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newsletters, December 2002
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The Effects of Processing on Food
The object of this newsletters is to take a broad look at the need
for food processing and its effect on food. Understanding the history
of processing food can be important to us, as consumers, to help us see
where we go from here.
Why is food processed?
The term "food processing" encompasses many types of food preservation
in the form of drying, smoking, pickling, salting, fermenting, canning,
freezing and vacuum packing. The objective of preserving food is to stop
the biological deterioration of foodstuffs, from either bacterial or parasitic
contamination, or from particular enzymes in some foods that can bring
about unwanted changes in the appearance, texture, smell, taste or nutritional
quality of the food.
Food processing also involves refining, filtering, cooking, chopping
and any other fate that may occur to it.
The processing and preserving of food has been happening for centuries.
Historically, most foods have been grown according to the seasons in each
part of the world. Man has discovered methods of preserving food to enable
his survival during fallow times. Meat has been hung, dried or smoked.
Grains can be dried, fermented or ground into flours for baking. To help
keep milk during the months when the cows no longer lactate, milk has
been made into butter, cheese or fermented into yoghurt or kefir. Vegetables
have been dried, pickled in vinegar, or salted to preserve them for use
during 'out of season' months.
In very hot countries for example, it is realised that meat should be
kept fresh until the cook requires it; consequently the local markets
sell live animals and poultry. This makes good sense especially if there
is no electricity available or only a limited supply for running a refrigeration
unit. Whilst we may think it cruel, it is in fact good food-hygiene practice
to buy a live beast or bird, then take it home to butcher and cook. In
Britain we are fortunate - we have access to frozen food, ready-made meals
and a (virtually) constant supply of power to ensure our food remains
safe. What happens to it after we have bought it is up to us: food-poisoning
is on the increase largely brought about by incorrect storage and incorrect
cooking - at home! How many shoppers buy frozen/chilled food and leave
it in the car while they go for a coffee and a chat, then 'just stop off'
at the DIY store, then eventually get home and put the frozen/chilled
food away? Other common occurrences are:
- not having the oven hot enough when the food goes in, giving bacteria
a chance to reproduce;
- not cooking/heating food long enough to ensure that it has reached
the correct temperature in the centre of the dish. All the research
into food safety and preserving food so that it is safe to eat amounts
to nothing if the consumer is lacking in care.
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Other reasons
for preserving food include:
- The transportation of food across continents
- Export or expedition
- The need for women to work increased the need for food to 'keep'
- The increasing size of towns to cities due to the growth of the industrial
revolution. Without the use of food processing methods it would be very
difficult for most industrialised, developed countries to maintain the
size and nutritional status of their populations.
As distances that food travels become greater, different preservation
methods are employed, however, virtually all have some negative effect
on the vitamin, mineral and carotenoid value of the food.
All the time that food was grown close to communities, people ate fresh
produce or at least local produce that had been kept and preserved locally.
Today, we consume produce grown in different countries all over the world
by people who, ironically, often do not have enough to eat.
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The use of
additives in food (2)
The use of salt, alcohol and vinegar in domestic kitchens
for preserving food has been known for several centuries. Their use today
continues in industry. The processes themselves rely on reducing the activity
of water or to change the pH to inhibit the growth of detrimental organisms
(bacteria, fungi, microbes etc). Additives in food are defined as substances
added to foods for technological purposes, not for adding nutritional
value.
Some common food additives include:
- Antioxidants - help prevent oxidation of cells in food. If we consume
oxidised food it may be harmful in large quantities and is not as nutritious
- Flavours - added to 'replace' flavour that may have been lost during
processing
- Preservatives - used to help reduce bacterial growth in food, for
example - Sodium nitrite inhibits the growth of Clostridium botulinum
found in meat
- Colours - added to 'enhance' the look of food after it has been processed
in some way, for example - homemade strawberry jam is dull red, whereas
supermarket jam is bright red
- Emulsifiers and stabilizers - help retain the texture of processed
food
- Flour improvers - used to improve elasticity of the dough and the
stability of the structure of the loaf
- Sweetening agents - sugar has been used historically, but it is a
bulky substance and does not supply the intensity of 'sweetness' that
the palate today expects
Freezing and refrigerating food (3)
All processes of life and chemical reactions take place
more slowly at low temperatures. The Romans were aware of the preserving
effect of cold, and used to chill perishable foods by packing them in
snow brought from the Alps. Straw was used to insulate the snow to prevent
it melting during the journey.
