i. Factors Affecting Homogenization Efficiency
Type of Homogenizing Valves: Design of homogenizer valve affects homogenization efficiency. Grooved valves require less homogenization pressure to attain same degree of homogenization pressures as compared to either simple valve with flat seat or needle valve.
Homogenization Pressure: The recommended pressure ranges for homogenization of milk is 140-175 kg/cm2. If the homogenizer is in perfect working condition i.e.the homogenizer valves are not worn out and are well seated, a homogenizer pressure of 175 kg/cm2 should give good homogenization efficiency. Some modern valves may, however, give satisfactory performance at lower homogenization pressure as well. Higher pressure of homogenization however does not improve the efficiency any further.
Single or Two Stage Homogenization: Two stage homogenization is often recommended because broken fat globules after first stage homogenization (175 kg/cm2) may have a tendency to agglomerate. In order to re-disperse them,homogenization at reduced pressure (35 kg/cm2) may be thus necessary in the second stage. A homogenization process of two or more stages does not however affect the mean particle size of the fat globules in any significant way. Modern homogenizer designs permit two stage homogenization with a single machine.Effect of Fat Content in Milk: Homogenization becomes less effective with increasing fat content. When high fat milk is homogenized, the newly created total fat globule surface becomes so large that materials required to form new membranes for all the fat globules is not sufficiently availably in the serum phase. Thus the newly formed fat globules may have a tendency to agglomerate and rise to the surface during storage.
Effect of Temperature of Homogenization: Milk can be homogenized over a wide range of temperature provided the homogenization temperature is above the melting point of milk fat (32OC). However, a temperature in excess of 50OC is often recommended which is necessary to inactive nature lipases. It lipase is not inactivated;it acts as a surface active agent and becomes incorporated into the newly formed membranes thereby causing hydrolytic rancidity in the product. Raw milk is therefore not to be homogenized. The recommended temperatures for attaining high degree of homogenization (80-90%) are therefore between 60 and 70 OC. Higher homogenization temperatures are also recommended for high fat milk. This is so because at higher temperatures, less protein is adsorbed during the formation of a new fat globule membrane. Furthermore, the membranes are formed more rapidly and thus the tendency of the fat globules to agglomerate is significantly reduced.
ii. Effect of Homogenization on Physico-Chemical Properties of Milk
Effect on Fat: Homogenized milk drains more freely out of the glass container leaving less milk sticking to the sides. This lack of adhesion is attributed to the reduction in size of the fat globules and the protection provided to these globules by the adsorption of higher proportion of casein. Homogenized milk with normal fat content does not have marked clustering of fat globules. This lack of clustering is attributed to:
- destruction of natural agglutinin of milk during homogenization.
- resurfacing of the fat globules.
- increased brownian movement resulting from greatly increased number of fat globules.
Proper homogenization however, does not cause any change in important fat constants or physico-chemical properties.
Effect on Protein: The fat globule membrane is composed of approximately 1/3 phospholipids and 2/3 protein. The membrane acts as an emulsifier to keep the emulsion stable. During homogenization, the original membrane is destroyed and the surface active agents in the serum phase get adsorbed on the fat globules to form a new membrane. The new membrane consists mainly of casein as well as serum proteins. While only 2% casein in milk is adsorbed on the fat globules in un-homogenized milk, in homogenized milk almost 25% of casein is adsorbed as part of fat globule membrane. Homogenization is often associated with destabilization of proteins. This destabilization effect is reflected in reduced alcohol stability, increased feathering of cream in coffee and in coagulation during the manufacture of evaporated milk. This destabilization effect is partly attributed to adsorption of citrates and phosphates on the newly formed fat globule membrane, which lowers their concentration in the serum phase thereby adversely affecting the protein stability.
Colour of Milk: Homogenization results in more uniform, opaque and whiter milk which make the product more acceptable to the consumers. The increased whitening is due to the increase in number and total surface area of fat globules, which reflect and scatter more light.
Emulsion Stability: It is practically not possible to churn homogenized milk.However, with increasing fat content, the emulsion stability decreases.
Curd Tension: Homogenized milk has greater tendency to form coagulum and requires less coagulating agent. The resultant coagulum has lower curd tension and a soft, spongy body. Homogenization at recommended pressure of 175 kg/cm2 causes the curd tension to be lowered by more than 50%. The possible reason for this effect of homogenization on curd tension is attributed to the increase in the number of fat globules, which serve as the points of weakness in the coagulum.Further, nearly 25% of the casein get adsorbed on the fat globules during the formation of new fat globule membranes as against only 2% of the total casein adsorbed on the surface of the fat globules in un-homogenized milk. This results in lower casein concentration in the serum phase thereby lowering the curd tension.Fat losses in the cheese whey are however low as the finely divided fat globules are retained in the curd due to adsorption of casein micelles on their surface.
Viscosity: Single stage homogenization causes increase in viscosity. This is brought about by formation of fat clusters, which results from membranes of newly formed fat globules joining together although fat itself is not in contact. When the milk is subjected to second stage homogenization, the fat globule clusters are disintegrated/broken down resulting in decrease in viscosity. The degree of clustering of the fat globules is directly proportional to the viscosity. A high fat content, a high homogenization pressure and a low homogenization temperature can significantly increase the fat clustering and hence the viscosity of milk. Preheating of milk at temperatures that promote whey protein denaturation also reduces membrane formation and hence increases agglomeration of fat globules.
iii. Problems/ Defects Associated with Homogenized Milk
Curdling During Cooking/Sterilization: Homogenized milk is some times more susceptible to curdling when it is used in certain food preparations requiring cooking.This is in part related to reduced protein stability of homogenized milk as also to the seasoning salts added as an ingredient in the new food formulation.
