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Lecture-5
CAUSES OF FOOD SPOILAGE
Ever wonder why your bread molds, why your milk spoils, or why some meats may
make you sick? Well, the culprit behind this is food spoilage, which can be caused by
microbes (which nclude yeasts and moulds), fungus, or bacteria. Essentially, food
spoilage indicates that the riginal nutritional value, color, odor, texture, and/or flavor of
the food are damaged and that the food ecomes unacceptable to eat. Microbial
spoilage of food often involves the degradation of protein, carbohydrates, and fats by the
microorganisms or their enzymes. Food spoilage can result from a ariety of factors. It
affects meat, dairy and even fruits and vegetables.
Meat Spoilage
First, lets specifically explore the causes of meat spoilage. Not cooking food
thoroughly plays a large part in much of the food poisoning due to meat consumption.
There are typically thousands of bacteria that reside in and on meat. Cooking the
meat to extremely high temperatures can effectively kill these pathogenic bacteria.
However, when meat is left raw the bacteria that live ithin it simply exponentially
multiply. Furthermore, once the meat has been properly prepared, it must be
maintained in a bacteria-unfriendly environment. Here, refrigeration is key in keeping
such environment as it suspends bacterial growth. For instance, when meat is left out on
a counter for hours bacteria in the air can settle on it and grow considerably.
Contamination of food happens more often than people realize. Here are the most
heard of ways that contamination occurs. Often, people mix cooked food with raw
meat or poultry. This is a rather big error as bacteria from the raw food can easily
contaminate the cooked food. With that, utensils used for raw food should not be
brought near cooked food. Once the food is cooked, it should not be placed on
unclean, chipped, or cracked food surfaces, for bacteria are abundant in crevices.
Spoiled dairy products pose a great threat to humans. Milk and eggs are somehow
incorporated into virtually every food entree. However, if improperly prepared,
spoiled milk and raw eggs are associated with a number of pathogens, Escherichia
coli, Salmonella, and Staphylococcus aureus, to name a few. All milk should be
correctly pasteurized and subsequently refrigerated. Pasteurization is effective in
killing pathogenic bacteria, but sometimes does not eradicate all spoilage bacteria. To
prevent precarious growth, refrigeration is also necessary.
Fruit and Vegetable Spoilage
Microorganisms can also attack fruits and vegetables, making them susceptible to rotting.
Contamination occurs when those that are picking and handling the fruits and vegetables
use improper hygiene. In addition, pathogenic organisms may contaminate the animal
manure used for fertilizing the soil the vegetable is grown in. The microorganisms could
also be introduced via irrigation ducts. These are only a few of the causes of fruit and
vegetable spoilage.
Storage rot in strawberry caused by Botrytis cinerea.
Blue mould rot in tomato caused by Penicillium spp. (also by Fusarium spp.)
Watery soft rot in apple caused by Sclerotinia sclerotiorum
Bread microbiology
Bread is a simple fermentation of sugar to C0 2 and alcohol. The baker first
combines flour, sugar, milk and other ingredients with a microorganism, usually a
bread yeast such as Saccharomyces cerevisiae, but not always. The ingredients
are mixed and then allowed to incubate at 27C for a few hours. During this time the
yeast convert the sugar present to ethanol and CO 2. Most incubations are for less
than 4 hours not leaving enough time for the yeast to increase in number. The C0 2
produced causes the bread to rise (leaven) and become porous. The success of
leavening is dependent upon the rate of gas production. This can be increased by
adding more yeast, more sugar, or dough conditioners (various salts that the yeast
need).
Tweaking a recipe by manipulating these factors can speed C0 2
production, within reasonable limits. Adding too much of anything can
either kill the yeast or cause the bread to rise too quickly. The
temperature of incubation is another critical consideration.
Saccharomyces grows best at 26 to 28C and deviations from that
temperature will usually result in slow or complete lack of leavening.
Failure as a baker can normally be attributed to either not adding the
exact amounts of ingredients or inappropriate incubation temperatures
during leavening.
Bread mold grows
Like all living things, molds (or fungi) need certain things in order to grow.
Among these are food, water, and proper temperature. Molds get their food by
extra cellular (outside the body) digestion, which is another interesting story
which we won't go into today.
Just like humans, molds are mostly water. Water is used in some processes,
but all of the biochemical reactions necessary for life must take place in a
watery solution (the cytoplasm) of the cell. (More on these reactions later.)
The water environment of the cell allows all of the components to move and
mix properly. It also prevents the mold from drying out, since water is critical
for normal survival.
As for temperature, molds are different from us. We control the temperature
of our body so that all of the reactions have the proper conditions to go
forward. We are called "warm blooded". Molds cannot control their
temperature, so they must grow and develop at whatever temperature their
environment is. In the watery solution inside mold, cells are proteins called
"enzymes" which direct the biochemical processes of the cells. These enzymes
work faster when the temperature is higher (as long as it does not get too
hot!) If you can do a growth experiment at several temperatures, you can
probably find the temperature at which these enzymes work best for molds.
For humans, it is 98.6 degrees Fahrenheit, which is our body temperature.
(When biochemists take these enzymes out of the body, they still work best at
98.6 degrees.) For most molds, the optimal temperature is around 80 degrees.
Molds do not need light for normal growth, because they do not make their
own food like plants do. Molds are eating (using extra cellular digestion) food
that was at some point produced by plants. At some point in their life, most
molds do need light to start a different life stage, such as spore production.
The main reason for growing molds in the dark is that they might get dried out
by sun or other bright lights