Dr. Vijay Kumar Gupta Principal Scientist Dairy Technology Division NDRI, Karnal-132001 1.0 INTRODUCTION
In India, souring of a large amount of milk particularly during summer and monsoon months is not uncommon. A good amount of this milk may often be curdled while reaching the dairy plants. The low per capita milk availability in the country warrants the proper utilization of milk constituents even from curdled milk. Skilled technicians can, on smelling and sometimes on testing, reject sour milk from the main processing line to avoid the possible difficulties in processing such milk. They may also perform clot-on-boiling test, titratable acidity test, developed acidity test, alcohol test, alcohol alizarin test and other heat stability test to confirm the unsuitability of milk for heat processing.
Most common practice to utilize curdled milk, whenever received, is to churn it directly into butter for heat clarifying into ghee. A few plants accumulate small amounts of curd obtained daily, until sufficient curd is available for churning. However, only the plants equipped with butter churn can use the direct churning method. Dairies without a butter churn sometimes neutralize curdled milk for separation into cream. The practice of mixing small amounts of curd with a large portion of milk/cream for ghee making is also not uncommon.
In any case, there are several difficulties in manufacturing ghee from curdled milk.
Since the amount of curdled milk to be handled on a daily basis cannot be anticipated, the technical management often finds it difficult to develop an organized production plan for handling curdled milk. Currently prevalent power breakdown and other Industrial operational difficulties make the matter worse. Moreover, the fat recovery and quality of ghee obtained from curdled milk have received little attention. Making ghee from butter obtained by direct churning of curdled milk is not very different from the desi ghee making technology. However, use of power churn and uncontrolled fermentation during curdling may create problems in fat recovery and ghee quality, respectively. Reports on ghee from curdled milk are rather limited.
2.0 MANUFACTURING PROCESS
Curdled milk can be converted either to cream or butter, both of which are heat clarified to ghee. Cream can be obtained by the separation of neutralized curdled milk, while butter can be made from either curdled milk by direct churning or cream obtained from it.
The direct churning of curdled milk appears to be the simplest and the shortest route for ghee making, followed by direct cream method and creamery butter method. The latter two methods, involving additional steps, must be justified in terms of advantages gained in product recovery and/or quality.
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2.1 Direct Churning of Curdled Milk
For desi ghee preparation, a smooth curd is set to about 0.8% lactic acidity. On the other hand, milk at the reception dock represents a varying state of coagulation ranging from a completely curdled mass, distinctly wheying off, to sour milk with definite signs of curd flakes. No systematic study has been reported concerning the effect of these variations on the churning efficiency of the curdled milk. There may be a question of losing efficiency in handling curdled milk in power churn designed specifically for 35-40% fat cream. It is not known if the process yields itself favourably to butter grain development and washing steps.
Thus, dairies churning curdled milk directly to butter might not have yet optimized the process.
2.2 Neutralization of Sour/Curdled Milk
Curdled milk may be neutralized to impart enough physical stability for centrifugal cream separation. The neutralized curdled milk should have also curd particles dissolved, enabling its warming up to a temperature of about 40°C for separation. The neutralization of cream for butter making has been studied with reference to the level of treatment, the use of sodium and calcium neutralizers and the means of incorporating neutralizing solutions. The neutralization of curdled milk has not received similar attention.
Food grade alkali neutralizers can be used for reducing the acidity in sour milk. Soda as well as lime and magnesium neutralizers are available for this purpose. Soda neutralizers have the distinct advantage that they dissolve readily and are completely soluble in water and their action on the acid in the sour milk is more speedy than that of lime neutralizers.
Brownish colour, however develops more rapidly with soda neutralizers during heat processing of such milk. A sodium type neutralizer such as sodium hydroxide is preferred for products requiring maximum solubility. The alkali should be dissolved in six to twenty times water by weight and added to the milk with sufficient agitation so that no excessive localized over neutralization takes place. The milk temperature should be less than 35°C and at least 15 minutes should be allowed for the reaction before heating.
