FST 222 INTRODCUTION TO FOOD SCIENCE AND TECHNOLOGY
PRINCIPLES OF FOOD PROCESSING AND PRESERVATION
Introduction
The limiting factors to continuous food processing are availability of raw materials (crops and
animals) and demand for final products. Hence, food processing is naturally seasonal. Most
agricultural produces have specific harvest times in the year. Also, there are usually gluts during
harvest times. Most produces are wasted during this period due to their perishable nature. Above
factors coupled with the ever-rising human population and reduction in the quantity of land
available for agricultural activities have placed challenges on man to come up with solutions that
will ensure sustainable availability of foods at affordable prices any time of the year. Knowledge
and practice of the principles of food processing and preservation are needed to achieve the
above objective.
Food processing is set of methods and techniques use to transform raw food materials into edible
food or to transform food into other forms for consumption by humans and animals either in the
home or by the food processing industry. Food processing typically takes clean, harvested crops,
butchered animal products or marine materials and uses these to produce attractive, marketable
and often long shelf-life food products. Processing of foods is a segment of manufacturing
industry that transforms animal, plant, and marine materials into intermediate or finished valueadded food products that are safer to eat. This requires the application of labor, energy,
machinery, and scientific knowledge to a step (unit operation) or a series of steps (process) in
achieving the desired transformation. Value-added ingredients or finished products that satisfy
consumer needs and convenience are obtained from the raw materials.
The aims of modern food processing could be considered four-folds:
(1) Extending the period during which food remains wholesome (microbial and biochemical),
(2) Providing (supplementing) nutrients required for health,
(3) Providing variety and convenience in diet, and
(4) Adding value.
The scope of food processing is broad and it involves series of unit operations. Unit operations
are stages or steps performed to transform raw agricultural materials into foods suitable for
human consumption. A typical processing operation includes raw material handling, ingredient
formulation, heating and cooling, cooking, freezing, shaping, and packaging. These could
broadly be categorized into primary and secondary processing. Primary processing is the
processing of food that occurs after harvesting or slaughter to make food ready for consumption
or use in other food products. Primary processing ensures that foods are easily transported and
are ready to be sold, eaten or processed into other products (e.g. after the primary processing of
peeling and slicing, an apple can be eaten fresh or baked into a pie). Secondary processing turns
the primary-processed food or ingredient into other food products. It ensures that foods can be
used for a number of purposes, do not spoil quickly, are healthy and wholesome to eat, and are
available all year (e.g. seasonal foods).
Most food processes utilize six different unit operations such as heat transfer, fluid flow, mass
transfer, mixing, size adjustment (reduction or enlargement), and separation. During food
processing, food material may be combined with a variety of ingredients (sugar, preservatives,
acidity) to formulate the product and then subjected to different unit operations either
sequentially or simultaneously. Food processors often use process flow charts to visualize the
sequence of operations needed to transform raw materials into final processed product. The
process flow diagrams may include quality control limits and/or adjustment and description of
any hazards.
Important operations/considerations in food processing
Raw material handling
Raw material handling is the very first step in the food processing. Raw material handling
includes postharvest transportation (farm to plant), sorting, cleaning or sanitizing before loading
into equipment in the plant. These could also be considered as part of primary processing of the
food materials. Appropriate raw material selection and handling affect microbial safety and final
product quality. Further food preservation studies need to consider the impact of raw material
(including postharvest handling prior to preservation) on the final processed product.
Cleaning and sanitation
Cleaning and sanitation of raw food material could be considered the first step in controlling any
contamination of foreign materials or micro-organisms during food processing. Cleaning
removes foreign materials (i.e. soil, dirt, animal contaminants) and prevents the accumulation of
biological residues that may support the growth of harmful microbes, leading to disease and/or
the production of toxins. Sanitization is the use of any chemical or other effective method to
reduce the initial bacterial load on the surface of raw materials or food processing equipment.
Engineering properties of food, biological and packaging material
Knowledge of various engineering (physical, thermal, and thermodynamic) properties of food,
biological, and packaging material is critical for successful product development, quality control,
and optimization of food processing operations. For example, Knowledge of thermal properties
of food (thermal conductivity, specific heat, thermal diffusivity) is useful in identifying the
extent of process uniformity during thermal processes such as pasteurization and sterilization.
Thus, food scientists and process engineers need to adequately characterize or gather information
about relevant thermophysical properties of food materials being processed.
Microbiological considerations
Most raw food materials naturally contain micro-organisms, which bring both desirable and
undesirable effects to processed food. Many fermented foods such as yoghurt (nunu) and ripened
cheese have extended shelf-life, developed aroma, and flavor characteristics over those of the
raw materials arising from microorganisms such as Lactobacillus, Lactococcus, and
Staphylococcus bacteria. On the other hand, raw food material also contains pathogens and
spoilage organisms. Different foods harbor different pathogens and spoilage organisms. Different
pathogenic and spoilage microorganisms offer varied degrees of resistance to thermal treatment.
