Fish Microbiology and Quality Assurance

Unit 2 - Growth and survival of microorganisms in food

2.2.1. Temperature of storage
Storage temperature refers to temperature at which the food is handled and stored. Microorganisms growing over a wide range of temperature have been reported from food. The minimum temperature supporting the growth of microorganism was found to be well below the freezing temperature of water, and the highest temperature close to boiling temperature of water. However, no single microorganism is capable of growing over this wide temperature range. Thus, selecting a proper temperature for the storage of different food types helps in maintaing quality. Each microorganism exhibits minimum, optimum and maximum temperature at which growth occurs, called cardinal temperature. This temperature is characteristic for an organism and is influenced by factors such as nutrient availability, pH of growth medium, water activity etc.

Grouping of microorganisms based on temperature requirement for growth
Microorganisms are grouped in to 4 broad types based on cardinal temperature.

Groups	Minimum temperature for growth  (0C)	Optimum temperature for growth  (0C)	Maximum temperature for growth  (0C)
Psychrotrophs	-5 to +5	25-30	30-35
Psychrophiles	-5 to +5	12-15	15-20
Mesophiles	5-15	30-40	40-47
Themophiles	40-45	55-75	60-90

Psychrophiles
Microrganisms capable of growing at low temperature are called psychrophiles. These are further divided in to 2 types based on their optimum temperature for growth.
Obligate psychrophiles (cold loving) – These have temperature optima of 12-15^oc, but unable to grow above 20^oc. These are confined to polar regions and deep marine environment.

Psychrotrophs (facultative psychrophiles)
These have same minimum temperature for growth as psychrophiles but have higher optimum and maximum growth temperature. Thus are found in most diverse habitats, grow well in refrigerated temperature and cause spoilage of chilled food.

Ex: Alcaligenes, Cornybacterium, Flavobacterium, lactobacillus, Pseudomonas, Enterococcus etc. These bacteria grow well at 5-7^oc (refrigerator temperature ) and cause spoilage of meat, fish, poultry, eggs etc.

Mesophiles
These grow well under normal temperature conditions of 30-40^oC. As a rule mesophiles grow quickly at their optimum temperature than psychrotrophs. Thus the spoilage of food at mesophilic temperature range is rapid than at chill temperature. Mesophiles are found in most genera of bacteria.

Thermophiles
These are high temperature loving microorganisms with the optimum temperature in the range of 55- 75^oC. Thermophiles are generally less important in food microbiology. But thermophilic spore formers of the genus Bacillus and Clostridium cause spoilage of canned foods.

Molds and yeasts
Like bacteria, many molds and yeasts are associated with food. Many spoilage molds are able to grow over a range of temperature. Some molds grow at refrigerated temperature (Aspergillus, Cladosporium, Thamnidium Sp).
Yeasts also grow and involve in spoilage of food under appropriate conditions. These grow over psychrophilic and mesophilic temperature range.

Storage temperature and spoilage

• Temperature of storage is most important parameter affecting the spoilage of highly perishable foods (Ex. fish).
• Nature of food need to be considered while selecting storage temperature. Maintaining all foods at refrigerated temperature is not advisable as it affects the quality of food.

Ideal temperature for storage of most vegetables is 10^oc , banana: 13-17^oc and refrigerator temperature for most foods.