Khoa making process

Khoa making process

• Khoa is prepared by different methods depending on the location, quantity etc. Khoa is manufactured by the following four basic methods viz. traditional method, improved batchmethod, mechanized method and use of membrane technology.

Traditional Method

Generally buffalo milk is preferred for manufacture of khoa as it results in higher yield, smooth texture and soft body with sweet taste. Where buffalo milk is not available, cow  milk is used for khoa making but it results in pasty body and slightly saltish taste in the product. Filtered milk is taken in a thick wide mouth iron pan (karahi) and boiled on a brisk non – smoky fire. An iron scraper (kunti) is used for stirring the milk during boiling and also to scrape the milk films forming on the surface during boiling. A rapid stirring and scrapping is carried out throughout boiling to facilitate quick and rapid evaporation of water from milk and also to prevent scorching of milk film on surface. Due to continuous evaporation of water, the milk progressively thickens. Some researchers observed that at 2.8 fold concentration of cow milk and 2.5 fold concentration of buffalo milk, heat denaturation of milk proteins take place and the proteins will not go into solution again. As the concentration is progressing the product slowly tends to leave the sides of the pan and starts accumulating at the bottom and at this stage the pan has to be removed from the fire. The contents are worked up and allowed to cool and the residual heat in the product helps in further evaporation of moisture. At this stage, stirring and scraping is increased to obtain good quality product. If the milk is subjected to high heat treatment with less stirring and scraping at this stage results in dark colored khoa that does not fetch good market value as that of white or cream colored khoa which is more preferred for sweets making.

Production of Three Varieties of Khoa:

Dhap

• For preparing dhap variety of khoa the heating should be stopped at rabri stage (thick mass) and leaving theproduct without much working results in dhap variety of khoa which carry soft grains and high moisture content

Pindi

• For production of pindi variety of khoa, heating is continued after rabri stage and with the help of a wooden ladle the soft grains are crushed and the mass is worked out to a smooth textured product. After cooling, the khoa is moulded in to hemispherical molds to give its shape.

Danedar

• Sour milk is preferred for production of khoa.Generally the milk that is left over after the preparation of other varieties of khoa is converted into danedar variety of khoa. Sometimes citric acid (0.05 to 0.1%) or sour whey is added to milk at boiling stage to get granular texture.

Improved Batch Method

A stainless steel double jacketed, steam heated pan or kettle is used to provide greater control on the heating process and to ensure a non smoky heating. Five liters of standardized cow milk with 4.5% fat or four liters of standardized buffalo milk with 5.5% fat is taken per batch and milk is brought to boil in the kettle. During boiling, bottom and the surfaces of the kettle are scraped and milk is stirred vigorously by a stainless steel stirrer to avoid burning of milk solids. About 2 kg/cm2 pressure is used for boiling milk. When the milk attains a rabri stage, slow heating is necessary at this stage to prevent burning of solids on the surface, discoloration of the product, development of burnt flavour and hard body and coarse texture. The rate of stirring should be increased during last stages to obtain good quality product. As soon as the product shows signs of leaving the sides of the kettle and accumulating in the centre in a pat form, heating is stopped. It takes about 15 -20 minutes to prepare a batch of khoa (1 kg approx) by this method. As steam will not be available at village level the above method has its application only in dairies having steam production lines. However a village level khoa pan was developed to overcome the above problem. The set up consists of hemispherical mild steel pan joined to a cylindrical jacket. Water is taken in the jacket and heated by placing the whole unit over a Chulla. Steam is generated in the jacket and the pressure is indicated by a gauge. The milk is taken in the steel pan and heated by the steam and after loosing the latent heat, returns to water phase. Water in liquid and vapour form exists in phase equilibrium at adjusted pressure because of the closed system. Good quality khoa can be prepared in the pan from 2.5 liters milk in 8 minutes.

