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Fat oxidation: A literary review

  • September 1, 2021
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Fat Oxidation in Ghee

#Fat Oxidation is also known as Lipid oxidation. It 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.

Fat oxidation differs from that of bulk lipids because of the 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 ones react with oxygen, moisture, or heat during processing or storage.

In the case of butter, it is not solely milked fat and is 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 has a keeping quality of about 8 months without refrigeration.

#Fat #Oxidation

Food lipids are principally #triglycerides, phospholipids, and sterols found naturally in most 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-flavors of rancidity
  • Degrades proteins, vitamins, and pigments
  • Crosslink lipids and other macromolecules into non-nutritive polymers

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. Still, the presence and activity of catalysts, the integrity of tissues and cells, the number of polyunsaturated lipids, and the structural properties of the final food product, including the total surface area of lipids and the nature of surfactant material, all play important roles in final product stability.

Fatty acids are long aliphatic chains consisting of carbon and hydrogens. The carbon chain varies 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 others have different degrees of unsaturation. However, only the polyunsaturated fatty acids are of interest when talking about lipid oxidation. Polyunsaturated fatty acids contain two or more double bonds, and it is these double bonds that are prone to oxidation.

Consequently, the risk of fat oxidation increases with the number of double bonds 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 if the raw material used for ghee making is rancid, the freshly prepared ghee may have this defect.

Rancidity in ghee is caused by the formulation of volatile compounds, which exhibit unpleasant odors 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 a liquid than in a solid state. Because of the more solid fat in buffalo milk, its rate of fat hydrolysis is slower than cow milk fat. Therefore, cow ghee is more prone to developing rancid flavor during storage.

Hydrolytic rancidity:

The fat-splitting enzyme lipoprotein lipase, found in milk fat globule membrane, is responsible for the hydrolysis of milk fat and the production of lower molecular weight fatty acids (butyric, caproic, and caprylic).

These fatty acids, particularly butyric, impart a rancid flavor to ghee. Very high heat treatment is employed during the manufacturing of ghee, which inactivates the lipase enzyme. Therefore, the hydrolytic rancidity in ghee is not of many problems, provided raw material of good quality (having no rancidity) is used. 

Oxidative rancidity:

 Fat oxidation of butterfat (ghee) is a more common problem and is caused by the oxidation of polyunsaturated fatty acids in the presence of oxygen.

The reaction of oxygen with poly-unsaturated fatty acids involves free radical initiation, propagation, and termination.

Fat Oxidation
Fat oxidation: A literary review 1

In ghee and butter oil, the chain reaction is catalyzed by heat, light, ionization, trace metals (copper and iron), etc. The end products of lipid auto-oxidation are ketones, aldehydes, alcohols, hydrocarbons, acids, epoxides, etc.

 The fat oxidation process begins virtually as soon as the membranes around the MF globules in milk are ruptured, allowing air to come in contact with the fat.

During the manufacturing 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 the use of equipment containing these metals result in rapid fat oxidation of lipids producing off-flavors, typically cardboard, metallic, tallow, oily and fishy.

The problem of fat oxidation can somehow be minimized in modern ghee-making methods.  The problem is reduced now due to the widespread use of stainless steel and the elimination of copper from dairy equipment.

Protection from light and air is very important to prevent fat oxidation. Fat oxidation leads to the formation of free radicals, peroxides, and oxidized cholesterols.

Light-induced flavors can develop when the cream is exposed to sunlight, fluorescent light, or even diffused daylight. The most damaging wavelength is between 440 to 490nm in the UV range, while 310 to 440nm and 490 to 500nm also contribute to accelerated degradation due to fat oxidation.

Homogenization and too vigorous agitation may increase fat oxidation and off-flavor production of dairy products like ghee.

 Fat oxidation: preventive methods

Use of good quality raw material: Raw materials used for manufacturing ghee and butter oil should be of good quality. Any off-flavor, such as acidic, oxidized, and rancid in raw material, shall be carried over to the final product.

The raw material should also be checked for copper and iron, which should not be more than permissible limits, which prevents fat oxidation to a large extent.

Method of ghee manufacturing:

Ghee prepared by the desi method has higher moisture content and shows higher acidity. This is the reason that produced ghee suffers from lower shelf-life. If ghee is to be stored longer, then the desi method should be best avoided.