The influence of sodium bicarbonate is two fold; it has a balancing effect on calcium and it changes the reaction. Correct way of neutralizing sour milk with sodium bicarbonate under plant situations was investigated at National Dairy Research Institute, Karnal (Gupta and Mulay, 1984). Lapses at different stages of neutralization were avoided or temperature of milk lowered to 4°C or below, otherwise additional acidity developed. Neutralized milk called for heating to 80°C (Flash) for stopping further development of acidity. Carbon dioxide evolution during the process caused frothing which needed careful handling.
Minimum boiling was required for completing the neutralization. Expulsion of carbon dioxide continued even during the storage of boiled neutralized milk. After the sour milk is neutralized to the normal acidity, it behaves mostly like normal milk as far as heat processing and other operations are concerned.
Conversion of cream obtained from neutralized curdled milk to butter should not be much different from the well known procedure of making butter from the neutralized cream.
However, butter for ghee making may require less rigorous attention in terms of composition control and working required for the table butter. Although high fat cream (75°C and above)
obtained from fresh milk can be directly heat clarified into good quality ghee, this may not be possible without washing and reseparation of cream obtained from neutralized milk. Such a treatment may be desirable to reduce the neutralized serum portion. Butter obtained either by direct churning or through the cream route can be converted into ghee in a manner similar to the creamery butter method of ghee making. The amount of moisture to be removed and quantity of ghee residue formed may be slightly higher in case of directly churned butter than creamery butter. Further economy in heating energy and heat clarification time can be achieved by using the prestratification technique. In this regards, conversion of butter into ghee claims a distinct edge over the direct cream method.
Depending upon the situation, dairy may adopt any one of the above three methods for making ghee out of curdled milk. Direct churning of curdled milk being the simplest process, may be used in dairies having enough curdled milk for their power butter churn.
Dairy without a butter churn must resort to direct cream method via neutralization. There may be several instances where creamery butter method will be most suitable. A typical situation may arise out of accidental curdling of a large quantity of milk such as a tanker load.
In this case, elimination of a considerable amount of serum solids through neutralization and cream separation followed by its conversion to butter would be more convenient. Here, butter making will cause a further reduction in volume of the product to be handled. Of course, for large scale ghee manufacturing, other innovative practices (Chakraborty, 1980) may also be applicable for curdled milk.
3.0 FAT RECOVERY
Fat being the costliest milk component, its increased recovery is bound to improve the economy of handling curdled milk for ghee making. Fat recovery is essentially related to the various manufacturing steps employed in the process of pre-concentration of fat and its heat clarification for ghee making. Pre-concentration of fat may be achieved from neutralized milk or during butter making.
3.1 Separation Step
Milk which is stale and partly sour or curdy, tends to lower the skimming efficiency largely because it increases the amount of separator slime which collects in the bowl and this in turn impedes the free passage of milk and cream and causes excessive loss of fat. If the milk is on the verge of curdling, the chances of incomplete separation are augmented by the fact that each particle of curd locks up a small amount of fat and the curd passing into the skim milk on account of its higher sp. gravity, carries this fat with it. If it is necessary to run curdy milk through the separator, it should be stirred sufficiently to break up the curd as finely as possible, taking care to see that the separator is slightly underfed.
In some dairy plants, the sour milk is separated in the cold milk separator with poor skimming efficiency while in others it is neutralized and separated quite efficiently. For a given efficiency of a cream separator, factors requiring special attention for the neutralized curdled milk would be the complete dissolution of curd particles releasing entrapped fat and minimum fouling of separator with fine curd particles. The extent of fat loss would, therefore, depend on the satisfactory neutralization. Other factors considered important to the efficiency of separating normal milk are also applicable for neutralized curdled milk.
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3.2 Churning Step
The ideal recovery of fat by desi method is 88-90%, but it is much lower in actual practice. Modern power churns on the other hand, are reported to give a fat recovery of 88-92% with creamery butter process. Data on fat recovery of curd by direct churning are, however, not available. In principle, fat recovery will be increased by improving the churning efficiency. The factors that promote the concentration of fat globules and their increased frequency of collision during the churning process result in a high fat recovery.
