity but negatively correlated with meat color, protein content, drip loss and cooking loss.
       
Colour of meat is an important quality attribute for the consumer. In this study a* (redness) value, b* (yellowness) value and L* value (lightness) were studied as the colour measurement parameters for pig meat samples. It was observed that a* (redness) value of pig meat samples was significantly affected by storage temperature, but there was no significant difference between a* (redness) value measured at different storage temperatures (Table 3). Two-way interaction of storage temperature and storage duration was significant in b* (yellowness) value (Fig 8) and L* value (lightness) of pig meat (Fig 9). But simple effect analysis indicated significant increase only in L* value (lightness) as a function of storage duration. The rate of change in L* value (lightness) of pig meat as a function of storage duration was found to be higher at higher storage temperature. These observations are consistent with reports by (Geesink et al., 2000; Bekhit et al., 2007; Kim et al., 2014b) that colour measurement values are affected by higher temperature coupled with rapid pH decline.
       
Overall consistent lower core temperature, maintenance of ultimate pH, lower percentage expressible water and preferred L* value as function of storage duration at 4±1°C were found in our study. These observations are in accordance with independent studies that the accelerated chilling regimes have an ability to improve the quality of fresh pig meat by increasing ultimate pH and percentage expressible water and, also by preventing negative changes in colour (Janiszewski et al., 2018; Zybert et al., 2019b; Zybert et al., 2020a).

The present study showed that at higher storage temperature (37±2°C), rapid pH decline, higher per cent expressible water, higher cooking loss and decreased colour stability were noticeable in semitendinosus muscle of pig meat. Although, shear force value was lowered at this temperature but due to undesirable changes in other quality parameters. It can be inferred that keeping the pig meat at higher temperature reduces its eating quality. Additionally, higher temperature is more conducive for microbial growth. Therefore, it is emphasized that the proper chilling regimes should be followed to prevent the quality deterioration of pig meat due to higher post slaughter temperature exposure and to maintain its uniform desirable eating quality during all weather conditions.

Authors will like to acknowledge Director ICAR-IVRI and Head, LPT Division for providing necessary facilities and financial assistance to complete this work.

 All authors declare that there is no conflict of interest.