The sulfhydryl and phospholipid contents have antioxidant properties in ghee butter. The methods which release higher amounts of these natural antioxidant components should ideally be adopted for ghee making.

Heating butterfat with higher amounts of solids-not-fat, as in the case of the direct cream method, at a higher temperature of clarification, will produce more sulfhydryl and thus results in a better shelf-life of the produced ghee.

Probably due to this reason, the keeping quality of ghee is better 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 at 1% to the ghee enhances its keeping quality.

Effect of species of mammals:

Cow ghee is more shelf-stable than buffalo ghee due to the former product’s higher content of natural antioxidants.

Although buffalo ghee has been reported to be more resistant to fat oxidation or lipolysis than cow ghee, ghee prepared from cottonseed-fed animals showed that the fat offers better keeping quality, presumably because of the antioxidant properties of gossypol.

Effect of method of preparation:

The quality of ghee is affected by the fat oxidation process, which is impacted by the method of ghee manufacturing. It is usually 9 months for the DC method and 4 months for the 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 been well established, and it has been shown that the presence of 0.1mg 100g phospholipids improves the quality of ghee, preventing unwanted fat oxidation.

Phospholipids may exhibit antioxidant activity by binding metals, regenerating other antioxidants, and synergizing with phenolic antioxidants. The main fraction of phospholipids, which exerted antioxidant properties, was found to be cephalin.

This fraction also showed maximum browning, which presumably was correlated with antioxidant properties. It was demonstrated that phospholipids act synergistically with tocopherol and have 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 hydroxytoluene (BHT)
  • Tertiary butyl hydroquinone (TBHQ), and many more.

PFA rules do not allow any synthetic antioxidant in ghee, whereas permitting the addition of gallates up to a level of 0.01% and BHA & BHT up to 0.02% in butter oil.

Naturally occurring antioxidants: Many plants and herbs have antioxidant properties and may be added to ghee to extend the keeping quality and lower the fat oxidation process. 

Some examples of such natural sources are as below:

The seeds of soybean and safflower are rich sources of phospholipids. Their addition to ghee and butter oil at a 0.5% level during boiling may delay the oxidative rancidity.

Juices of Amla (Phyllanthus Emblica) at a level of 1.25% in ghee can retard fat oxidation, possibly due to the high content of ascorbic acid and gallate in amla.

 It has been found that adding betel and curry leaves (at a rate of 1% of ghee) during heat clarification of butterfat improves the oxidative stability and the color and flavor of ghee.

The antioxidant properties of these plants are attributed to their phenolic compounds, predominately hydroxy charcoal. The betel and curry leave also contain carotene and ascorbic acid, which tend to undergo fat oxidation by consuming all free oxygen that may be present in the headspace of the ghee container.

Packaging and storage conditions:

Tin cans are best to protect ghee against oxidative spoilage. The reason is that hot ghee filling is possible in tin cans, which will exclude most of the oxygen from the product and enable the replacement of oxygen with nitrogen gas.

The headspace in such containers should also be minimized to prevent fat oxidation.

Ghee should not be exposed to direct sunlight or irradiation.

It should preferably be stored at about 22 degrees C. Nitrogen blanketing of any tanks where ghee is held is one method of preventing oxidation. Still, 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 long shelf life, a maximum level of 3% is recommended. During packing, any air, for example, in drums, should be replaced with nitrogen before the filling is commenced, and packing should ideally be filled from the bottom to prevent any incorporation of air during filling.

If You Wish to partner with us to Import/Buy/Distribute/Trade Our Milkio Ghee products, or for developing Private label ghee products, or contract ghee manufacturing support, Please Feel Free To Contact Us Via Our Email, We Will Be In Touch With You Within the next 48 Hrs.

References:

  • Deeth, H. C., & Fitz-Gerald, C. H. (1983). Lipolytic enzymes and hydrolytic rancidity in milk and milk products. In Developments in dairy chemistry—2 (pp. 195-239). Springer Netherlands.
  • Dairy fats and related products by A.Y Tamime page no:104,111,286-301
  • Deeth, H. C., Fitz-Gerald, C. H., & Wood, A. F. (1979). Lipolysis and butter quality.Australian Journal of Dairy Technology34(4), 146.
  • Advanced dairy chemistry, P.F Fox, vol 2lipids
  • Achaya, K. T. (1949). Rancidity in Indian butterfats (ghee).Biochemical Journal44(5), 561.

Credit : Anso Jo Mathai

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