The basic principle of churning cream into butter are well known. However, the factors important to the churnability of curd have not been studied in detail. These involve the standardization of the churning process in terms of (a) controlling the total solids contents of the curd, (b) the extent of filling the churn, (c) the RPM of the butter churn and (d) the temperature of churning.
The loss of fat in the buttermilk obtained by desi method is reported to be at least 10%. This may not be a real loss in farm situations where butter milk produced in small quantities in consumed by the farm family. But in the dairy situation, this represents not only a low fat recovery but also a problem in casein manufacturing. For this reason, improving the fat recovery from buttermilk should also merit due consideration.
3.3 Heat Clarification Step
Essentially the amount of ghee residue, its fat content and the process of recovering fat from ghee residue influence fat recovery associated with the heat clarification process. In general, ghee residue remaining after pressure filteration and/or centrifugal clarification is subjected to a hot water treatment for collecting most of the entrapped free fat. Not much improvement is envisaged at the present time in increasing the recovery of this fat. The overall practice of handling and storage of various intermediate products such as cream and butter and ghee in bulk storage or small containers also affect the fat recovery through varying amount of stickage loss. The conditions of handling curdled milk ghee poses no different problem in this aspect.
The process for maximum fat recovery from buffalo curdled milk have been standardized at National Dairy Research Institute, Karnal (Gupta at al, 1986b) by (a) direct churning method and (b) reprocessing method (neutralization and cream separation). In direct churning method, ageing of curdled milk for 3-4 hr at 5-8°C, its 50% dilution with chilled water (8-10°C) and churning at 10-12°C are helpful in getting optimum fat recovery (88.87% in butter and 85.0% in ghee). 70% of buttermilk fat is also recovered after neutralization and warm separation. In reprocessing method, 91.77% fat is recovered in cream and 85.60% in ghee. Optimized reprocessing method is: neutralization of curdled milk to 0.08-0.10% T.A., boiling, filteration and warm cream separation.
4.0 QUALITY OF GHEE
In most cases, a small amount of ghee obtained from curdled milk is blended with the bulk of dairy ghee. The influence of ghee made from substandard material remains unnoticed due to its blending with a large bulk. The nature of ghee spoilage, particularly oxidation and rancidity, is such that even a small amount of catalytic agent such as FFA or copper ions,
hastens the deterioration process. Thus, it is necessary to ascertain the quality of ghee obtained from a high acid and neutralized material such as curdled milk with particular reference to analytical constants, sensory attributes, shelf life and public health.
4.1 Analytical Constants
The major analytical constants of ghee remain unaffected by the method of preparation.
In case of manufacturing ghee from curd, this is possible only when butter has undergone little deterioration on storage. However, in practice, desi ghee made from collected butter of indifferent quality, subjected to neutralization, washing (prestrafication) and refining treatment may cause a varying amount of losses in water soluble and volatile fatty acids affecting several analytical constants. Deterioration to such an extent is not normally expected in ghee made by direct churning route, if butter is clarified without delay. Other routes of ghee making involving neutralizing of curdled milk should also not affect analytical constants.
Physico-chemical qualities of buffalo sour and curdled milk ghee, prepared through direct churning and reprocessing methods with 0, 1, 2 and 3 washings of cream/butter, were evaluated at NDRI, Karnal (Gupta et al, 1986a). All experimental ghee samples had R.M.
value, Polenske value, Iodine value, Saponification value, B.R. reading, % free fatty acids and Peroxide value within reasonable limits comparable with those of fresh buffalo milk ghee and conformed to PFA and Agmark Standards.
4.2 Sensory Quality
The sensory quality of ghee centres on flavour, texture and colour, of which flavour is considered to be the most important one. Ghee develops its characteristics flavour during heat clarification. Ghee associated with lactic fermentation possesses most appealing flavour characteristics. Curdled milk ghee is richer in flavour as compared to dairy ghee prepared from fresh products. Treatment standardized for neutralized cream for table butter may be suitably modified for neutralized curdled milk. Desi ghee has been claimed to have better textural properties than the dairy ghee that is known to exhibit often an excessive layering. It is not certain if this phenomenon is related to a more complete extraction of fat in direct cream and creamery-butter process used in dairies as against a partial removal of butterfat, particularly the high melting glyceride fraction, in butter obtained under practical village condition for desi ghee. Studying the nature of fat obtained from buttermilk after the direct churning process may provide a better explanation. Souring has been demonstrated to influence the whitish colour of buffalo ghee due to the conversion of Biliverdin to Bilirubin, imparting a yellowish greenish tinge (Chandravandana et al., 1977).
