Fundamental Food Microbiology, Third Edition - Fuad Fathir

458 FUNDAMENTAL FOOD MICROBIOLOGY
A. Nature of Food
Composition (amount of carbohydrates, proteins, lipids, and solutes), A w (moisture),
pH, and antimicrobial content (natural or added) greatly influence microbial destruction
by heat in a food. In general, carbohydrates, proteins, lipids, and solutes provide
protection to microorganisms against heat. Greater microbial resistance results with
higher concentrations of these components. Microorganisms in liquid food and food
containing small-sized particles suspended in a liquid are more susceptible to heat
destruction than in a solid food or a food with large chunks in liquid. Microorganisms
are more susceptible to heat damage in a food that has higher A w or lower pH. In
low-pH foods, heating is more lethal to microorganisms in the presence of acetic,
propionic, and lactic acids than phosphoric or citric acid at the same pH. In the
presence of antimicrobials, not inactivated by heat, microorganisms are destroyed
more rapidly, the rate differing with the nature of the antimicrobials.
B. Nature of Microorganisms
Factors that influence microbial sensitivity to heat include inherent resistance of
species and strains, stage of growth, previous exposure to heat, and initial load. In
general, vegetative cells of molds, yeasts, and bacteria are more sensitive than spores.
Cells of molds, yeasts, and many bacteria (except thermoduric and thermophilic),
as well as viruses, are destroyed within 10 min at 65ºC. Most thermoduric and
thermophilic bacterial cells important in foods are destroyed in 5 to 10 min at 75
to 80ºC. Yeast and most mold spores are destroyed at 65 to 70ºC in a few minutes,
but spores of some molds can survive at as high as 90ºC for 4 to 5 h. Bacterial
spores vary greatly in their sensitivity to heat. Generally, heating at 80 to 85ºC for
a few minutes does not kill them. Many are destroyed at 100ºC in 30 min, but there
are bacterial species whose spores are not destroyed even after boiling (100ºC) for
24 h. All spores are destroyed at 121ºC in 15 min (sterilization temperature and
time). Below this temperature (and time), spores of some bacterial species can
survive; however, this depends on the initial number of spores and the nature of the
suspending medium. Species and strains of bacterial cells and spores also differ in
heat sensitivity. This is especially important if a food is heat treated on the basis of
results obtained by using a heat-sensitive species or strain but contains heat-resistant
variants.
Cells at the exponential stage of growth are more susceptible to heat than resting
cells (stationary phase). Also, cells previously exposed to low heat become relatively
resistant to subsequent heat treatment (due to synthesis of stress proteins). Finally,
the higher the initial microbial load in a food, the longer the time it takes at a given
temperature to reduce the population to a predetermined level. This is because the
rate of heat destruction of microorganisms follows first-order kinetics, which is
discussed later. This suggests the importance of lower initial microbial loads (through
sanitation and controlling growth) in a food before heat treatment.