Introduction

The preservation and export of meat is an essential process in the meat industry, involving the application of techniques and measures that guarantee its quality, product safety during transport and storage. Meat preservation refers to the implementation of methods and technologies that make it possible to prolong shelf life, avoiding deterioration and preserving its organoleptic characteristics.

According to Luzardo-Butria¹, vacuum packaging helps prevent oxidation and the development of microorganisms by eliminating oxygen from the package² and can also maintain the sensory quality of the meat, such as color and texture, for a prolonged period of time³.

The use of controlled or modified atmospheres is also common in the preservation of meat, these techniques involve the modification of the gaseous composition of the environment in which the meat is found, with the objective of prolonging its shelf life, Brody et al.⁴, can help inhibit the growth of microorganisms and delay the oxidation of meat components⁵.

Refrigeration allows meat to be kept at low temperatures, which reduces the development of microorganisms and delays spoilage processes. On the other hand, freezing involves reducing the temperature of the meat to below its freezing point, which stops microbial activity and preserves the quality of the product⁶.

Meat exports imply complying with the requirements, sanitary and quality regulations established by the importing countries. According to the Ministry of Rural Development and Lands of the Plurinational State of Bolivia, there are specific protocols for exporting meat to different countries³.

Livestock farmers and government representatives from Bolivia and China indicated that the Processing Plants of Frigorífico del Oriente S.A (FRIDOSA) and Matadero Frigorífico Santa Cruz (FRIGOR S.A.) were the first to be registered for the export of deboned meat to the Asian country, with an export of 40000 t in 2019 and 117000 t projected for the year 2025⁷.

Annual consumption in 2019 of beef in Bolivia was 217687 t, and per capita annual consumption of 21 kg, the main producing departments were Santa Cruz and Beni, among the cities with the highest consumption, Santa Cruz, La Paz and Cochabamba with 71 % of the national total. Bolivia needs markets to generate added value to its economy by diversifying the supply of meat processing plants with specialized vacuum processed cuts. Another strategy of the meat industry in growing development is to offer safe, hygienic, good quality products².

Vacuum-packed metabolically activated foods, such as meats, continue respiratory activity, consuming a small amount of oxygen present in the product's tissues, which increases the vacuum producing carbon dioxide and water vapor. The increase of CO₂ in the package has advantages as an inhibitor against many microorganisms^1,4.

Quality is a very complex term, and is not the only definition valid for all levels of meat production, involving characteristics of carcass composition as determinants of market value, and the most recent ones consider its nutritional, organoleptic, technological and sanitary hygienic properties⁸.

The purpose of the present study was to determine the tolerable limits of fecal coliforms in vacuum-packed meat preserved under refrigeration, by means of total mesophilic aerobic bacteria counts, sensory analysis of the organoleptic characteristics of beef and shelf life of beef in 3 pH ranges.

Materials and methods

The research work was carried out at FRIDOSA, located 10 km east of the city of Santa Cruz. For the study, carcasses were obtained from 10 Nelore breed steers that entered the deboning room, 120 samples were collected and the pH was measured 48 h post mortem.

A quantitative experimental investigation was proposed with 2 variables under study: pH of beef in 3 ranges: pH < 5.8 (normal), 5.9 (intermediate) and 6 to 7 (high)⁸ and its storage for a period of 3 months at 0±1° C⁴, vacuum-packed in cardboard boxes. The samples were taken in FRIDOSA's deboning room. The organoleptic characteristics of flavor, color, odor and texture were evaluated in cooked meat from the beginning to the end of the test. The raw material used was meat from 10 beef steers. We worked with the front cut (shoulder), which corresponds to the triceps brachii muscle, 30 samples were selected for each pH level, within 48 h after slaughter. A pH-meter with a Testo 230 electrode was used to measure the samples. Subsequently, the shoulder cut obtained with a sterilized knife was divided into 4 to make a total sample of 120 cuts⁸. For the statistical analysis, the randomized block design was applied, followed by the minimum significant difference (LSD) and Tukey tests with a significance of 5% and 1%^9,10,11.

In the laboratory, the microbiological analysis method Petrifilm Plate NB 32020^12,13 was used for the CFU count of Escherichia coli. The total mesophilic aerobic count was carried out according to NB 32003 and NB 32016^14,15.

Sensory analysis (flavor, color, odor and texture) was evaluated on 3 cm thick samples, cooked according to the ISO 22000¹ Food Safety Management System standard (Table 1).

