Freezing Technology

1.4.2 Factors that influence the growth of microorganisms

There are a number of external and internal factors (intrinsic to the substrate, that is, the foods in which they grow) that affect the microbial growth.

Of the different external factors, temperature is the most important one.

(1)  Thermophiles (high-temperature loving kind): growth optimum above

45 ^o C (35 ^o to 65 ^o C).

(2)  Mesophiles (medium- temperature loving kind): growth optimum above

25^o to 37 ^o C (15 ^o to 45 ^o C).

3)  Psychrotrophs (cold-temperature tolerant kind): growth temperature is between 0 ^o - 25^oC (optimum 15 ^o - 25 ^oC).

The bacteria on temperate water fish are all classified according to their growth temperature range as either psychrotrophs or psychrophiles. Psychrotrophs (cold-tolerant) are bacteria capable of growth at 0^oC but with optimum around 25^oC.  Psychrophiles (cold-loving)are bacteria with maximum growth temperature around 20^oC and optimum temperature at 15^oC (Morita, 1975). In warmer waters, higher numbers of mesophiles can be isolated. The microflora on temperate water fish is dominated by psychrotrophic gram-negative rodshaped bacteria belonging to the genera Pseudomonas, Moraxella, Acinetobacter, Shewanella and Flavobacterium. Members of the Vibrionaceae (Vibrio and Photobacterium) and the Aeromonadaceae (Aeromonas spp.) are also common aquatic bacteria and typical of the fish flora. Gram-positive organisms are Bacillus, Micrococcus, Clostridium, Lactobacillus and Coryneforms can also be found in varying proportions, but in general, Gram-negative bacteria dominate the microflora.

Interaction between growth factors

1)    Microbial growth is arrested even when one of the growth factor is controlled. For example, bacterial growth slows down  under reduced pH or acidic conditions.

(2)   If more than one factor becomes limiting, microbial growth is drastically curtailed or even stopped.  For example bacterial growth would stop under acidic conditions combined with low temperature and lower water activity(a_w).