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Milk is one animal product obtained from cattle and goats whose by-products are both nutritious and have commercial value from milk, cream, butter, ice-cream and yoghurts are obtained. Apart from these, there are other varieties of milk referred to as homogenised, vitamin-D type, filled and imitation, evaporated, condensed (sweetened), special, certified, low-sodium, flavoured and low fat milk, all rich in protein, fats and minerals.
Fermented milk
Generally, fermented milk is made by the celchition of micro-organism cultures to milk. Buttermilk is produced through the action of streptococcus lacti on whole or skin milk. This is pasteurised to take care of pathogenic vegetative cells of bacteria and within 24 hours with formulation and acid. Buttermilk may be stabilized with gelatin to avoid the separation of casein.
Yoghurt
This is popular with by-product delicacy enjoyed all over the world. To produce yoghurt milk is standardised to 3 percent or 1.7 percent fat, homogenised and heated to 900C for between 15 to 30 minutes. It is then cooled to between 43 to 450C, and inoculated with 1 to 2 percent mixed culture of Lactobacillus bulgaricus and Streptococcus thermophilus. The mixture is uncubated at this temperature for 3 hours until the desired acidity of between 0.85 to 0.95 percent and pH 4.0 to 4.2 are obtained. Mature yoghurt lasts for between 10 and 14 days.
However, changes can be made to the basic procedure which includes the addition of 3-5 percent skin milk before the inoculation to raise the intuitive value. This gives the product a better body and consistency. Addition of stabilisers such as gelatin of between 0.1 to 0.3 percent, algamates and agar improve consistency and stability of the product. Sucrose of 4-11 percent, fruit flavours and fruits may be added prior to or after fermentation. This is responsible for such flavours as strawberry and vanilla.
The Streptococcus thermophilus bacterium initiates the fermentation of lactose to lactic acid, reduces product redox potentials by removing oxygen and cause some milk protein breakdown through the production of proteolytic enzyme. This creates a favourable condition for the development of Lactobacillus bulgaricus which begins to develop when the pH has dropped to about 4.5. The characteristic flavor is due to the lactic acid and trace amounts of ethanol, dimethyl propanol, ethanoic acid and other volatiles produced by bacterial fermentation. Lactobacillus bulgaricus is mainly responsible for the production of acetal dehyde which contributes both to the taste and shelf life of the yoghurt.
CREAM.
When cream or fat globules are separated from whole milk and standardised to the prescribed fat content cream results. Creams may be prepared to have fat from 12 to 80 percent, depending on the speed of the separator, the temperature of the milk, the rate of milk flow and the number of times the product is passed through the separator. Marker cream falls into discrete classes, depending on fat centenl. These are 10, 12, 18, 20, 25, 30, 34, 36 and 48 percents.
Whipping cream
This product of milk contains 35 percent fat and is obtained when cream is whipped by beating or using high speed food mixer or blowing air into the cream. Whipped cream increases in volume and the percentage increase is called over-run.
Butter
As a favourite milk product for those who take it along with bread, butter is produced from milk fat to which salt and alkali may be added and the pH adjusted. It may also contain lactic acid eultines, annto, carotene andor turmeric as colouring agent.
Principally butter contains about 80 percent milk fat, 2 percent milk solids and not more than 16 percent water. In the manufacture of butter, two or three days old milk is neutralised with sodium bicarbonate or sodium sequicaboriator. The milk is then pasteurised and cooled immediately to 4-50C. It is then held for several hours to solidify or crystallise fat.
The cream from milk is separated at high speed of 6000 revolutions per minute (rpm). The churning inverts the cream emulsion so that fat forms the continuous phase.
Churning expanded milk and gases are given off. Churning continues until there is a break after 30 to 40 minutes churning time. It is at this point that fat globules are formed. They begin to separate from the cream-liquid-butter-milk mixture. At this stage water is added at 2-30C at about 5 percent volume of cream and run-off butter milk through a strainer. This is washed with water while the churning and run-off continue. About 1.52 percent salt is added into the mass and churned for further 15 to 17 minutes to work the butter.
At this time the residuant water and salt become incorporated into the fat particules which separate as a continuous compact mass of butter. The final moisture is removed and stored at -100C.
Ice cream
Obviously ice-cream is another people’s favourite milk product which has been a money-spinner for those engaged in the processing and sales. Ice-cream is a frozen milk product on milk or cream, sugar, stabliliser, emulsifies and flavouring materials.
The ingredients include skim milk powder, butter or vegetable fat, sugar, emulsifier, flavouring and colouring agents. Natural emulsifiers used include egg yolk, whole egg, lecithin and menoglyceryl stearate while stability ingredients are gelatin, sodium alguiate, gum (carrageenan) and agar. Typical composition of commercial ice cream may contain 12 percent milk fat, 11 percent milk solids, non-fat, 15 percent sugar, 0.2 percent stabiliser, 0.2 percent emulsifier and a trace of flavour. This would ultimately give total solids of 38.4 percent while the rest is water.
The flow-chard or processing stages start with the mixing and pasteurising of the above-stated ingredients. They are homogenised to break down the fat globules and ensure uniformity in mixing. This can be done before or after pasteurisation.
Chilling and Ageing follow, before freezing and incorporation of air. Slow freezing brings about large crystals and does not allow it to melt easily. After freezing it becomes solid. It should freeze at a – 50C. The incorporation of air is to increase the volume of the cream and the quality of air incorporated is called over-run.
The advantage for incorporating air is that ice-cream is in frozen state and the air allows it to melt easily when licked.
The frozen ice-cream is filled into containers and hard-frozen. The mixed ingredients pasteurised at 650C for 30 minutes and then homogenised. Fruits and nuts, if desired are added after homogenisation. The mixture is cooled to 20C while being agitated then frozen and held at this temperature for between 3 to 24 hours.
This process is called ageing and provides adequate time for the melted fat to solidify. During this period the gelatine and other stabilisers swell and combine with water. Milk protein also swells with water and the viscosity of the mix is increased. These exchanges lead to quicker whipping to desired over-run in the freezer, smoother ice-cream body and texture and slower ice-cream melt.
The cold milk is then pumped into a freezer with air incorporated into the mix. Both are chilled with the refrigerant circulating between the double walls. The mix is further frozen to -5.50C. Air is whipped in to create subdivided air cells. Air is incorporated in the cream so as to prevent it from being too dense, too hard and too cold in the mouth. The range of over-run in the cream is between 70 to 100 percent. Thus 100 percent over-run is made from equal volumes of mix and air.
Essentially, the freezing must be rapid to prevent the growth of large crystals and air cells must be small and evenly distributed. The freezing chamber is provided with a mixer to scrape the wall. The mixing and freezing are done in about 30 seconds to about 5.50C temperature. It should be noted that not all the water is frozen at this temperature. And the ice-cream obtained is semi-solid. Under this condition, it is easily pumped out of the cylinder as a continuous extrusion by the incoming unfrozen unit. The semi-solid ice-cream emerging from the freezer goes directly into the packaging cans.
In this era of increasing interest in self-employment, the production of milk products such as yoghurt, butter and ice-cream is a veritable avenue for earning millions of naira, of the various afore-stated stages are adhered to strictly. The financial implications of starting and operating these businesses will be provided later.
Ayo Oyoze Baje
