Fundamental Food Microbiology, Third Edition - Fuad Fathir

MICROBIAL GROWTH CHARACTERISTICS 63
the food, but one having a shorter generation time than the other. After a storage
period, the one with shorter generation time becomes predominant. Many foods
most often get spoiled by bacteria than by yeasts and molds because, in general,
bacteria have shorter generation time. Thus, in a mixed population, the intrinsic and
extrinsic environments dictate which one, two, or a few in the initial mixed population
will become predominant and produce specific changes in a food. These
aspects are very important in the control of microbial spoilage of foods and in the
production of bioprocessed (or fermented) foods. 3,4
B. Sequence of Growth
Among the different microbial types normally present in a food, different species
(strains) can become predominant in sequence during storage. 3,4 Initially, depending
on the environment, one or two types may grow optimally and create an environment
in which they can no longer grow rapidly. However, another type in the mixed
population can find this changed environment favorable for growth and grow rather
rapidly. This shift in predominance can occur several times during the storage of a
food. The sequential microbial growth can be seen particularly in foods stored for
a long time. If a food is packaged in a bag with a little air (e.g., ground meat), the
aerobes grow first and utilize the oxygen. The environment then becomes anaerobic,
in which the anaerobes (or facultative anaerobes) grow favorably. In the natural
fermentation of some foods, such as sauerkraut, four different bacterial species grow
in succession, one creating the favorable growth condition for the next. The desirable
characteristics of the final product are dependent on the growth of all four species
in specific sequence. To identify the existence of such a situation, it is necessary to
enumerate and determine the microbial types at different stages of storage or fermentation.
3,4
C. Growth in Succession or Diauxic Growth
Microorganisms that can metabolize two or more nutrients in a food, one preferred
over the other and present in limiting concentrations, show growth in stages separated
by a short lag phase. 1 Initially a bacterial strain grows by utilizing the preferred
nutrient and after a short lag of adaptation grows by utilizing the other nutrient.
During each stage, the growth curve has exponential and stationary phases with the
lag phase in-between. An example is the growth of certain bacterial strains (such as
some lactic acid bacteria and Gram-negative bacteria) in fresh meat. A strain grows
initially by utilizing the limiting concentrations of carbohydrate present, followed
by utilization of nonprotein nitrogenous (NPN; such as amino acids) substances.
D. Symbiotic Growth
Symbiosis, or helping one another, during growth often occurs in food containing
two or more types of microorganisms. 3,4 One type may produce metabolic products
that the second type needs for proper growth, but cannot produce by itself. In turn,
the second species produces a nutrient that stimulates the first one to grow better.
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