GENERAL AND FOOD MICROBIOLOGY PART II - L. V. Krasnikova - 2016

TOPIC 2. MICROBIOLOGICAL EXAMINATION OF CHILLED, FROZEN, AND CURED MEAT AND BRINES

Chilling, freezing, and curing are essential technological processes designed to preserve the quality of meat and meat products and impart the desired organoleptic characteristics to them. Product quality verification via microbiological control is required at all stages of these technological operations.

2.1. Procedure for Microbiological Examination

Chilled meat is tested for total microbial load, the presence of pathogenic microorganisms, coliform Bacteria, Salmonella, staphylococci, sulfite-reducing clostridia, and bacteria of the genus Proteus. The testing methodology is similar to that used for the MICROBIOLOGICAL EXAMINATION OF fresh meat.

Sampling. Samples of chilled and frozen meat for microbiological analysis are collected similarly to those of fresh meat. Cured meat samples are taken in an amount of 400 g. Brine is sampled into sterile flasks with a capacity of at least 500 cm³.

The surface of chilled meat is typically dominated by psychrophilic microorganisms of the genus Pseudomonas and others. They account for about half of the total bacteria detected in the meat. These are small, motile, non-spore-forming, non-capsulated Gram-negative rods. Chilled meat must be free of pathogenic microorganisms, including: Salmonella in 25 g of product, coliforms in 0.1 g, and bacteria of the genus Proteus in 0.01 g.

Frozen meat contains a negligible amount of microorganisms, as some non-spore-forming bacteria die off at low temperatures (-23 to -25 °C). During the storage of frozen meat at temperatures above -10 °C, certain mold species of the genera Cladosporium and Thamnidium, as well as some Yeast species, can develop and cause product spoilage. At storage temperatures around -5 °C, mold Fungi of the genera Penicillium, Aspergillus, and Mucor are capable of growth.

The control of mold and yeast content in meat is carried out using standard Methods covered in the general microbiology course.

During meat curing, the quantitative and species COMPOSITION OF THE microflora in the brine and meat products changes. Quality brine contains A large number of various microorganisms. As a result of the multiplication of microbes adapted to curing conditions, the total microbial count in the brine increases tenfold by the end of the curing process.

Salt-tolerant (or halophilic) microorganisms can develop in cured meat (corned meat). Typically, sodium chloride acts as a preservative against many bacteria. At a 12-15% concentration of table salt in the medium, only halophiles capable of causing product spoilage can multiply. Halophiles include many mold species, certain Yeasts, and many pigment-producing rod-shaped bacteria.

Salt-tolerant microorganisms are capable of growing in media with a salt concentration of 6-8% and maintaining viability in media with a high salt content (20% or more).

Unspoiled corned meat and brines are dominated by salt-tolerant lactic acid bacteria, micrococci, and certain species of Gram-negative bacteria.

Good-quality brine must not contain coliforms in more than 50 cm³; in pickling brines after heating at 100 °C for 5 min, coliforms must be absent in 500 cm³, and spores of anaerobic clostridia and aerobic bacilli must be absent in 50 cm³.

2.2. Determining The Effect of Sodium Chloride Concentration on Bacteria

Using a sterilized bacteriological loop, a piece of cured meat the size of a match HEAD is inoculated into four test tubes containing nutrient broth (NB) medium with 10, 15, and 20% NaCl, as well as one without NaCl (control). The test tubes are labeled and placed in an incubator at 25-30 °C for 24-48 hours.

Upon completion of incubation, the cultures are analyzed (observing Changes in the nutrient medium, Cell shape, and presence of spores). The turbidity of the medium is assessed using conventional symbols: (-) - no growth; (+) - weak growth; (++) - moderate growth; (+++) - abundant growth. To examine the Morphology of the grown bacteria, fixed smears are prepared from the suspension, stained with methylene blue, and examined microscopically.

Assignment for this topic:

1. Determine the effect of table salt concentration in the medium on bacteria.

2. Perform a MICROBIOLOGICAL ANALYSIS OF salted food raw Materials. Inoculate nutrient media with samples of meat and brine to detect coliforms and total viable count (TVC).

3. Study the cultural and morphological properties of microorganism colonies grown on nutrient media. Describe the grown colonies, prepare fixed smears from them, stain them using the Gram method, and sketch the microscopic view. Draw Conclusions regarding the wholesomeness of the examined samples.

Review Questions

1. What microbiological indicators are used to assess the wholesomeness of chilled, frozen, and cured meat?

2. Which microorganisms are capable of multiplying in chilled meat, and which in cured meat?

3. What constitutes a good-quality brine?