Fundamental Food Microbiology, Third Edition - Fuad Fathir

CONTROL BY A COMBINATION OF METHODS (HURDLE CONCEPT) 531
The concept can also be explained with another example (Figure 40.1B). Suppose
a food has several types of psychrotrophic spoilage bacteria that can grow rapidly
at 30ºC or at pH 6.0 or at an A w of 0.99 and spoil the food rather quickly. If any
one of the factors is reduced (e.g., if the temperature is reduced to 5ºC, keeping the
other two unaltered), the growth rate will be slightly reduced, but the product can
still be spoiled before the desired shelf life time (say 60 d). However, if all three
parameters are reduced slightly, which will allow the target microorganisms to grow
if used individually, such as 5ºC, pH 5.0, and A w 0.93, their growth will either stop
(C 2) or occur at a very slow rate (C 1), or they might even show some death during
storage (C 3). With this combination, the product may remain stable during its
expected shelf life. Each of the three parameters used in this example can be
ineffective individually, but when used together, their effect becomes additive, which
the target organisms fail to accommodate for during growth. If more such parameters
are added, the growth of microorganisms in foods can be reduced drastically for a
considerable period of time.
The different parameters or factors that are used in combination include intrinsic
factors (e.g., A w, pH, Eh, and natural inhibitors), processing factors (e.g., heating,
drying, fermentation, and preservatives), and extrinsic factors (e.g., temperature and
aerobic or anaerobic environment). Competitive flora (e.g., lactic acid bacteria) and
nonthermal processing methods (Chapter 39) can be added to them. Food preservation
by combining several of these factors has been used for a long time. An example
is the preservation of jams and jellies, for which high heat, low pH (of fruits), low
A w (sugar in fruits and added), and anaerobic packaging are used to reduce microbial
numbers as well as the growth of survivors. However, the greatest challenge will be
the long-term preservation of meat, dairy, and fish products at refrigerated temperature
or even at ambient temperature, where refrigeration is not always available,
such as during a military operation, a work relief during a natural disaster, or in the
rural areas of many developing countries. To achieve this goal, it will be necessary
to study the effectiveness of combining different factors to control target microorganisms
in a food. In conducting such studies, one should also recognize that the
factors could have additive, synergistic, or even adverse (neutralizing) effects when
used in combination.
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III. CURRENT STATUS
These aspects have been studied in a rather limited way. Results of some of these
studies are briefly described here. 3,4
A. Low-Heat Processing
Low heat (£100ºC) does not kill many pathogenic and spoilage bacterial spores.
They can be heat activated, which leads to germination and outgrowth. However, if
the pH of the food is reduced to 4.5 or NO 2 and NaCl are added, the heat-shock
spores will not germinate.