Results

Microbiological analysis. The total mesophilic aerobic bacteria count from day 0 to 3 months showed a significant difference (p < 0.05) between the 3 pH levels and the lowest development was at pH < 5.8 with 6.8x10⁶ CFU/g^4.8 (Figure 1). When comparing the CFU count of vacuum-packed meat with the limits set by the Bolivian IBNORCA standard 1x10⁵ CFU/g¹⁰ for fresh, refrigerated and/or frozen red meat, it was observed that the meat samples presented counts above the permissible level, the limit being 10x10⁶ CFU/g, so the risk of spoilage is higher¹¹.

Figure 1 Development of mesophilic aerobes in CFU/g according to pH range 

Figure 2 Development of E. coli in CFU/g according to pH range 

E. coli at pH 5.9 presented the lowest CFU count with 1.75 and the highest at pH 6 to 7 with 2.25 CFU (Figure 2), a value that is within the levels established by the NB 310017 standard. No significant statistical difference by month was observed in any of the 2 microorganisms (p>0.05).

Sensory analysis. The organoleptic characteristics of flavor, color, odor and texture were evaluated in cooked meat from the beginning to the end of the test (Table 2). At pH < 5.8 there were no variations in the first month, but in the second month a change in color was observed (it became an opaque red) and in the third month there were variations in the three parameters.

At pH 5.9 from month 2 the samples showed variations in 3 parameters and in month 3 a change in odor (Table 2). At pH 6 to 7 from month 2, there were variations in color and odor, and in month 3 in texture and odor. At pH 5.9 and pH 6 to 7 after 2 months of storage, there were evident signs of deterioration (Table 2).

The E. coli CFU count showed low values under the conditions of pH, storage, time and temperature in which the study was carried out¹⁶, indicating that no significant growth was observed for E. coli in the meat cuts stored at 0 and 4° C for 3 months.

Discussion

In the study carried out, it was found that meat with intermediate pH (5.9) and high pH (6 to 7) should not be vacuum-packed or stored for a period of 3 months because it does not maintain its organoleptic characteristics that ensure quality and safety for human consumption¹⁷.

The storage of vacuum-packed cuts with a pH < 5.8 and kept at 0° C contributes to the quality of the product, controls microbial development and maintains its sensory characteristics in commercially acceptable conditions for 3 months, which allows its transport to distant destinations^17-20.

FRIDOSA uses innovative techniques for the preservation of vacuum-packed meat products, thus improving the shelf life and quality of its cuts, for which it conducts studies on texture, water retention, microbial development and color⁵.

Research carried out at FRIDOSA is related to the control of meat quality with the pH of live muscle, which is normally above 7, with the pH of the meat after slaughter reaching values of 5.4 to 5.7 in normal meats.

The pH of meat has an effect on its organoleptic characteristics such as color and water retention capacity, but has only a slight effect on flavor and tenderness, which is why it is difficult to predict changes in the latter through pH⁶.

Maturation time has a significant influence on pH. Frozen meat does not show increases in pH in relation to storage time. Vacuum packaging was able to neutralize the increase in pH, but at the time of packaging, the product should not have a pH higher than 5.85. The variation of pH values is the effect of many factors, some of which are specific to the animal (genetics, metabolism, susceptibility to stress, etc.), but normally the most relevant ones have to do with the environment in which the animal was handled and its carcass during the 24 h before and after slaughter⁸.

In relation to mesophytic bacterial, significant variations in CFU/g were observed in the samples analyzed, coinciding with Martinez et al.⁷ who mentioned that these variations are due to factors such as the final pH of the meat, temperature and storage time; it is worth noting that pH values <5.8 showed low microbial development.

In the sensory analysis of the 1 month cooked meat, no variations were observed in the organoleptic characteristics at the 3 pH levels, but in the 2nd and 3rd months it showed signs of deterioration unfit for human consumption at pH > 5.9²¹. At high pH, the risk of deterioration (protein degradation, putrefaction) is greater. Meat and meat products with pH above 6.0 are particularly risky^1,22.

A fundamental parameter for ensuring the shelf life of fresh cuts is the refrigeration temperature, which has a direct effect on the speed of microbial growth; however, strict control of microbial load and temperature is not sufficient to achieve a limited marketing period²³.

In this sense, cuts stored at 4° C presented physicochemical and appearance alterations in month 3 pH <5.4¹⁶, loss of vacuum and presence of odors, while at 0°C no alterations were perceived until month⁵.

The color change occurred from month 2 at pH levels > 5.8. Meat color is the first quality aspect that directly influences consumer preferences18 and the decision of whether or not to buy beef, the first one perceived at the time of purchase^24-26.