Recovery of Fat During Centrifugal Separation: Milk fat is difficult to separate from the homogenized milk. If the milk has been homogenized at the generally accepted homogenization pressure of 175 kg/cm2, a significant portion (50-90%) of the fat remain in the skim milk after centrifugal separation. Even addition of homogenized milk with un-homogenized milk and then centrifugal separation does not yield a satisfactory result. The recovery of fat from homogenized milk is a serious problem for commercial dairies which receive significant quantities of processed milk as ‘returns’ from the market and need to salvage fat for economic operation of the plant.
Formation of Cream Plug: Appearance of scum or buttery particles on the surface of the homogenized milk is objectionable. Sometimes, fat rising in homogenized milk is to such an extent that a compact ring of creamy material is visible under the container closure often referred as cream plug. Several factors such as worn out or poorly maintained homogenizer valve, improper homogenization pressure, excessive foaming, improper cleaning of processing lines and failure to recycle the first few liters of milk coming out of the homogenizer lead to such defects in the product.
Sedimentation: Appearance of sediments in homogenized milk upon storage could be a serious problem. This defect is often ascribed to settling of the extraneous matters such as body cells and dirt as also to destabilization of proteins during homogenization. However, clarification of milk before homogenization reduces the amount of deposits significantly whereas clarification after homogenization prevents this defect entirely.
Foaming: Though not a serious problem, excessive foaming in homogenized milk poses handling difficulties. The two possible reasons for this could be inclusion of air as a result of splashing or excessive agitation of the homogenized milk or homogenizing the air into the milk during processing. However, improving the handling procedure during homogenization can largely eliminate this problem.
Flavour Defects of Homogenized Milk: The most important flavour defect associated with homogenized milk is ‘sunlight flavour’, sometimes referred as ‘tallowiness”, ‘burnt like’ or ‘activated flavour’. This develops due to oxidation of free methionine and formation of free SH compounds from sulfur containing amino acids. Development of ‘sunlight flavour’ also requires riboflavin. Probably, all of these compounds are together responsible for ‘sunlight flavour’. The possible reasons for sensitivity of homogenized milk for development of these flavour defects could be the effect of the light upon the increased protein surface following homogenization.Homogenized milk is however resistant to development of oxidized flavour defect.This could be attributed to the formation of new fat globule membranes resulting in ‘dilution’ of catalytic metals viz., copper and iron, which are concentrated in the native MFGM, thereby minimizing direct contact between the fat and the metal ions.
Recovery of Fat During Centrifugal Separation: Milk fat is difficult to separate from the homogenized milk. If the milk has been homogenized at the generally accepted homogenization pressure of 175 kg/cm2, a significant portion (50-90%) of the fat remain in the skim milk after centrifugal separation. Even addition of homogenized milk with un-homogenized milk and then centrifugal separation does not yield a satisfactory result. The recovery of fat from homogenized milk is a serious problem for commercial dairies which receive significant quantities of processed milk as ‘returns’ from the market and need to salvage fat for economic operation of the plant.
Formation of Cream Plug: Appearance of scum or buttery particles on the surface of the homogenized milk is objectionable. Sometimes, fat rising in homogenized milk is to such an extent that a compact ring of creamy material is visible under the container closure often referred as cream plug. Several factors such as worn out or poorly maintained homogenizer valve, improper homogenization pressure, excessive foaming, improper cleaning of processing lines and failure to recycle the first few liters of milk coming out of the homogenizer lead to such defects in the product.
Sedimentation: Appearance of sediments in homogenized milk upon storage could be a serious problem. This defect is often ascribed to settling of the extraneous matters such as body cells and dirt as also to destabilization of proteins during homogenization. However, clarification of milk before homogenization reduces the amount of deposits significantly whereas clarification after homogenization prevents this defect entirely.
Foaming: Though not a serious problem, excessive foaming in homogenized milk poses handling difficulties. The two possible reasons for this could be inclusion of air as a result of splashing or excessive agitation of the homogenized milk or homogenizing the air into the milk during processing. However, improving the handling procedure during homogenization can largely eliminate this problem.
Flavour Defects of Homogenized Milk: The most important flavour defect associated with homogenized milk is ‘sunlight flavour’, sometimes referred as ‘tallowiness”, ‘burnt like’ or ‘activated flavour’. This develops due to oxidation of free methionine and formation of free SH compounds from sulfur containing amino acids. Development of ‘sunlight flavour’ also requires riboflavin. Probably, all of these compounds are together responsible for ‘sunlight flavour’. The possible reasons for sensitivity of homogenized milk for development of these flavour defects could be the effect of the light upon the increased protein surface following homogenization.Homogenized milk is however resistant to development of oxidized flavour defect.This could be attributed to the formation of new fat globule membranes resulting in ‘dilution’ of catalytic metals viz., copper and iron, which are concentrated in the native MFGM, thereby minimizing direct contact between the fat and the metal ions.
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