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1. What is the importance
of fat in the diet?
Fats and oils are recognized as
essential nutrients in both human and animal diets. They provide
the most concentrated source of energy of any foodstuff, supply
essential fatty acids (which are precursors for important
hormones, the prostaglandins), contribute greatly to the feeling
of satiety after eating, are carriers for fat soluble vitamins,
and serve to make foods more palatable. Fats and oils are
present in varying amounts in many foods. The principal sources
of fat in the  diet
are meats, dairy products, poultry, fish, nuts, and vegetable
fats and oils.
2. What are "essential" fatty
acids and why are they important?
Experimental work in the 1930’s in
animals and humans demonstrated that certain long chain polyunsaturated
fatty acids, linoleic and arachidonic, are essential for growth
and good skin and hair quality. Now linoleic and linolenic
acids are termed "essential" because they cannot
be synthesized by the body and must be supplied in the diet.
The requirement for these essential fatty acids has been demonstrated
clearly in infants. While the minimum requirement has not
been determined for adults, there is no doubt that they are
essential nutrients. For the general population 3% of calories
as linoleic acid is considered to be a satisfactory minimum
intake. In the case of linolenic acid, the requirement for
humans has been estimated to be 0.5% of calories.
3. How much fat is in the diet?
Fats in the diet are often referred to as "visible"
or "invisible." Visible fats are those added to
the diet in foods such as salad dressings, spreads and processed
foods, whereas invisible fats are those that are naturally
occurring in foods such as meats and dairy products.
The guidelines given by USDA for fat call for a total fat
intake of no more than 30% of calories with a saturated fatty
acid intake of no more than 10% of calories.
4. What are trans fats? What are their
health effects?
Hydrogenation is the process of chemically adding hydrogen
gas to a liquid fat in the presence of a catalyst. This process
converts some of the double bonds of unsaturated fatty acids
in the fat molecules to single bonds, thereby increasing the
degree of saturation of the fat. The degree of hydrogenation,
that is, the total number of double bonds which are converted,
determines the physical and chemical properties of the hydrogenated
oil or fat. An oil that has been "partially" hydrogenated
often retains a significant degree of unsaturation (i.e.,
double bonds) in its fatty acids. Hydrogenation also results
in the conversion of some cis double bonds to the trans configuration
and in the formation of cis or trans positional isomers in
which one or more double bonds has migrated to a new position
in the fatty acid chain. The levels and types of these trans
fatty acids formed depend on the type of oil and conditions
(e.g., temperature, pressure, catalyst, and duration) of the
hydrogenation processing.
Small amounts of trans fatty acids occur naturally in foods
such as milk, butter, cheese, beef, and tallow as a result
of biohydrogenation in ruminants.
The hydrogenation process is very important to the food industry
to achieve desired stability and physical properties in such
food products as margarines, shortenings, frying fats, and
specialty fats. Examples of enhanced stability provided by
hydrogenation include increased shelf life of commercial snack
foods and prolonged frying stability of food service deep
frying fats.
Prior to 1990, there were numerous reviews and studies on
the nutritional and biological effects of trans fatty acids.
Most of these studies focus on the development of atherosclerosis
and on the effects of trans fatty acids on serum cholesterol
levels. Generally these studies indicated that trans fatty
acids were not uniquely atherogenic nor did they raise total
cholesterol compared to cis fatty acids. However, these findings
were challenged in 1990 by a Dutch study (27) which indicated
that a diet high in trans fatty acids (11.0% energy) raised
total and LDL cholesterol and lowered HDL cholesterol in human
subjects compared to a high oleic acid diet. A follow-up study
by these investigators using a somewhat lower level of dietary
trans fatty acids (7.7% energy) reported that dietary trans
acids raised total and LDL cholesterol and lowered HDL cholesterol
compared to a high linoleic acid diet but not compared to
a high stearic acid diet.
Recent epidemiological studies have reported that trans fatty
acids have a positive association with CHD risk (31). There
are a number of limitations of these studies including the
difficulty of measuring trans fatty acid intake through the
use of food frequency intake questionnaires, the lack of a
dose response relationship between trans fatty acid intake
and heart attack risk and the inconsistency of study results.
