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Microbes Associated With Food Spoilage |
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Microbes Associated With Food Spoilage - Fruits, vegetables, meats, poultry, sea foods, milk and dairy products and various other food products differ in their biochemical composition and therefore are subject to spoilage by different microbial populations.
Such changes depend upon the nature of the microbes involved in the spoilage. Thus degradation of apple juice by yeast gives an alcoholic taste to the juice. Yeasts convert the carbohydrate into ethanol.
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Bacteria which attack food proteins, convert these into .amino acids which are broken down again into foul smelling end products. Digestion of cystein, for example, yields hydrogen sulphide, giving a rotten egg smell to food. Digestion of tryptophan yields indole and skatole which give food a fecal odour.
Two other products of the microbial metabolism of carbohydrates are (a) acid that causes foods to become sour, and (b) gas which causes sealed cans to swell. Digestion of fats, as in spoiled butter, yields fatty acids giving a rancid odour or taste to food. Food may become slimy due to production of capsules in bacteria. There may be pigment development giving some colour to foods.
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1. Meats and fish. Microorganisms, which cause their spoilage are usually introduced during handling, processing, packaging and storage. For example if a piece of meat is ground, the surface organisms accumulate in the teeth of the grinder alongwith other dustborne organisms. Bacteria from the hands of the preparer or from an errant sneeze may add more microbes. Processed meats may become contaminated during handling.
Some as sausage etc. made from animal intestines may contain residual bacteria, especially botulism spores. Organ meats like liver, kidney etc. spoil quickly, and may contain many bacteria trapped in their filtering tissues.
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Greening on meat surface is usually due to Gram-positive rod, Lactobacillus, or the Gram-positive coccus, Leuconostoc.
2. Poultry and eggs. Species of Salmonella cause diseases in chickens and turkeys, passing to the consumer through the poultry or egg product. Processed foods as pot pies, egg salad, omelet etc. may be sources of salmonellosis.
Chicken product may become contaminated due to improper handling or from the water used for cleaning the product. Eggs may become contaminated by Proteus causing black rot (here H2S accumulates due to digestion of egg cysteine), by Pseudomonas causing green rot, and by Serratia marcescens, causing red rot. The main part of contamination is yolk (white is inhibitory to Gram-positive bacteria due to the presence of inhibitor enzyme, lysozyme).
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3. Breads and bakery products. The ingredients of bread products as flour, egg, sugar and salt are usually the sources of spoilage organisms. Some bacteria and molds are able to survive the baking temperatures. Some species of Bacillus give the bread a soft and cheesy texture with long, stringy threads.
This kind of bread is called ropy.Cream rolls, custards from whole eggs, whipped cream are good media for growth of Salmonella, Lactobacillus and Streptococcus species which produce acids.
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4. Other foods. Many cereals, fruits and vegetables are spoiled by microorganisms. Chief agents are molds and bacteria giving them unpleasant smell and odour. Grains are spoiled by a mold, Aspergillus flavus. It is also present in peanut products and other foods.
The mold forms a toxin aflatoxin. Another grain spoilage occurs by Claviceps purpurea, in rye, wheat and barley grains causing ergot disease. The mold toxin may induce convulsions and hallucinations. The drug LSD is derived from this toxin.
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