How to Test for Food Spoilage Organism

How to Test for Food Spoilage Organism

Food spoilage occurs when the appearance of the food becomes bad or there are textural changes on the food or when there is a change in taste and odor. The appearance changes when colonies of bacteria are visible on the surface or there is mycelia growth by molds and fungi. Textural changes also occur when slime formation takes place due to microbial cells or due to enzymatic degradation. Taste and odor are caused by the secretion of metabolic by products from the spoilage organism giving nitrogenous compounds, sulfides or organic acids. For food spoilage to occur, the organism has to get to the food from some source like environment, operation personnel or food ingredients. The food environment must also favor growth i.e. the temperature, pH, nutrients, oxidation reduction potential, and inhibitory agents. The time should be sufficient for the numbers to grow to a certain level of between 106 to 108 cells, giving food products its shelf life. Spoilage associated with H2S and amines may be detected at a lower level whilst lactic acid may be detected at high levels.

Important groups of food spoilage bacteria are: -

Psychotropic bacteria

These are bacteria that grow rapidly at temperatures of 10-25 degC but can survive at temperature below 7 degC. They are mainly responsible for food spoilage under non refrigerated aerobic conditions. Some of these bacteria are, pseudomonas, Acinetobacter, Moraxella, flavobacterium, and some molds and yeast.

Psychotrophic facultative anaerobic bacteria

A psychotropic facultative anaerobic bacteria is one that makes ATP by aerobic respiration when oxygen is present but can switch to fermentation when there is no oxygen and those that grow rapidly in the temperature range of 10-25 degC. Some of these types of bacteria are, lactobacillus, leuconostoc, some enterococcus, alcaligenes spp, enterobacter spp, and some microaerophilic yeast.

Thermoduric psychotrophs

Thermoduric psychotrophs are bacteria that can survive pasteurization temperature and can survive at below 7 degC and can grow rapidly at 10-25 degC. They are widespread in farm environments and important for the milk industry. They include facultative anaerobes like bacillus coagulants, lactobacillus viridescens.


These are bacteria that can survive without oxygen like clostridium. They can sporulate and can survive low heat treatment.

Thermophilic bacteria

These are bacteria that grow between 40-90 degC with an optimum of 55-65 degC. These include some lactic acid bacteria like pediococcus and streptococcus spp, bacillus and clostridium. They are commonly found in warm processed foods in restaurant and supermarkets.

Aciduric bacteria

These are bacteria that can grow at a pH of 4.6 or below. They are associated with spoilage of acidic food like fruit juice, pickles, salsa, salad dressing and fermented sausages. Yeast and molds are also aciduric.

Specific Microbial Spoilage Indicators

Refrigerated raw meats stored under aerobic conditions – enumeration of psychotropic aerobes like gram negative aerobes.

Refrigerated raw meats stored anaerobically (vacuum packed) – enumeration of psychotropic lactic acid bacteria, enterobacteriaceae under anaerobic conditions.

Raw milk: TPC, psychotropic gram negative and thermoduric bacteria.

Pasteurised milk: TPC, psychotropic gram negative and gram positive bacteria.

Butter: Lipolytic bacteria

Salad dressing: Lactobacillus spp.

Biosicence Diagnostics provide a whole range of media to test and identify food spoilage organism. Contact the friendly sales staff for more information.


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