FAT OXIDATION IN GHEE
Lipid oxidation is a major cause of quality deterioration in food. The design of foods with improved quality depends on a better understanding of the physicochemical mechanisms of lipid oxidation in these systems. The oxidation of fat differs from that of bulk lipids, because of the presence of droplet membrane, the interactions between ingredients, and the partitioning of ingredients between the oil, aqueous and interfacial regions. Free radicals are the product of oxidation in which particularly unstable one react with oxygen, moisture or heat during processing or storage. In case of butter it is not solely milk fat and is, in fact, only 40 to 60 % saturated. It also consists of water and easily singed milk solids, making it a lesser option for cooking on heat. On the other hand, Ghee is almost 100 % pure, with saturated milk fat. So it is so stable and resistant to oxidation and it have a keeping quality of about 8 months without refrigeration.
Food lipids are principally triglycerides, phospholipids and sterols found naturally in most of biological materials consumed as food and added as functional ingredients in many processed foods. As nutrients lipids, especially triglycerides, are a concentrated caloric source, provide essential fatty acids and are a solvent and absorption vehicle for fat soluble vitamins and other nutrients. The presence of fat significantly enhances the organoleptic perception of food. As a class, lipids are also one of the most chemically unstable food components and will readily undergo free radical chain reactions that not only deteriorate the lipids but also produce
- Produce oxidative fragments, some of which are volatile and are perceived as off -flavours of rancidity
- Degrades proteins, vitamins and pigments
- Cross link lipids and other macromolecules into non- nutritive polymers
The fat oxidation depends on all processing steps including raw product selection, storage, refining, manufacturing etc. Thus fat oxidation can be defined as changes in fat with 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. Certain key variables now known to influence oxidative processes can be targeted to increase food lipid stability during and after processing. retention of or addition of exogenous antioxidants is a well-known consideration, but the presence and activity of catalysts, the integrity of tissues and cells, the quantity of polyunsaturated lipids and structural properties of the final food product, including total surface area of lipids, and the nature of surfactant material all pay important roles in final product stability.
Fatty acids are long aliphatic chains consisting of carbon and hydrogens. The carbon chain vary in length, degree of unsaturation, and structure. In foods, fatty acids are mainly found in lipid complexes called triglycerides. Some fatty acids are saturated, while other have different degrees of unsaturation. However, when talking about lipid oxidation it is only the polyunsaturated fatty acids which are of interest. Polyunsaturated fatty acids contain two or more double bonds, and it is these double bonds which are prone to oxidation. Consequently, the risk of oxidation increases with the number of double bonds present in the fatty acid. For instance, EPA (C20:5) having five double bonds, is more prone to oxidation than linolenic acid (C18:3), having only three double bonds.
MAIN DEFECTS IN GHEE DUE TO OXIDATION
Rancidity: This is the most serious defect of ghee. It is of two types, viz. hydrolytic and oxidative rancidity. Normally this defect develops in ghee during storage, but in case the raw material used for ghee making is rancid, the freshly prepared ghee will also have this defect. Rancidity in ghee is caused by the formulation of volatile compounds, which exhibit unpleasant odours even when present in small quantities. The nutritive value of ghee is also adversely affected due to rancidity in ghee. Milk fat hydrolysis is faster in liquid state than in solid state. Because of more solid fat in buffalo milk its rate of fat hydrolysis is slower than cow milk fat. Therefore, the cow ghee is more prone to developing rancid flavour during storage.
Hydrolytic rancidity: The fat splitting enzyme, lipoprotein lipase found in milk fat globule membrane, is responsible for hydrolysis of milk fat and production of lower molecular weight fatty acids (butyric, caproic and caprylic). These fatty acids, particularly butyric, impart rancid off flavour in ghee. During manufacture of ghee a very high heat treatment is employed which inactivates the lipase enzyme. Therefore, the hydrolytic rancidity, in ghee is not of much problem, provided raw material of good quality (having no rancidity) is used. Rancid flavour defect is found more commonly in butter oil.
Oxidative rancidity: Oxidation of butterfat (ghee) is a more common problem and caused by oxidation of poly-unsaturated fatty acids in presence of oxygen. The reaction of oxygen with poly-unsaturated fatty acids involves free radical initiation, propagation and termination. In ghee and butter oil the chain reaction is catalysed by heat, light, ionization reaction and trace metals (copper and iron), etc. The end products of lipid auto-oxidation are ketones, aldehydes, alcohols, hydrocarbons, acids, epoxides etc. Oxidation process begin virtually as soon as the membranes around the MF globules in milk are ruptured, allowing air to come in contact with the fat. During manufacture and most importantly, during the packing and storage of MF products, it is essential that the fat is protected as far as possible ravages of oxidation.
Defects associated with oxidation: The contamination of cream by copper or iron through poor manufacturing practices and use of equipment containing these metals result in rapid oxidation of lipids producing off-flavours, typically cardboardly, metallic, tallow, oily and fishy. This defects is now comparatively rare because of the widespread use of stainless steel and elimination of copper from dairy equipment. Protection from light and air is very important to prevent oxidation. The oxidation lead to formation of free radicals, peroxides and oxidised cholesterols.
