BLBS102-c38 BLBS102-Simpson March 21, 2012 14:17 Trim: 276mm X 219mm Printer Name: Yet to Come
726 Part 7: Food ProcessingVarious food processing operations make the food products
more attractive, satisfying, safer, and easier to eat. Common
food-processing techniques include pasteurization, sterilization,
cooking, drying, cooling and freezing, fermentation, addition of
preservatives and reduction of water activity, and use of two or
more of the above techniques to inhibit or stop chemical, bio-
chemical, and microbiological activities. Most foods available in
the market are subjected to some form of thermal processing. Al-
though the canning process started from Nicholas Appert’s time
in 1809, food processing via the canning process still provides
a universal and economic method for preserving and process-
ing foods. Thermal processing of canned foods has been one of
the most widely used methods for food preservation during the
twentieth century and has contributed significantly to the nutri-
tional well-being of much of the world’s population (Teixeira
and Tucker 1997).
The two very common industrial conventional thermal food
processing technologies in production of canned products are
pasteurization and sterilization. Pasteurization is the process of
heating liquids and/or solid foods for the purpose of destroy-
ing principal pathogens capable of growing under aerobic con-
ditions and reducing the level of spoilage vegetative bacteria,
protozoa, and fungi from high acidic foods (pH<4.5). When
pH>4.5, foods produced by this process should be stored at
low temperature to avoid further growth spore forming micro-
organisms. However, commercial sterilization refers to an in-
tensive heat treatment process that effectively kills or eliminates
all pathogens and vegetative microorganisms as well as bulk
of spore-forming bacteria from the low acid foods (pH≥4.5).
Products produced through this method can be shelf stable for
up to two years. In order to reduce the process severity, the
thermophilic spore formers are not targeted to be completely
eliminated, instead the canned product is advised to be stored at
temperatures below 30◦C to prevent the growth of thermophiles.
Because of the high safety implications and severe process-
ing conditions, the conventional thermal sterilization has been
accepted to result in considerable product quality degradation.
However, many improvements have been implemented to im-
prove thermal processing operations and techniques, and novel
processing approaches have been introduced in recent years to
improve the quality of thermally processed foods without com-
promising safety of these products. Therefore, the main focus
of this chapter is to describe the basic principles of thermal
processing operation in terms of production of safe and better
quality products.THERMAL PROCESSING BASICS
IntroductionThe major objective of thermal processing is production of safe
and stable products that consumers are willing and able to buy.
To achieve this goal, it is necessary to understand the scientific
basis on which the process is established. Silva et al. (1992) in-
dicated that commercial thermal processing is a function of sev-
eral factors, such as product thermo-physical properties (prod-
uct heating rate), surface heat transfer coefficient, initial foodtemperature, retort temperature, heating medium (hot water or
steam), heating medium come-up time (CUT), temperature re-
sistance of food microorganisms and quality factors, and target
degree of lethality or safety level we need to achieve.
The success of thermal processing does not depend on the
elimination of the entire microbial population, because this
would result in low product quality due to the long heating
required. Instead, all pathogenic and most spoilage-causing mi-
croorganisms in a hermetically sealed container are destroyed,
bulk of the spore formers is killed, and an environment is created
inside the container that does not support the growth of remain-
ing spore formers. There are different mechanisms that enable
one to control the germination and growth of such type of spores
in canned foods. These are based on the microbial growth and
inactivation with respect to oxygen requirement, pH preference,
and temperature sensitivity.
In general, canned foods have a 200-year history and are likely
to remain popular in the foreseeable future owing to their conve-
nience, long shelf life, and low cost of production. The technol-
ogy is receiving increasing attention from thermal-processing
specialists to improve both the economy and quality of some
canned foods (Durance 1997). However, the sterilization pro-
cess not only extends the shelf life of the food, but also affects
its nutritional and sensorial qualities. Process optimization is
therefore necessary in order to promote better quality retention
without sacrificing safety.Reaction RateDuring thermal processing of foods, several types of chemi-
cal reactions occur. Some reactions result in a quality loss and
such type of reactions must be minimized, whereas others result
in the development and formation of a desirable flavor, taste,
or color, and these ones must be optimized to obtain the best
product quality (Toledo 2007). In order to maintain quality of
food products through optimization of processing conditions,
predictive mathematical models are very important. To realize
this goal, information is needed on the rates of destruction of
microbes as well as quality parameters and their dependence on
variables such as temperature, pH, light, oxygen, and moisture
content, which can be expressed by mathematical models. A
better understanding of kinetics of food products can provide
better opportunities for developing food processes to maximize
quality parameters and ensuring safety.
Each reaction undergoes on its own rate and the rate of reac-
tion is described by the reaction kinetics. Kinetics is the study
of the rate at which compounds react. Reaction kinetics (rate
theory) deals to a large extent with the factors that influence the
reaction velocity. The rate depends on several factors, including
the contact between the reacting components, their concentra-
tion, temperature, and pressure at which the reaction takes place.
The “collision theory” implies that the molecules need to collide
with each other in order for the reaction to take place. If there
are a higher number of collisions in a system, there is a greater
chance for the reaction to occur. The reaction will go faster, and
the rate of the reaction will be higher. In collision theory, two
main things are to be considered: the activation energy, which