TRADITIONAL DAIRY PRODUCTS

Lesson 4. METHODS OF PREPARATION OF KHOA

Module 2. Khoa

Lesson 4
METHODS OF PREPARATION OF KHOA

4.1 Introduction

Khoa is prepared by different methods depending on the location and quantity of milk available for conversion. Khoa is manufactured by the following four basic methods viz. traditional method, improved batch method, mechanized method and use of membrane technology.

4.2 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 due to higher chlorides in the product.

4 liters of buffalo milk or 5 liters of cow milk which approximately yields 1 kg khoa is used per batch. Filtered milk is taken in a heavy bottomed wide mouth iron pan (karahi) and boiled on a brisk non – smoky fire. An iron scraper (khunti) is used for stirring the milk during boiling and also to scrap the milk film forming on the surface during boiling. A rapid stirring and scrapping is carried out through out 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. The researchers have observed that at 2.8 fold concentration of cow milk and 2.5 fold concentration of buffalo milk, heat denaturation of milk proteins takes place and the proteins will not go into solution again. The heating is continued till the milk thickens considerably and at this stage heating is reduced and speed of 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 it results in dark colored khoa that does not fetch a good price in the market as white/ cream colored khoa is preferred for sweets making.

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 the residual heat of the vessel helps in further evaporation of moisture. The contents are transferred to the non corrosive metal moulds and allowed to cool.

There are several limitations of this method such as:

1) Time and labor consuming

2) Large variation in quality

3) Poor keeping quality

4) Small scale production

5) Smoky smell

4.2.1 Production of three varieties of khoa

4.2.1.1 Dhap

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

4.2.1.2 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 up to a smooth textured product. khoa is molded in to hemispherical molds to give its shape, and allowed to cool.

4.2.1.3 Danedar

Generally the milk that is left over after the preparation of other varieties of khoa during the day develops acidity which 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.

4.3 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 accumulates in the centre in a pat form, heating is stopped. It takes about 15 -20 min 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 outer jacket and heated by placing the whole unit over a Chullah. 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 litre milk in 8 min.

4.4 Mechanized Processes for Khoa Production

4.4.1 The first 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 covered steam jacketed pans with open holes provided with spring loaded reciprocating type scrapper knives operating at 30 strokes per min. 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 min. 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 min. The product then moves to the second cascading pan where its concentration is raised to the desired level i.e., 65-70 percent in 6 to 7 min. The steam pressures maintained in the two pans are 2 kg in this machine. The steam requirement is 50kg/cm² and electric power requirement is 4 KW per hour.

4.4.2 Use of roller drier

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 74ºC for 10 min 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 103ºC for 10-12 min to develop cooked flavour on drums with steam pressure of 0.7 to 1 kg/cm² and a roller speed of 10 rpm.

Limitations

1) Losses in the beginning is high

2) Uneven product

3) Lacks grainy texture

4) Need careful monitoring during working

5) Difficult to maintain constant steam pressure
(If more steam dried product will come and if less steam liquid product)

4.4.3 Use of scraped surface conical vat

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 min and 50 min respectively. Steam pressure used is 1.5 kg/cm². Product losses are high in this machine.

Fig. 4.1 Conical vat for khoa making developed at NDRI

4.4.4 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 95ºC and Convap 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.

Fig. 4.2 Contherm–Convap system (Alfa Laval)

Limitations

1. Lacks typical khoa flavour

2. Sticky, pasty and burnt particles

3. Lacks grainy texture and uneven colour

4. Inconsistent product quality

5. High loss of milk solids in the beginning (Due to lot of adjustments required in the initial stage of operation).

4.4.5 Thin film scraped surface heat exchanger (TSSHE)

A horizontal heat exchanger for khoa making was developed which consists of a hopper to accommodate the foam and provide milk concentration under atmospheric pressure. In this model, the steam jacket is subdivided into three parts to reduce the amount of heating as the product moves. The scraper speed is 40 rpm and the steam pressure maintained is 3 kg/cm² in the first compartment which is step wise reduced to 1.5 kg/cm² in the last compartment. The machine can convert 50 kg of milk into khoa per hour per batch. A continuous khoa making machine with three stage concentration has been developed. The machine has three jacketed cylinders placed in a cascade arrangement. This facilitates easy transfer of milk from one cylinder into the other. The scrapper speeds are 40, 55 and 69 rpm for the first, 2^nd & 3^rd stage respectively. The operating steam pressures used are 2.0 & 1.7 & 1.5 kg/cm² in respective stages. One roller is used in the last stage in place of scraper blade which kneads the khoa to improve its body and texture. The first stage raises the milk solids level from initial 15 to 25 percent, the second stage to 50 percent and the third stage to 65-70 percent. The machine converts 50 kg of milk into khoa per hour at the operating pressures, specified. However, the capacity depends on the milk flow rate, steam pressure, total solid concentration of feed and final moisture required in the product. It is claimed that use of concentrated milk improves the capacity of the machine.

4.4.6 Inclined scraped surface heat exchanger (ISSHE)

An inclined scraped surface heat exchange for continuous khoa making was developed. A scraper assembly is so built as to 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 previously vacuum pre-concentrated to 40 - 55% T.S and fed at the rate of 60-80 lit/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 time. It also improves the capacity of the machine.

• Increase in feed temperature enhances the production 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 change of heat transfer rate, colour and texture of khoa due to change in the temperature to which the milk constituents are subjected to different stages of khoa making.

• The man power requirements are reduced.

Fig. 4.3 Inclined scraped surface heat exchanger (ISSHE)

(Source: NDDB)

Advantages of the ISSHE

1) As the khoa making is continuous process it is possible to maintain uniform quality for khoa throughout the production run.

2) There are minimum operational losses.

3) The equipment gives flexibility to change the characteristics of khoa as per the requirement of the product formulations.

4) It is easy to go in for automation so that the entire khoa making process can be monitored by a microprocessor.

5) The plant is suitable for ‘in place cleaning’.

6) The man power requirements are less compared to that of batch method.

7) Due to small hold up raw material in the plant at any point of time there is no chance of whole batch getting spoiled.

8) Permits quick start up and close down.

9) Milk flows in enclosed system, thus permitting freedom from environment contamination.

Limitations

It requires concentrated milk as a feed which is available only in organized dairies. Hence the unit is not suitable in the situation where concentrated milk is not available.

4.5 Chemical Quality of Khoa

Wide variations exist in chemical quality of cow and buffalo milk khoa.

Table 4.1 Chemical composition of khoa

The market samples of khoa show wide variations in chemical composition. Certain times, the market samples fail to meet the minimum legal standards. To provide minimum legal standards in khoa, the minimum fat content of 4.4% in cow’s milk and 5.5% in buffalo milk should be maintained.

Suggested Readings

1) Punjurath, J. S., Veeranjeneyulu, H., Mathunni, M. I., Samal, S. K. and Aneja, R. P. (1990). Inclined scraped surface heat exchanger for continuous khoa making. Indian J. Dairy Sci., 43(3):225-230.
2) Dharam Pal et al (TO BE ADDED)
3) Arora (TO BE ADDED)
4) Dodeja (TO BE ADDED)
5) Agrawala (TO BE ADDED)
6) S. K. Singh (TO BE ADDED)