Light induced flavours can develop when cream is exposed to sunlight, fluorescent light or even diffused daylight. The most damaging wavelength are in UV range between 440 to 490nm, while 310 to 440nm and 490 to 500nm also contribute to accelerated degradation. Homogenisation and too vigorous agitation my increase oxidation and off flavour production.
Use of good quality raw material: Raw material used for the manufacture of ghee and butter oil should be of good quality. Any off flavour, such as acidic, oxidized, and rancid present in raw material shall be carried over to the final product. The raw material should also be checked for the presence of copper and iron, which should not be more than permissible limits. Method of manufacture of ghee: Ghee prepared by desi method has higher moisture and higher acidity and thus lower keeping quality. If ghee is to be stored for longer time than this method should be avoided. The sulfhydryl and phospholipid contents have antioxidant properties in ghee and butter oil. Those methods, which releases higher amounts of these natural antioxidant components should be adopted. Heating butterfat with higher amounts of solids-not-fat, as in case of direct cream method, at higher temperature of clarification will produce more sulfhydryl and thus better shelf life. Probably due to this reason the keeping quality of ghee is more than butter oil. The pre-stratification method produces ghee with higher amounts of phospholipids because its loss in ghee residue is minimum. Also the extraction of phospholipids from ghee residue and addition @ 1 percent to the ghee enhance its keeping quality.
Effect of species of mammals: Cow ghee is apparently more shelf-stable than buffalo ghee due to the higher content of natural antioxidants the former product. Although buffalo ghee has been reported to be more resistant to lipolysis than cow ghee. Ghee prepared from cottonseed-fed animals showed that the fat had better keeping quality, presumably because of the antioxidant properties of gossypol, a phenolic substance in cotton seed observed that ghee produced and packed in winter has longer shelf life than that packed in summer and rainy seasons.
Effect of method of preparation: The keeping quality of ghee is affected by the method of manufacture. It is 9 months for DC method and 4 months for creamery butter method. Higher temperatures, or longer periods of heating at a particular temperature, have been shown to impart better oxidative stability because of greater liberation of phospholipids from phospholipid-protein complexes. It has been suggested that during heating, especially after most of the moisture has been evaporated, antioxidants are produced from phospholipids. The antioxidative properties of phospholipids in ghee have well established, and it has been shown that the presence of 0.1mg 100g phospholipids improves the keeping quality of ghee. Phospholipids may exhibit antioxidant activity by binding metals, regenerating other antioxidants and providing a synergism with phenolic antioxidant. The main fraction of phospholipids, which exerted antioxidant property, was found to be cephalin. This fraction also showed maximum browning, which presumably was correlated with antioxidant properties. It was demonstrated that phospholipids acts synergistically with tocopherol, and it has also a metal-inactivating action with copper.
Addition of antioxidants: The antioxidants are added universally to anhydrous butterfat and high fat food products. There are two sources of antioxidants, namely synthetic and natural.
Synthetic antioxidants: These include
– Gallates (ethyl, propyl and octyl),
– Butylated hydroxy anisol (BHA)
– Butylated hydroxy toluene (BHT)
– Tertiary butyl hydro quinone (TBHQ), and many more.
PFA rules do not allow any synthetic antioxidant in ghee whereas permits the addition of gallates up to a level 0.01% and BHA & BHT up to 0.02% in butter oil.
Naturally occurring antioxidants: There are many plants and herbs, which have antioxidant properties and may be added particularly to ghee for extending the keeping quality. Some of the examples of such natural sources are as below:
– The seeds of soybean and safflower are rich source of phospholipids. Their addition to ghee and butter oil at 0.5% level during boiling may delay the oxidative rancidity.
Juices of Amla (Phyllanthus amblica) at level of 1.25% in ghee can retard the fat oxidation possibly due to high content of ascorbic acid and gallate in amla.
It has been found that addition of betel and curry leaves (at rate of 1% of ghee) during heat clarification of butterfat improves not only the oxidative stability but also colour and flavour of ghee. The antioxidant properties of these plants are attributed to their phenolic compounds, predominately hydroxy charicol. The betel and curry leaves also contain carotene and ascorbic acid, which have tendency to undergo oxidation by consuming all free oxygen that may be present in the head space of the ghee container.
Packaging and storage conditions: Tin cans are best to protect ghee against oxidative spoilage. The reason being that hot filling of ghee is possible in tin cans, which will exclude most of the oxygen from the product and also enable to replace oxygen with nitrogen gas. The headspace in such containers can also be minimized. Ghee should not be exposed to direct sunlight or irradiation. It should preferably be stored at about 22oC. Nitrogen blanketing of any tanks where ghee is held is one method of preventing oxidation, but it is also important that pipe unions and pump seals that may allow air to be sucked into the process stream are suitable for the purpose. Small bubbles of nitrogen are streamed into the liquid ghee to scavenge any dissolved oxygen that may be present. Measurement of dissolved oxygen at the time of packing is important, and to ensure a long shelf life a maximum level of 3% is recommended. During packing, any air, for example in drums, should be replaced with nitrogen before filling is commenced, and packing should ideally be filled from the bottom to prevent any incorporation of air during filling.
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- Advanced dairy chemistry, P.F Fox , vol 2lipids
- Achaya, K. T. (1949). Rancidity in Indian butterfats (ghee).Biochemical Journal, 44(5), 561.
Credit : Anso Jo Mathai