Above all it must be remembered that epidemiological studies
do not show cause and effect and are simply indicators of
where clinical studies may be needed.
In contrast to the amount of literature on trans fatty acids
in relation to coronary heart disease, relatively few investigators
have studied trans fatty acids with respect to cancer. Ip
and Marshall (35) published a comprehensive review of more
than 30 reports addressing this issue. With respect to breast
cancer, Ip and Marshall noted that epidemiologic evidence
shows only slight to negligible impact of fat intake in general
on breast cancer risk and no strong evidence that intake of
trans fatty acids is related to increased risk. In addition,
there is no evidence indicating that intake of trans fatty
acids is related to increased risk of either colon cancer
or prostate cancer. Overall, the available scientific evidence
does not support a relationship between trans fatty acids
and risk of cancer at any of the major cancer sites.
In summary, recent comprehensive reviews of the literature
indicate that trans fatty acids at their current level of
intake are a safe component of the diet. At relatively high
levels of intake, trans fatty acids appear to raise LDL and
lower HDL cholesterol. When substituted for unhydrogenated
oils high in unsaturated fatty acids, trans fats increase
total and LDL cholesterol. However, trans fats lower total
and LDL cholesterol when substituted for animal fats and vegetable
oils high in saturated fatty acids. Hydrogenated oils are
used mainly as a substitute for more highly saturated vegetable
oils and for animal fats containing both saturated fatty acids
and cholesterol.
5. What is hydrogenation and why is
it used by the food industry?
Hydrogenation is the process by which hydrogen is added directly
to points of unsaturation in the fatty acids. Hydrogenation
of fats has developed as a result of the need to (1) convert
liquid oils to the semi-solid form for greater utility in
certain food uses and (2) increase the oxidative and thermal
stability of the fat or oil.
Hydrogenation is an extremely important process as far as
our food supply is concerned, because this processing imparts
the desired stability and other properties to many edible
oil products. The level of unsaturated fatty acids present
in some oils such as soybean oil is reduced in order for the
oils to have functional properties in many food applications.
Hydrogenation is the only practical way to impart these properties.
In the process of hydrogenation, hydrogen gas is reacted with
oil at elevated temperature and pressure in the presence of
a catalyst. The catalyst most widely used is nickel supported
on an inert carrier, which is removed from the fat after the
hydrogenation processing is completed. Under these conditions,
the gaseous hydrogen reacts with the double bonds of the unsaturated
fatty acids.
The hydrogenation process is easily controlled and can be
stopped at any desired point. As hydrogenation progresses,
there is generally a gradual increase in the melting point
of the fat or oil. The partially hydrogenated oils are typically
used to produce institutional cooking oils, liquid shortenings
and liquid margarines. Further hydrogenation can produce soft
but solid appearing fats which still contain appreciable amounts
of unsaturated fatty acids and are used in solid shortenings
and margarines. When oils are more fully hydrogenated, many
of the carbon to carbon double bonds are converted to single
bonds increasing the level of saturation. If an oil is hydrogenated
completely, the carbon to carbon double bonds are eliminated
completely and the resulting product is a hard brittle solid
at room temperature.
The hydrogenation conditions can be varied by the manufacturer
to meet certain physical and chemical characteristics desired
in the finished product. This is achieved through selection
of the proper temperature, pressure, time, catalyst, and starting
oils. Both positional and geometric (trans) isomers are formed
to some extent during
hydrogenation, the amounts depending on the conditions employed.
6. Can peanut or soybean oils cause
allergies?
Food allergies are caused by the protein components of food.
Edible oils undergo extensive processing (sometimes referred
to as "fully refined") which removes virtually all
protein from the oil. Refined edible oils therefore do not
cause allergic reactions because they do not contain allergenic
protein. Food products containing refined edible oils as ingredients
are also non-allergenic unless the food products contain other
sources of protein.
Some edible oils may be extracted and processed by procedures
that do not remove all protein present. Mechanical or "cold
press" extraction is occasionally used which may not
remove all protein. These cold pressed oils are rarely used
domestically and are usually found only in health food or
gourmet food stores. Studies using cold pressed soybean oil
have shown it to be safe; however, insufficient testing has
been done to ensure that all cold pressed oils can be safely
consumed by sensitive individuals.