Accordingly, the design of an adequate process to produce safer products depends in part on the
resistance of such microorganisms to lethal agents, food material, and desired shelf life.
Role of acidity and water activity in food safety and quality
Intrinsic food properties (e.g. water activity, acidity, redox potential) can play a role in
determining the extent of food processing operations needed to ensure food safety and minimize
quality abuse. Higher acidity levels (pH <4.6) are often detrimental to the survival of
microorganisms, so milder treatments are sufficient to preserve an acidic food. Low-acid foods
(pH ≥4.6) support the growth and toxin production of various pathogenic microorganisms,
including Clostridium botulinum. Products such as milk, meat, vegetables, and soups are
examples of low-acid foods and require more severe heat treatment than acid foods such as
orange juice or tomato products. Knowledge of availability of water for microbial, enzymatic or
chemical activity helps predict the stability and shelf life of processed foods. This is reported as
water activity (aw), and is defined as the ratio between partial pressure of water vapor (pw) of
the food and the vapor pressure of saturated water (pw’) at the same temperature. The water
activity concept is used in food processing to predict growth of bacteria, yeast, and molds.
Bacteria grow mostly between aw values of 0.9 and 1, most enzymes and fungi have a lower aw
limit of 0.8, and for most yeasts 0.6 is the lower limit. Thus, food can be made safe by lowering
the water activity to a point that will not allow the growth of dangerous pathogens.
Goals of food processing
The goals of modern food processing can be summarized as follows:
Formulations. A logical basic sequence of steps to produce acceptable and quality food product
from raw materials.
Easy production procedures. Develop methods that can facilitate the various steps of
production.
Time economy. A cohesive plan that combines the science of production and manual labor to
reduce the time needed to produce the product.
Consistency. Application of modern science and technology to assure the consistency of each
batch of products.
Product and worker safety. The government and the manufacturers work closely to make sure
that the product is wholesome for public consumption, and the workers work in a safe
environment.
Buyer friendliness. Assuming the buyer likes the product, the manufacturer must do everything
humanly possible to ensure that the product is user friendly (size, cooking instructions, keeping
quality, convenience, etc.).
FOOD DETERIORATION AND SPOILAGE
Food deterioration refers to adverse changes in food quality or a reduction in food quality. Food
deterioration may be manifested physically in the form of changes in colour, texture, taste and
smell. For example, a bunch of plantain at its optimal quality will be firm to the touch and yellow
in colour. When deterioration sets in, it may feel very soft and soggy and turn black in colour.
Deterioration can also be manifested chemically in the form of changes in composition leading to
changes or loss/reduction in nutrient content. For example, beans infested with weevils will have
fewer nutrients compared with uninfected beans and over ripening in fruits can cause sugars to
change to starch. Although food deterioration causes food quality to drop, it does not render the
food unfit for consumption. Ultimately, food deterioration causes food spoilage. Food spoilage
refers to deterioration in food quality, making it unfit or unsafe for consumption. Food spoilage
is a natural process that cannot be stopped but can only be delayed. Once food has been
harvested, gathered or slaughtered it begins to deteriorate until eventually it becomes unfit for
consumption
It is safe to say that food spoilage is the ‘end point’ of food deterioration. On the basis of ease of
spoilage, foods can be placed in three groups:
1. Stable or imperishable foods: these are foods that do not spoil unless handled carelessly
e.g sugar, flour, dry beans etc
2. Semi-perishable foods: if these foods are properly handled and stored they will remain
unspoiled for fairly long periods e.g sweet potatoes, yams, some fruits etc
3. Perishable foods: this group includes meat, fish, milk and most fruits and vegetables.
4. They readily get spoilt unless special preservative methods are used on them. A common
feature of this group of foods is their high moisture content.
Spoilage affects different foods in different ways. Protein foods (beans, fish, meat and egg) go
putrid and produce very bad smell. Fats and oils go rancid and begin to smell and taste bad.
Carbohydrate foods go stale,become slimy and smell and taste unpleasant. Fruits and vegetables
ferment, decay and become rotten.
Factors responsible for food deterioration and spoilage are:
1. Biological: this includes activities of micro and macro organisms. The microorganisms
include bacteria, yeast and mould while the macro organisms include insects, rodents,
birds and other pests.
2. Chemical: this includes atmospheric oxygen and chemical reactions involving the
constituents of the food. This also includes the activities of enzymes present in the food.
3. Physical: this includes drying out, mechanical injuries, bruises and wounds that may be
sustained by the food material during harvesting, transportation, storage and processing.
4. Other factors such as rain, light, heat and cold also cause food deterioration and spoilage.
Food deterioration and spoilage can be prevented or minimized by the proper application of the
principles of food preservation.
