Spoilage And Associated Chemical Physical Changes In Food

We all know that food is the basic necessity of all the living entities. Needless
to say that such a commodity has to be absolutely safe and of highest possible
quality especially free from toxins and spoilage. A food is said to be spoiled if
it has been damaged or injured making it unsuitable for human use. “A product
is fit as a food if a discriminating consumer, knowing the story of its
production and seeing the material itself, will eat it, and conversely, the
same product is spoiled when such an examiner refuses it as a food”.

All of
us would agree that a food is spoiled if it is not harvested at proper maturity, is
contaminated with dirt, handled by dirty or diseased person, is fertilized with
sewage and has objectionable changes due to the activity of microorganisms or
action of enzymes of the food. The major causes of spoilage are: the
microorganisms or their enzymes, the native enzymes of food, rodents,
environmental factors and purely chemical reactions.

It must be admitted that despite of the improvement in the methods of
production, handling and processing, the microbiological quality still remains
the most important factor. This aspect assumes significance from toxin
production, spoilage of fresh and processed products and quality control and as
sanitation indicators in a processing unit. Microbial quality is also on the top of
the different hazards which are associated with the safety of food for
consumption by human beings.

Various fruits, vegetables and their products
may be spoiled by one or more factors like unsuitable packaging, chemical
changes or action of microorganisms, tissue enzymes, insects, rodents or
improper methods of processing, under processing, etc. Different spoilage
causing agents for various fruit and vegetable products, their prevention and
health hazards associated with spoilage are also discussed here

PRINCIPLES OF FOOD PRESERVATION

In accomplishing the preservation of food by various methods, three main
principles are involved:

I. Delay or prevention of microbial decomposition of food

II. Delay or prevention of self-decomposition of food

III. Prevention of damage caused by insects, rodents, birds, mechanical causes
etc.

Various principles and sub-principles of preservation of food are summarized
in Table 4.1. First principle of food preservation is based mainly on the
following considerations:

• By delaying the microbial decomposition of food

• By preventing the microbial decomposition of food

Most of the methods of food preservation depend not only on the
destruction or removal of microorganisms but also on the delay in the
initiation of their growth, and hindrance to growth once it has begun.
Knowledge of growth curve of microorganisms is very helpful for developing
the appropriate technique to delay the microbial decomposition of the food.

Detailed principles of preservation

I. Delay or prevention of microbial decomposition of food

i) by keeping out microorganisms (asepsis).

ii) by removal of microorganism.

iii) by hindering the growth and activity of microorganisms.

iv) by killing the microorganisms.

II. Delay or prevention of self decomposition of food

i) by inactivation of food enzymes.

ii) by delay or prevention of purely chemical reactions.

III. Prevention of damage to food caused by insects, rodents, birds and
mechanical causes

CLASSIFICATION OF FOODS BASED ON
PERISHABILITY

1. Perishable foods: The foods which spoil readily unless special preservation
methods are used such as meat, fish, most of fruits and vegetables, egg,
poultry and milk etc.

2. Semi-perishable foods: Semi-perishable foods like waxed potatoes and
some varieties of apple, if handled and stored properly, shall remain
unspoiled for a fairly long period.

3. Non-perishable foods: These foods do not spoil until and unless they are
handled carelessly. Such foods are also called as stable foods such as
cereals, sugar

FACTORS GOVERING SPOILAGE

Since microorganisms are the main spoilage causing factors, major
emphasis remains on the factors related to microbial spoilage of the
foods. Here bacteria cause most of the problems since these are not killed at
ordinary temperatures. The yeasts and molds have low resistance to heat
(processing temperatures). The main factors responsible for such spoilage are
described here.

Number and Kind of Microorganisms

The initial number of microorganisms present on the food has direct
relationship with its spoilage. More the number of microorganisms present,
rapid is the spoilage. The effect of initial number of spores on time required to
kill them is shown in Table 4.2.

The heat resistance of bacteria involved in
food poisoning is of major concern from public health view point. Usually,
only one type of microorganisms will be there because of the particular
environmental conditions involved. However, contamination may increase the
number as well as new kinds of microorganims.

Effect of initial number of spores on time required to kill them

In addition to destruction or removal of microorganisms, the delay in
the initiation of growth also prevents microbial spoilage. This is done by
keeping the microorganisms in lag phase as long as possible. Once the
microorganisms enter the log phase, it is very difficult to control them.

Suitability of Temperature

Different microorganisms grow at different temperatures. Psychrophiles have
affinity for low temperatures (8-10o
C), mesophiles grow best at medium
temperatures (25-40o
C), while thermophiles appear at higher temperatures
(50-55o
C).