The vast preponderance of edible oils consumed are highly
refined and processed to the extent that allergenic proteins
are not present in detectable amounts. The majority of well-designed
and performed scientific studies indicate that refined oils
are safe for the
food-allergic population to consume (53).
7. What is a smoke point and how is
it affected in cooking oils?
The "smoke," "flash," and "fire points"
of a fatty material are standard measures of its thermal stability
when heated in contact with air. The smoke point is the temperature
at which smoke is first detected in a laboratory apparatus
protected from drafts and provided with special illumination.
The temperature at which the fat smokes freely is usually
somewhat higher. The flash point is the temperature at which
the volatile products are evolved at such a rate that they
are capable of being ignited but not capable of supporting
combustion. The fire point is the temperature at which the
volatile products will support continued combustion. For typical
fats with a free fatty acid content of about 0.05%, the smoke,
flash, and fire points are around 420º, 620º, and
670º F, respectively. The degree of unsaturation of an
oil has little, if any, effect on its smoke, flash, or fire
points. Oils containing fatty acids of low molecular weight
such as coconut oil, however, have lower smoke, flash, and
fire points than other animal or vegetable fats of comparable
free fatty acid content. Oils subjected to extended use will
have increased free fatty acid content resulting in a lowering
of the smoke, flash and fire points. Accordingly used oil
freshened with new oil will show
an increased smoke, flash and free points.
| 8. Why do cooking
oils go bad? How do you prevent them from going rancid?
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I. |
Autoxidation
Of particular interest in
the food field is the process of oxidation induced
by air at room temperature referred to as "autoxidation."
Ordinarily, this is a slow process which occurs only
to a limited degree. In autoxidation, oxygen reacts
with unsaturated fatty acids. Initially, peroxides
are formed which in turn break down to hydrocarbons,
ketones, aldehydes, and smaller amounts of epoxides
and alcohols. Heavy metals present at low levels in
fats and oils can promote autoxidation. Fats and oils
often are treated with chelating agents such as citric
acid
The result of the autoxidation of fats and oils is
the development of objectionable flavors and odors
characteristic of the condition known as "oxidative
rancidity." Some fats resist this change to a
remarkable extent while others are more susceptible
depending on the degree of unsaturation, the presence
of antioxidants, and other factors. The presence of
light, for example, increases the rate of oxidation.
It is a common practice in the industry to protect
fats and oils from oxidation to preserve their acceptable
flavor and shelf life.
When rancidity has progressed significantly, it is
apparent from the flavor and odor of the oil. Expert
tasters are able to detect the development of rancidity
in its early stages. The peroxide value determination,
if used judiciously, may be helpful in measuring the
degree of oxidative rancidity in the fat.
It has been found that oxidatively abused fats can
complicate nutritional and biochemical studies in
animals because they can affect food consumption under
ad libitum feeding conditions and reduce the vitamin
content of the food. If the diet has become unpalatable
due to excessive oxidation of the fat component and
is not accepted by the animal, a lack of growth by
the animal could be due to its unwillingness to consume
the diet. Thus, the experimental results might be
attributed unwittingly to the type of fat or other
nutrient being studied rather than to the condition
of the ration. Knowing the oxidative condition of
unsaturated fats is extremely important in biochemical
and nutritional studies with animals.
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II
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Oxidation at Higher Temperatures.
Although the rate of oxidation
is greatly accelerated at higher temperatures, oxidative
reactions which occur at higher temperatures may not
follow precisely the same routes and mechanisms as
the reactions at room temperatures. Thus, differences
in the stability of fats and oils often become more
apparent when the fats are used for frying or slow
baking. The more unsaturated the fat or oil, the greater
will be its susceptibility to oxidative rancidity.
Predominantly unsaturated oils such as soybean, cottonseed,
or corn oil are less stable than predominantly saturated
oils such as coconut oil. Methylsilicone often is
added to institutional frying fats and oils to reduce
oxidation tendency and foaming at elevated temperatures.
Frequently, partial hydrogenation is employed in the
processing of liquid vegetable oil to increase the
stability of the oil. Also oxidative stability has
been increased in many of the oils developed through
biotechnological engineering. The stability of a fat
or oil may be predicted to some degree by the oxidative
stability index (OSI).
Source: www.iseo.org
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