The use of icehouses was developed in the Middle East,
then the idea travelled into Renaissance Europe and later to North America.
Years ago, ships would carry blocks of ice across the
sea to buyers in ports to sell to hotels or the private market. Ice was
a product for the affluent!
More recently, in the 19th century early attempts to
produce a refrigerator were tried in Europe, but proved too bulky for
use in the home. However, in 1918, the first practical model of the refrigerator
was manufactured in the USA. This revolutionised food storage and food
safety for those who could afford it.
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Effects of
processing on food
Fats are affected when heated and by hydrogenation.
As fats get hot, they progressively darken, the products of the oxidation
accumulate and the fat in this state becomes 'rancid' with an unpleasant
flavour. Heating polyunsaturated fatty acids is not recommended as concern
has been expressed at the toxic effects of potentially harmful chemicals
produced through high temperature frying (4). The process of hydrogenation
causes the naturally occurring cis form of fats to form trans fats. Trans
fats block the receptor sites for the beneficial essential polyunsaturated
fatty acids in the digestive tract.
Carbohydrates are affected by heat and also by refining.
Heating sugars results in browning and caramel forms. The heating process
on starchy foods makes starches as a whole more digestible, but cooling
of the food renders part of the starch resistant to digestion in the small
intestine, although it is degraded by micro-flora in the large intestine
(2).
Protein is affected by heat. If carbohydrate is present
with the protein (which it usually is) then the browning of the carbohydrate
can reduce the bioavailability of the protein. Baking and toasting of
cereal products and the resulting browning process contributes to lysine
loss by about 10 - 15%.
Minerals are largely unaffected by heat processes.
Losses generally occur through cell-wall breakdown and minerals leaching
into the cooking medium. Any major effects are likely to be on the organic
components that may impair the availability of Iron, Calcium and Zinc
for example (2).
Vitamins can suffer substantial losses during the processing
of food. For example, water-soluble B and C are easily lost through preparation
and cooking techniques using water. Vitamin C is easily oxidised and destroyed;
and Thiamine decomposes if heated in alkaline conditions. Air, heat, light,
cooking, acid and alkaline all have an effect on vitamin status of food
(2).
Irradiating food has a minor effect - nutritionally
speaking it is no worse than conventional 'cooking', however, changes
occur in fats that affect the structure and consequently the taste, so
irradiation of fats is very limited (2).
Heat processing affects the chemical structure of food
components. Cooking processes are normally scaled-up versions of those
in the domestic kitchen but with a much closer control of heating and
other operations. The nutrition value of proteins and fats are much more
affected than that of carbohydrates. Vitamin losses can be great, depending
on the methods employed. Of all the processing methods used, deep-freezing
has the least damaging effect on vitamins and minerals.
The majority of effects of processing on food emphasises
the importance of underpinning the diet with a good quality multinutrient
food supplement.
Minerals Lost in Refining Bread Flour (1)
Calcium 60%, Phosphorus 70.9%, Magnesium 84.7%, Potassium 77%, Sodium
78.3%, Chromium 98%, Manganese 85.8%, Iron 75.6%, Cobalt 88.5%, Copper
67.9%, Zinc 77.7%, Selenium 15.9%, Molybdenum 48%
Minerals added back into white or brown flour are
Calcium carbonate 260mg/100g and Metallic Iron 1.65mg/100g
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Some of the
benefits of food processing include:
- Development of heat processes for drying, pasteurisation and canning
of food proves helpful in reducing the deterioration of food by bacteria
and the risk of Food Poisoning.
- Refrigeration was developed as a method of controlling food spoilage,
especially meat.
- Deep-freezing developed as a method of retaining the quality of perishable
foods such as meat, fish and vegetables.
- Fermentation has some benefits in that food may become easier to
digest.
- Food has been preserved in some way for centuries, to extend the
availability throughout the year. Processing methods generally 'improve'
the acceptability and nutritional value of food consumed on an annual
basis.
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References
- Ministry of Agriculture Fisheries & Food, 1989.
- "Human Nutrition and Dietetics", James, Ralph & Garrow et al.
Churchill Livingstone 2000.
- "Oxford Dictionary of Food". Alan Davidson. Oxford University
Press 1999.
- Grootveld et al 1998.
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