Mechanized Processes for Khoa Production

Use of Roller Driers

A roller drying process was adopted in the preparation of khoa. Process variables such as steam pressure, flow rate and distance between rollers and scrappers were adjusted to get the desirable product. Vacuum concentrated milk with 50% T.S. was heated to 74 ºC for 10 minutes to develop cooked flavour in khoa. Steam at a pressure of 25 -30 psi and roller speed of 17-19 rpm gives good results. It was concluded that satisfactory khoa can be prepared on drum driers using 50% T.S. vacuum concentrated milk. The concentrate is preheated to 101 to 103 ºC for 10-12 minutes to develop cooked flavour and drums with steam pressure of 0.7 to 1 kg/cm2 and a roller speed of 10 rpm.

Use of Scrapped Surface Heat Exchangers

Attempts have been made to use scrapped surface heat exchangers in the preparation of khoa. A mechanized scraped surface heat exchanger with a conical vat process is developed for the production of khoa. Forty kg concentrated or 80 kg whole milk can be taken per batch which takes about 14 minutes & 50 minutes respectively. Steam pressure used is 1.5 kg/cm2. Product losses are high in this machine.

A Continuous Khoa Making Machine

A continuous khoa making machine was developed which consists of a preheating cylinder and two cascading pans. The preheater is a steam jacketed cylinder containing rotary scrapers which rotate at 120 rpm. The cascading pans are open steam jacketed pans provided with spring loaded reciprocating type scrapper knives operating at 30 strokes per minute. The milk is taken into the preheater and heated by steam at 3 kg/cm2 pressure. Here the milk is concentrated to about 30 to 35 per cent of total solids within 10 to 12 minutes time. From the preheater, the milk enters the first cascading pan. Here the milk is further concentrated to about 50 to 55 per cent total solids within 7 to 8 minutes. The

product then moves to the second cascading pan where its concentration is raised to the desired level i.e., 70-75 per cent in 6 to 7 minutes. The steam pressures maintained in the two pans are 2 kg/cm2 and 1.5 kg/cm2 respectively. 50 liters of milk can be converted into khoa within an hour time in this machine. The steam requirement is 50 kg per hour and electric power requirement is 4 KW per hour.

Use of Inclined Scraped Surface Heat Exchanger

An inclined scraped surface heat exchange for continuous khoa making was developed. They used a scraper assembly which combines the functions of scraping and conveying. The SSHE has 3 jackets which operate at 1.0, 1.5 and 1.0 kg/cm3 respectively. Milk is pre-concentrated to 40 - 55% T.S and fed at the rate 60-80 lts/hr. Feed temperatures between 10 – 80°C can be employed. Rotor speed used is 40 to 80 rpm. 

The advantages claimed by this unit are:

• Increased solids content in feed improves the colour of khoa because of reduced
• residence times. It also improves the capacity of the machine.
• Increase in feed temperature enhances the capacity of the plant.
• By increasing rotor speed there will be significant increase in the heat transfer
• rate.
• Variation in steam pressure in separate sections of steam jacket results in changeof heat transfer rate, colour and texture of khoa due to change in the temperatureto which the milk constituents are subjected to different stages of khoa making.
• The man power requirements are less.

Contherm – Convap System

Attempts were made to prepare khoa on contherm- convap system which was developed by Alfa -Laval. This unit consists of two parts, a contherm for heating the feed to about 95 ºC and convap (a SSHE) for concentrating milk to desired milk solids level. Concentrated milk with 35- 40%T.S. at the rate of 300-350 kg per hour can be fed to the machine. The steam pressures employed are 3 kg /cm2 in contherm and 4 kg/cm2 in convap.

Factors Affecting Quality of Khoa

Higher amount of free fat (>60%) will be released in buffalo milk khoa than cow milk khoa. Higher free fat content contributes to soft body and oily or greasy appearance to buffalo milk khoa. 

The quality of khoa depends on:

• Type of milk used in khoa preparation
• Chemical quality of milk
• Method of preparation
• Degree of heat treatment
• Manner of handling the product
• Packaging employed
• Duration, temperature and humidity of storage.

Due to compositional differences in cow and buffalo milks, the physical attributes of khoa

will vary.