Rancidification, the product of which can be described as rancidity, is the process
which causes a substance to become rancid, that is, having a rank, unpleasant smell or
taste. Specifically, it is the hydrolysis and/or autoxidation of fats into short-chain
aldehydes and ketones which are objectionable in taste and odor. When these processes occur
in food, undesirable odors and flavors can result. In some cases, however, the flavors
can be desirable. In processed meats, these flavors are collectively known as warmed-over
flavor. Rancidification can also detract from the nutritional value of food, and some vitamins
are highly sensitive to degradation. Akin to rancidification, oxidative degradation
also occurs in other hydrocarbons, e.g. lubricating oils, fuels, and mechanical cutting fluids.
Rancidification pathways Three pathways for rancidification are recognized:
Hydrolytic rancidity Hydrolytic rancidity refers to the odor that
develops when triglycerides are hydrolyzed and free fatty acids are released. This reaction
of lipid with water sometimes requires a catalyst, but results in the formation of free fatty
acids and salts of free fatty acids. In particular, short-chain fatty acids, such as common butter
fats, are odorous. Rancidity in foods may be very slight, indicated by a loss of freshness
to very severe, indicated by objectionable odors and/or flavors. Slight degrees of rancidity
are much more common in foods than severe rancidity, yet slight rancidity is a much
more a practical concern. A slight degree of rancidity may not be objectionable to consumers,
but products which do not seem fresh will not attract repeat purchases. If customers
do not return to a product, the long-term effects of a slight degree of rancidity can
be very serious. Oxidative rancidity
Oxidative rancidity is associated with the degradation by oxygen in the air. Via a free
radical process, the double bonds of an unsaturated fatty acid can undergo cleavage, releasing
volatile aldehydes and ketones. Oxidation primarily occurs with unsaturated fats. For
example, even though meat is held under refrigeration or in a frozen state, the poly-unsaturated
fat will continue to oxidize and slowly become rancid. The fat oxidation process, potentially
resulting in rancidity, begins immediately after the animal is slaughtered and the muscle,
intra-muscular, inter-muscular and surface fat becomes exposed to oxygen of the air.
This chemical process continues during frozen storage, though more slowly at lower temperature.
This process can be suppressed by the exclusion of oxygen or by the addition of antioxidants.
Thus, airtight packaging will slow rancidity development.
Microbial rancidity Microbial rancidity refers to a process in
which microorganisms, such as bacteria, use their enzymes such as lipases to break down
fat. This pathway can be prevented by sterilization. Health effects
Consuming rancid food products is unlikely to cause immediate illness or harm. Rancidification
can reduce the nutritional value of food, and some vitamins are highly sensitive to
degradation. In addition, rancidification can produce potentially toxic compounds associated
with long-term harmful health effects concerning advanced aging, neurological disorders, heart
disease, and cancer. Reducing rancidification
Antioxidants are often used as preservatives in fat-containing foods to delay the onset
or slow the development of rancidity due to oxidation. Natural antioxidants include polyphenols,
ascorbic acid and tocopherols. Synthetic antioxidants include butylated hydroxyanisole, butylated
hydroxytoluene, TBHQ, propyl gallate and ethoxyquin. The natural antioxidants tend to be short-lived,
so synthetic antioxidants are used when a longer shelf-life is preferred. The effectiveness
of water-soluble antioxidants is limited in preventing direct oxidation within fats, but
is valuable in intercepting free radicals that travel through the aqueous parts of foods.
A combination of water-soluble and fat-soluble antioxidants is ideal, usually in the ratio
of fat to water. In addition, rancidification can be decreased,
but not completely eliminated, by storing fats and oils in a cool, dark place with little
exposure to oxygen or free radicals, since heat and light accelerate the rate of reaction
of fats with oxygen. Antimicrobial agents can also delay or prevent rancidification
by inhibiting the growth of bacteria or other micro-organisms that effect the process.
Oxygen scavenging technology can be used to remove oxygen from food packaging and therefore
prevent oxidative rancidification. Measurement of oxidative stability
Oxidative stability is a measure of an oil or fat's resistance to oxidation. Because
the process takes place through a chain reaction, the oxidation reaction has a period when it
is relatively slow, before it suddenly speeds up. The time for this to happen is called
the "induction time", and it is repeatable under identical conditions. There are a number
of ways to measure the progress of the oxidation reaction. One of the most popular methods
currently in use is the Rancimat method. The Rancimat method is carried out using an
air current at temperatures between 50 and 220 °C. The volatile oxidation products
are carried by the air current into the measuring vessel, where they are absorbed in the measuring
fluid. By continuous measurement of the conductivity of this solution, oxidation curves can be
generated. The cusp point of the oxidation curve gives the induction time of the rancidification
reaction, and can be taken as an indication of the oxidative stability of the sample.
The Rancimat method, the oxidative stability instrument and the oxidograph were all developed
as automatic versions of the more complicated AOM, which is based on measuring peroxide
values, for determining the induction time of fats and oils. Over time, the Rancimat
method has become established, and it has been accepted into a number of national and
international standards, for example AOCS Cd 12b-92 and ISO 6886.
See also Fermentation
Food preservation Preservative
Putrefaction Lipid peroxidation
References
Further reading Imark, Christian; Kneubühl, Markus; Bodmer,
Stefan. "Occurrence and activity of natural antioxidants in herbal spirits". Innovative
Food Science & Emerging Technologies 1: 239–243. doi:10.1016/S1466-8564(00)00018-7.
External links Olive oil storage and rancidity