Gupta et al., (1986a) evaluated the sensory qualities of buffalo sour and curdled milk ghee prepared by direct churning and reprocessing methods with 0, 1, 2 and 3 washings of cream/butter. Though direct churning method ghee samples were judged to have highly significantly (P<0.01) better appealing sensory qualities than the reprocessing method ghee, all the samples were graded between “good to “excellent”. Washing of cream/butter did not much improve the quality of ghee and was thus found unnecessary. Some of the fermented products, particularly the water soluble ones, contributing to desirable flavour in ghee, might be getting flushed away during washing treatment. Total sensory score of curdled milk ghee prepared through both the standardized methods (without washing of cream/butter) was found comparable with that of fresh 0.5% T.A. buffalo milk and NDRI ghee. Colour wise,
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ghee prepared through direct churning method was highly significantly (P<0.01) more liked than the one prepared through reprocessing method. Probably sodium bicarbonate added during the reprocessing method imparted relatively higher degree of brownish tinge to ghee.
Washing of cream/butter was observed to improve the colour characteristics of ghee prepared through both the methods, which may also be due to the partial removal of serum portion through washing of cream/butter. A greater amount of serum portion is understood to impart brownish colour to the ghee due to the interactions, particularly, between casein and lactose during heat clarification.
4.3 Keeping Quality
Several factors, associated with the preparation of ghee from the fermented products affect its keeping quality. The most important is the FFA content of ghee, known to accelerate the development of tallowiness. However, washing of cream as well as prestratification of melted butter significantly reduce FFA with a resultant improvement of shelf life.
4.4 Safety and Nutrition
From the public health view point, the quality of ghee prepared from naturally curdled raw milk as against by lactic fermentation after an adequate heat treatment, may be questionable. This concern is based on the possibility of bio-toxin production during the uncontrolled curdling process and their subsequent transfer to ghee. However, on a closer scrutiny, the chance of bio-toxin production in milk during natural souring process may be narrowed down essentially to bacterial toxins from coliform group of organism and mycotoxins due to yeast and mould growth. It may be further realized that only fat soluble toxins would be of any consequence in the manufacture of ghee. Furthermore, the fat-soluble toxins must be able to withstand the heat clarification treatment. Under these conditions, the probability of the production of fat-soluble, heat tolerant bio-toxins during the normal milk curdling process can be established only on the basis of detailed toxicological studies conducted on a wider industrial basis. To date, no toxic effect accruing to ghee has been reported.
Ghee from direct churning method contains comparatively lower amount of phospholipids. High acidity in ghee may also affect vitamin A potency. Thus, a lower phospholipid content and slight loss in vitamin A potency may lower the nutritive value of curdled milk ghee in comparison to the best available product. But ghee need not be considered a major source of these nutrients under Indian conditions. Of course, ghee samples of questionable safety value or which are nutritionally substandard can be put to several profitable non-edible uses, such as ceremonial lamp burning or in havans etc. About 2% of the ghee produced in India is used for such purposes.
5.0 REFERENCES
Chakraborty, B.K. (1980) “Industrial Ghee Production-Trends and Innovations” Paper presented at IDA Ghee Conference, Sept. 12-13, 1980, New Delhi.
Chandravandana, M.V., Daniel, E.V. and Dastur, N.N. (1977) Indian Dairyman, 29: 233.
Gupta, V.K., Arora, K.L. and Chakraborty, B.K. (1986a) Physico-chemical and sensory qualities of ghee from curdled buffalo milk. Asian J. Dairy Res., 5(1): 49-55.
Gupta, V.K., Arora, K.L. and Chakraborty, B.K. (1986b) Recovery of ghee from curdled buffalo milk. Asian J.
Dairy Res., 5(3): 143-148.