PRINCIPLES OF FOOD PRESERVATION
Introduction
The food industry utilizes a variety of technologies such as thermal processing, dehydration,
refrigeration, and freezing to preserve food materials. The goals of these food preservation
methods include eliminating harmful pathogens present in the food and minimizing or
eliminating spoilage microorganisms and enzymes for shelf life extension. Common food
preservation/processing technologies includes addition of heat, removal of heat, removal of
moisture, and packaging of foods to maintain the desirable aspects established through
processing.
Food preservation is the act of extending the shelf life of a food item. It is the process or
practice of treating food in order to slow down natural decay or spoilage. The basic principles of
food preservation are
1. Prevention or delay of autolysis: autolysis (self destruction) of food may be delayed or
prevented by the destruction or inhibition of the enzymes present in the food. This is done by
taking advantage of the fact that enzymes are very sensitive to changes in their environment as
they are only able to operate under specific environmental conditions such as temperature, pH
and moisture content. A change in any of these requirements will have adverse effect on the
enzymes.
2. Prevention or delay of microbial spoilage: microorganisms are responsible in the greatest part
for the losses of world food supply. They are extremely small living things. They are ubiquitous
(everywhere) and can break down the complex organic components of food into simpler ones
thereby causing them to deteriorate and become spoilt.
3. Prevention or inhibition of non-enzymatic chemical reactions
Methods of inhibiting enzymes’ activities (autolysis)
(i) Blanching or scalding: this is applied to solid food products to inactivate specific enzymes in
a food material. Raw food is immersed in hot water at temperatures of 88-1000C or exposed to
live steam for a short time (few seconds to few minutes). In addition to inactivating enzymes,
blanching also expel respiratory gases, soften food products, set natural colour of raw food
products, removes raw flavour from foods etc.
(ii) Low temperatures: at low temperatures (40C – 100C), the activities of enzymes are retarded.
Consequently, in a refrigerator, the ripening process of fruits is delayed. Freezing at -180C and
below inactivates and sometimes destroys enzymes. Hence, frozen foods (fish, poultry, etc) have
a much longer shelf life than unfrozen foods.
(iii) Change in pH: enzymes are sensitive to pH. Processors take advantage of this factor by
lowering the pH of foods thereby preserving them. pH can be lowered through fermentation by
lactic acid bacteria. The accumulation of acid as a result of the fermentation process lowers the
pH of the food thereby inhibiting the enzymes.
(iv) Removal or lowering of moisture content: enzymes require moisture for their biochemical
activities. Absence of water means they cannot carry out their activities. The moisture may be
removed by dehydration or bound with the help of sugar, salt, sorbitol or glycerol.
Methods of preventing or delaying microbial activity
(i) Use of protective covering: nature endowed food materials with some protective cover to
ensure freedom from microorganisms in the form of peel, husk, shell etc as is the case with
different food products. If the cover is compromised, microorganisms will invade the food
material and commence their spoilage activities. To prevent this, apart from the natural covering,
artificial packaging materials (bottles, cans, flexible pouches, cartons) are used in the food
industries to protect processed foods against microorganisms.
(ii) Removal of moisture: removal or binding of moisture plays an important role in preventing
or delaying microbial activities because they require moisture for their growth and activities. If
the moisture is less than they require, their growth and activities will be inhibited and the shelf
life of the food will be extended. This is the reason why foods with low moisture content such as
powdered milk, grains and flours keep longer than high moisture foods such as fresh milk,
vegetables, fish etc.
(iii) Use of chemical preservatives: the use of preservatives such as pepper, ginger, benzoic acid,
sulphur (iv) oxide (SO2) etc inhibit the growth and activities of microorganisms thereby
preserving the food material. These compounds have anti microbial properties and are able to
inhibit their activities. The addition of these compounds to foods results in extension of shelf
lives. Benzoic acid is used in preserving soft drinks, beer and margarine.
(iv) Low temperature: use of low, chilling and freezing temperatures inhibits the growth and
activities of microorganisms. The activities of microorganisms are inhibited at chilling
temperatures and completely stopped at freezing temperatures. Some microbes are killed at
freezing temperatures. This is why refrigerated and frozen foods keep longer than unrefrigerated
and unfrozen foods
(v) High temperature
(a) Blanching: apart from inactivating enzymes (as discussed earlier) blanching also destroys the
vegetative forms of microorganisms.
(b) Pasteurization: this is the use of high temperatures (below 1000C) to destroy pathogenic
organisms including bacteria and yeast in some liquid foods. Pasteurization is employed in the
processing of various foods such as milk, fruit juices, beer etc.
