chemistry of foods
TRANSCRIPT
Lipids are chemically diverse group of compounds with different structural
features.
oLipids are a group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E, and K), monoglycerides, diglycerides, triglycerides, phospholipids.
o They are widely distributed in all foods in good quantities. The common property of lipids is that they are insoluble in water but, soluble in organic solvents like hexane, petroleum ether, chloroform, acetone etc.
o Food lipids are generally referred as fats and oils.
oAt ambient temperature (room temperature) fats are solids, and oils are liquids. This property may be due to the reason that fats contain more saturated fatty acids and oils contain more unsaturated fatty acids.
oFats and oils constitute about 95% of the plant and animal lipids. Both oils and fats are made up of triacylglycerols. Triacylglycerols are glycerol esters of fatty acids.
•Lipids undergo various complex chemical changes during processing and storage of foods.
• During processing and storage, they also react with other constituents present in food and from different compounds, which are desirable and undesirable for food quality.
Occurrence Lipids are present in all the animal and plant tissues.
In plants, they are more concentrated in seeds and nuts.
In animals, it is more concentrated in adipose tissue. Generally, cereals, fruits and vegetables are poor in lipids while nuts, legumes like soybean, peanut are rich in lipids.
Lipid content in different plant and animal sources
Lipids have two important functions in living organisms:
i) it is a major structural component of membrane and
ii) it serves as a storage form of energy-rich fuel. In addition, lipids play an important role as electron carriers, light absorbing pigments, hormones and intracellular messengers. They also facilitate the absorption of fat-soluble vitamins and provide essential fatty acids. Fats are added to foods as a method of transferring heat in frying, for flavour, mouth feel and to enhance the texture, palatability .
They are broadly classified into simple lipids, compound (or complex) lipids and derived lipids.a. Simple lipids (neutral lipids)Fats and oils, waxes, sterol esters and ether lipids are categorized in this class of lipids. Common structural feature of the simple lipids is that these are esters of fatty acids with alcohols.b. Compound lipidsThese are also called complex lipids. Like simple lipids, these are also esters of fatty acids with alcohols, but these lipids contain other groups such as phosphate, nitrogenous base etc.
Cont… Phospholipids glycolipids and lipoproteins are
grouped in this class of lipids.
c. Derived lipidsAs the name suggests, these lipids are obtained upon the hydrolysis of simple and complex lipids. This class of lipids contains fatty acids, sterols, hydrocarbons, alcohols, monoacylglycerol (MAG), and diacylglycerols (DAG), carotenoids and fat-soluble vitamins.
Schematic representation of classification
o Acylglycerols are the most abundant group of naturally occurring lipids. Almost all (99%) of the fatty acids present in the living system exist in the form of acylglycerol.
o Acylglycerols are the fatty acids esters of glycerol.
o They exist in three forms namely,
i) monoacylglycerol,
ii) diacylglycerols and
iii) triacylglycerols.
Fatty acids Fatty acids contain aliphatic chain (tail) with a
terminal carboxylic acid group (head). Most fatty acids have even number of carbons in a straight chain. Majority of fatty acids contain 4 - 24 carbon.
Although the presence of free fatty acids in foods is low, they are associated with both major and minor lipids. Therefore, fatty acids are the major components of the lipids. Fatty acids present in plants and animals are esterified to glycerol (example: triacylglycerol).
Fatty acids can be classified into
Saturated (i.e. without any double bonds) unsaturated fatty acids (i.e., with one or more double bonds). Unsaturated fatty acids are further classified into mono and polyunsaturated fatty acids. If fatty acid contains one double bond, it is called monounsaturated fatty acid (MUFA). If the fatty acid contains more than one double bond, it is called polyunsaturated fatty acid (PUFA).
Examples
Example for Saturated fatty acids: butyric acid
CH3- CH2- CH2- COOH
Example for MUFA: oleic acid
18CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-10CH2 = 9CH2-
8CH2-CH2-6CH2-
CH2-4CH2-CH2-
2CH2-1COOH
The most common unsaturated fatty acid present in fats is oleic acid.
Example for PUFA: Linoleic acid
18CH3-CH2-CH2-CH2-CH2-CH2=12CH2-CH2-
10CH2 = 9CH2-8CH2-CH2-
6CH2-CH2-4CH2-CH2-
2CH2-1COOH
Trivial and systematic names of some fatty acids
Oxidative and hydrolytic rancidity of lipids
The word rancidity is a general term commonly refers to off-flavours resulting from oxidative deterioration (auto oxidation) and hydrolytic cleavage (lipolysis) of triacylglycerols in fats and oils.
Rancidity of fats and oils is caused due the presence of prooxidative enzyme like lipoxygenase and lipolytic enzyme like lipase, exposure to light, oxygen and moisture, and presence of transition metals like copper and iron.
Lipolysis Lipolysis is the breakdown of lipids and involves
hydrolysis of triglycerides into glycerol and free fatty acids
Hydrolysis of esters in lipids is called lipolysis. For example, triacylglycerol is hydrolyzed to free fatty acid and glycerol.
Free fatty acids released in food by lipolysis produce off-flavours. This is also termed as ‘hydrolytic rancidity'. For example, rancidity flavour in raw milk due to lipolysis of fat in milk.
Cont… Though free fatty acids cause rancidity of foods, they
are also required for flavour generation in certain foods. For example, in cheese, bread, yogurt lipases are added to release free fatty acids.
Hydrolysis of ester will occur in two different ways-
1) Enzymatic or
2) heat and moisture
Enzymatic lipolysis:During processing and storage of food,
lipases present in the food hydrolyze the ester bonds of triacylglycerols of fat and release fatty acids. These free fatty acids produce off-flavours.
During deep fat, frying of foods temperature is very high and water present in the foods is released. Thus, lipolysis is a more favourable reaction during deep fat frying of foods. Presence of free fatty acids in deep-fat fried foods decreases the quality of foods.
Auto-oxidation Lipid oxidation is a major concern to the food industry
because it generates off-flavour and off-odour in edible oils and foods containing oils. Lipid oxidation is often referred to as auto-oxidation. In auto-oxidation lipids get oxidized via a self-catalyzed reaction in presence of molecular oxygen. Auto-oxidation in fats is a free radical reaction and it consists of three steps.
Initiation The lipid auto-oxidation initiates with removal of
hydrogen from the polyunsaturated fatty acid (LH). This removal of hydrogen from fatty acid results in the formation of lipid alkyl radical (L•).
LH→ L• + H•
Propagation Lipid alkyl free radicals (L•) formed reacts with O2
molecule and forms peroxy radical (LOO•). The peroxy radical can then abstract a H atom from a different polyunsaturated fatty acid to form lipid hydroperoxide (LOOH) and another new alkyl radical on another fatty acid (LH).
L• + O2 → LOO•
LOO• + LH→LOOH+ L•
The hydroperoxides formed can degrade to volatile aldehydes and ketones. These volatiles produced cause strong off-flavour, which is termed as rancidity.
Termination The propagation reaction (chain reaction) terminates
due to the combination of two free radicals as shown below. The termination reaction yields stable non-reactive products like L-L or LOOL
L• + L•→ L - L
L•+ LOO• → LOOL
Lipid Oxidation
Rancidity can be prevented in different ways. They are
i) They should be stored in closed containers to minimize oxygen exposure.
ii) They should be packed under vacuum to prevent from oxygen exposure.
iii) They should be stored in coloured jars or wraps to control exposure to light,
iv) Metal chelating agents like EDTA should be added to control metal induced oxidation
v) They should be kept away from moisture and heat to prevent hydrolytic cleavage.vi) Addition of antioxidants.
