MICROBIOLOGY Study Guide - 2012

CHAPTER 15. FOOD MICROBIOLOGY

15.1. MICROBIOLOGY OF MILK AND DAIRY PRODUCTS

15.1.5. Canned Dairy Products

Canned dairy products are divided into two groups According to the preservation method: sterilization (the abiogenesis principle); sweetened condensed milk relying on high osmotic pressure created by added sucrose or glucose (osmobiogenesis); and dry milk products where Microbial growth is halted by Water removal (xeroabiogenesis).

Evaporated (sterilized) milk belongs to group "A" canned foods. This product is manufactured by concentrating whole milk to a specific solids content (28% for concentrated milk) and sterilizing it in tin cans. The primary factors ensuring the shelf life of this type of canned product are the elevated solids content, high-Temperature Processing, and hermetic sealing. Raw milk intended for this type of production must meet stringent quality requirements. It must be at least Class 1 according to the reductase test; the spore count of mesophilic and thermophilic bacilli and clostridia must not exceed 100 per 1 cm³.

As a result of intensive heat Treatment of sterilized canned milk, only isolated spores of aerobic spore-forming Bacteria of the genus Bacillus may survive and multiply in the finished product. The most common causative agents of spoilage are mesophilic bacilli of the species: B. subtilis, B. megatherium, B. circulans, B. cereus, B. licheniformis, as well as thermophilic bacilli: B. coagulans and B. stearothermophilus, which are capable of multiplying at 50–55 °C. Most Bacillus species cause coagulation (curdling) of condensed milk. B. subtilis promotes "sweet curdling" as well as bitterness. Coagulation caused by B. megatherium is accompanied by gas production and a cheesy odor. The proliferation of anaerobic gas-producing bacteria leads to gas accumulation in the sealed can and The Development of flat sour or swell spoilage (bombage).

Evaporated milk meets commercial sterility requirements if it contains mesophilic aerobic and facultatively anaerobic microorganisms of the B. subtilis group in an amount not exceeding 11 Cells per 1 g (cm³) of product. If the canned food contains spore-forming mesophilic aerobic and facultatively anaerobic microorganisms of the B. cereus and/or B. polymyxa group or mesophilic clostridia per 10 g, the product fails to meet commercial sterility requirements.

Sweetened condensed milk is a product obtained from pasteurized whole or skimmed milk by evaporating part of the moisture with The addition of sucrose. The addition of a high-concentration sugar syrup facilitates milk preservation and replaces thermal sterilization. The predominant microflora of sweetened condensed milk consists of micrococci, and less commonly spore-forming rods and Yeasts.

The following defects may appear during storage of these canned products.

✵ "Button" formation. This defect is characterized by the appearance of compact spots of various colors (white, yellow, brown) on the product surface, consisting of casein clots. The mold fungus Catenularia fuliginea is most frequently the causative agent. At the Base of the mold colony, the milk first thickens and then turns into a chocolate-brown "button".

✵ Swelling (bombage) is caused by gas-producing microorganisms, typically yeasts of the genus Torulopsis, which can ferment lactose and sucrose. Their proliferation is promoted by increased milk acidity and decreased sugar concentration.

✵ Thickening, especially after prolonged storage, is caused by micrococci characterized by osmotolerance, The ability to ferment lactose, and the secretion of a rennet-like enzyme. As a result of the combined action of lactic acid and the enzyme, an acid-rennet clot is formed, accompanied by a sour and cheesy odor in the product.

✵ Molding results from the growth of Molds On the surface of the product or the inner side of the lid. Colonies of the green mold Penicillium glaucum appear most frequently, and less often Cladosporium herbarum.

Dry milk (Milk powder). During The production of dry milk, the thermal processing of raw Materials does not completely destroy microorganisms. Spores of bacteria belonging to the genera Bacillus and Clostridium survive pasteurization. Upon re-contamination, B. subtilis and B. cereus often enter the milk. Contamination of milk by microorganisms after pasteurization can occur in the storage tank if cleaning and disinfection are inadequate. Thermophilic spore-forming and non-spore-forming microorganisms may multiply in the vacuum evaporator. During spray drying, the microbial destruction effect is rather weak. During cooling and packaging, microorganisms can enter the product from the air.

Psychrotrophic bacteria, if allowed to multiply in raw milk during prolonged refrigerated storage, are capable of accumulating active thermostable proteinases, which adversely affects the quality of the finished product during extended storage.

A wide spectrum of microorganisms enters the product accidentally. Pathogenic bacteria, particularly salmonellae and enterotoxigenic staphylococci, are especially hazardous.

In dry milk products with a moisture content of 4 to 7%, microorganisms cannot multiply; the shelf life of these products is 8 and 3 months, respectively. During storage, a gradual decrease in the total microbial count is observed due to Cell die-off. In top-grade dry milk, the total aerobic plate count (TVC) must not exceed 5 x 10⁴ CFU/g, coliforms must be absent in 0.1 g, S. aureus in 1 g, and pathogens in 25 g.

Breach of storage conditions and product humidification promote mold growth. Surviving microorganisms multiply very rapidly once the milk is reconstituted.