A large number of microorganisms grows at mesophilic temperature
therefore processed foods are immediately cooled to arrest microbial activity /
spoilage of foods. So, storage temperature is very important in relation to
microbial growth and hence, the spoilage behaviour of foods.
Different types of microorganisms require different times to kill their cells or
spores. The time required to kill all the spores of flat sour bacteria (Bacillus
stearothermophilus) in relation to temperature.

Effect of temperature of heating on the time needed to kill
spores of flat sour bacteria

Suitability of food

Different microorganisms prefer different kinds of foods. Some grow best on
proteinacious foods, others on starchy or fatty foods. The physical state of the
food whether heated, frozen, moistened or dried, also has an important influence on the spoilage it will undergo. The moisture content also influence
the type of microorganisms in the foods since the requirements of moisture for
their growth are different.

pH of Food

The pH of the food influences the kind and growth of microorganisms. The
composition of the vegetable, its pH and moisture contents affect their type of
spoilage. As a general rule, foods having pH<4.5 (acid foods) do not require
heat processing (particularly cooking under pressure), but those with pH > 4.5
(low acid foods) always require processing under pressure.It is because of the
reason that thermophilic bacteria may not be killed at normal temperatures as
most bacteria thrive best at pH of 4-7.5, while the yeasts and molds require a
pH of 2.5-8.0 and 1.5-8.5 for their growth.

Presence of Air

Aerobic or anaerobic microorganisms can be found in the spoiled fruit and
vegetable products, depending upon the presence or absence of air (oxygen) in
the container or package.

CHEMICAL AND PHYSICAL CHANGES
ASSOCIATED WITH FOOD SPOILAGE

Changes in nitrogeneous organic compounds: Most of the nitrogen in the food
present in the form of proteins which are hydrolysed by enzymes to produce
amino acids. The anaerobic decomposition of the protein, peptide or amino acid results in the production of obnoxious odour which is called as
putrefaction.

When the microorganisms act on amino acid they deaminate or
decarboxylate.
E. coli produces glyoxylic acid, acetic acid and ammonia from glycine and
from serine it produces pyruvic acid and ammonia.

Alanine is degraded into αketo acid, ammonia and carbon dioxide by E. coli; acetic acid, ammonia, and
CO2 by Psuedomonas; and propionic acid, acetic acid, ammonia and CO2 by.
Clostridium nigrificans. Other nitrogenous compounds like amide, urea,
guianidine and creatine, etc. are also decomposed to ammonia, carbon dioxide
and other products.

Changes in Carbohydrates: Carbohydrates are preferred by the microorganisms
as energy yielding foods. They hydrolyse the polysaccharides to
monosaccharides before utilization such as to glucose which is then, oxidized
to CO2 and H2O. Anaerobically, these undergo decomposition involving one or
more types of fermentation.

• Alcoholic fermentation by yeast with ethanol and CO2 as products.

• Lactic fermentation by homofermentative lactic acid bacteria with lactic
acid or by heterofermentative lactic acid bacteria with lactic acid, acetic
acid, ethanol, glycerol and CO2 as chief products.

• Coliform type of fermentation by coliform bacteria with lactic acid, formic
acid, ethanol, CO2, hydrogen and perhaps aceton and butanediol as likely
products.

• Propionic acid fermentation by propionoic bacteria producing propionic
acid, acetic and succinic acid and CO2.

• Butyric- butyric isopropyl fermentation by anaerobic bacteria producing
butyric acid, acetic acid, CO2, H2 and in some cases, butylenes glycol,
butanol and 2-propanol

They are present as salts and are oxidized by the microorganisms to carbonate
and cause the food medium to become alkaline. Organic acid aerobically are
oxidized to carbon dioxide and water as is done by the film yeast.
Changes in other compounds: Other compounds also undergo changes as
detailed here:

• Ethyl alcohol is oxidized to acetic acid.

• Glycoside is hydrolysed to sugars.

• Acetaldehyde is oxidized to acetic acid or reduced to ethanol.

• Protopectin are acted upon by pectinesterase – pectic acid + methanol
(water soluble) by hydrolysis of methyl ester. Polygalacturonases destroys
the linkage between galactouronic acid unit of pectin or pectic acid to yield
smaller chain and ultimately, free D-galacturonic acid, which may be
degraded to simple sugar.

Changes in Lipids: Fats present in the media are hydrolysed by lipase into
glycerol and fatty acid.Phospholipids may be degraded to their constituents
phosphate, glycerol, fatty acid, and nitrogenous base e.g. chlorine.

One or more such changes can be produced in the food undergoing spoilage.
The physical and sensory qualities of the food also undergo changes, thus
making the product unfit for human consumption.