(c) Sterilization: this employs the use of higher temperatures (above 1000C) to destroy all
microbial life forms (both pathogenic and spoilage organisms). In food processing, it is neither
possible nor practicable to destroy all forms of microbes. Hence, commercial sterilization or
processing is employed. Commercial sterilization renders food free of viable forms of microbes
of public health significance as well as any microbe of non-health significance capable of
reproducing in the food. In order words, commercially sterilized foods contain microorganism
but they are unable to multiply and cause any harm i.e they are not viable.
(vi) Use of ionization irradiation: ionization radiation is used to keep foods fresh and longer. This
radiation destroys disease causing microorganisms with radiation energy instead of heat energy.
The UV rays have germicidal properties and are able to kill off the microbes. Radiation from UV
source, when absorbed by the purines and pyrimidines of the nucleic acid in microorganisms
causes mutation or death. Irradiation can only be used on a limited range of and its safety is still
under investigation. It is also expensive.
(vii) Manipulation of the environment’s pH and oxygen content (modified atmosphere) can also
be used to control microbial growth and activities. The pH of food scan be manipulated by
increasing the acid content either by fermentation or direct addition of organic acid as a
preservative. Fermentation of milk by lactic acid bacteria to produce yoghurt is a typical
example. The lactic acid bacteria convert the milk carbohydrate (lactose) to CO2 and lactic acid.
These two products render the environment acidic making it non-conducive for a lot of microbes
thereby preserving the food. The carbonation of alcoholic and non-alcoholic beverages
eliminates or reduces the oxygen present in the food environment thereby making it
uninhabitable for aerobic organisms.
(viii) Use of pulsed electric field (PEF): this is a non-thermal method of food preservation. It
uses short bursts of electricity to inactivate microbes. High energy pulsed electric field break
open vegetative cell walls in a process called electroporation. PEF can be used in processing
liquid and semi-liquid foods. It can also be used in treating waste water.
It should be noted that many of the methods discussed above can also cause the food to
deteriorate. It is therefore a matter of balance. The amount of heat that will destroy microbes may
also be harmful to some nutrients and the dosage of UV rays that will destroy the microbes may
also have adverse effects on the food material. It is therefore the responsibility of the food
processor to choose the method and dosage that has the maximum effect in destroying the
microbes but has minimal adverse effect on the quality of the food.
Methods of preventing or inhibiting non-enzymatic spoilage reactions
(i) Use of appropriate packaging materials: the use of dark coloured plastic and bottles in the
packaging of beverages and oil foods inhibits photo catalysed deteriorative reactions. This is so
because the darkly coloured containers prevent or reduces the penetration of light rays into the
food material thereby preventing or reducing the incidence of deteriorative reactions catalysed by
light. Furthermore, the use of high density polyethylene and other appropriate packaging
materials with low oxygen permeability also inhibit deteriorative oxygen-catalysed reactions.
(ii) Use of chemicals: chemicals such as butylated hydroxyl toluene (BHT) and butylated
hydroxyl anisole (BHA) are used to inhibit oxidation of oil foods thereby preventing or delaying
the onset of rancidity.
Other means of preventing food deterioration and spoilage includes
(iii) Harvesting at the right time
(iv) Harvesting under favourable conditions
(iii) Careful handling during harvesting, packing, transportation and storage
(iv) Safeguarding against pests, insects, rodents and birds
(v) Fermentation
Fermentation causes desirable biochemical changes in foods in terms of nutrition or digestion, or
makes them afer or tastier through microbial or enzyme manipulations. Examples of fermented
foods are cheese, yogurt,most alcoholic beverages, salami, beer, and pickles. Representative
vegetative bacteria in the fermentationsare Lactobacillus, Lactococcus, Bacillus, Streptococcus,
and Pseudomonasspp. Yeast and fungi (e.g. Saccharomyces, Endomycopsis, and Monascus) are
also used for fermentation. Food supports controlled growth of these microorganisms, which
modify food properties (texture, flavor, taste, color, etc.) via enzyme secretion.
ROLES OF FOOD TECHNOLOGISTS IN THE SOCIETY
Food technology on the other hand is the application of food science to the selection,
preservation, processing, packaging, distribution, and use of safe food.
The roles of food technologist in the society cannot be over emphasized and it is but not limited
to the following:
1. Document available raw materials, good agricultural practices, good hygienic practices
and measures to reduce post-harvest losses and food wastes.
2. Promote adaptation and improvement of traditional foods and processes, while respecting
the traditional, ethical, cultural and religious aspects involved and supported by total diet
studies related to these parameters
3. Develop sustainable and flexible food processing technologies that can be scaled up as
needed.
4. Develop food packaging materials and processes that will extend shelf life, reduce waste,
and are sustainable and economical
5. Improve food systems and food and nutrition security through capacity building and
dissemination of knowledge. Food Technologist help in preventing food insecurity by
developing a good food processing and preservation methods as inadequate method of
processing and preservation of food is one of the major challenges or causes of food
insecurity.