By Theresa Siebert, Petrus Britz, Pr Eng and Agrelek
Soya yoghurt (soghurt) is a cultured soya milk product produced by fermenting the fortified soya milk with a mixed culture of Lactobacillus bulgaricus and Streptococcus thermophilus. The unflavoured product has a soft, fragile, custard-like consistency and a distinct acidy flavour. The composition of formulated soya milk is very similar to dairy milk and therefore the manufacturing of soya yoghurt is the same as dairy yoghurt. Soya yoghurt is also described as a dairy analogue.
We thank the ARC Institute for Agricultural Engineering in South Africa who made the information available to the readers of ProAgri Zambia.
There are many different types of dairy yoghurt on the market, including stirred, set, sweetened and fruit yoghurts. Yoghurt may also be frozen to produce a product similar to ice cream.
Soya yoghurt can also be prepared in different forms and flavours. The texture of the yoghurt is determined by the soya milk protein and solids content. The richer the soya milk, the firmer the final yoghurt will be. This report describes a flavoured set soghurt.
The soya yoghurt production process
Soya milk preparation
Good quality soya milk is required to produce good quality yoghurt. Any off-flavours that may be present in milk will also be noticeable in the yoghurt. Cooking, formulation and homogenisation are necessary steps in the production of soya milk intended for yoghurt. The addition of specific sugars to soya milk is crucial for culture development and growth. A combination of dextrose and sucrose is usually added. The addition of lactose is optional.
Soya milk is naturally lactose-free and if no lactose is added, the yoghurt is safe for consumers with lactose intolerance. Soya milk is fortified with soya isolates to obtain a solids content of 7,5% and protein content of 3,8%. In part 8 and 9 in ProAgri Zambia 32 and 33 we discussed the preparation of soya milk.
Preparation of starter cultures
Starter cultures are prepared by mixing commercial freeze-dried cultures with a small quantity of concentrated milk at 45°C. The milk is concentrated by heating for one hour in a boiling water bath followed by a cold-water bath to cool the milk to the required temperature. The added 3% starter culture and milk are mixed thoroughly before being incubated at 41°C for five hours. The prepared (active) starter culture may then be stored under refrigerated conditions until it is used, but it must be used within two days.
Sweetening of milk
The milk is sweetened with 4% sucrose and 3% dextrose and stirred continuously until the sugar has dissolved.
Heating of soya milk
A small batch of soya milk (less than 2 500 litres per day) is heated in an open steam-jacketed kettle equipped with an agitator. The milk is held at 80 to 85°C for 30 minutes to pasteurise it and to modify the proteins so that they will provide the proper viscosity and gelation with minimal syneresis (extraction of liquid) in the product. Large processors use continuous high temperature short time (HTST) technology.
In this process, the milk is quickly heated in a plate heat exchanger to 90 to 95°C and held for 15 to 40 seconds before being cooled.
Cooling of the milk
The milk is cooled to 50°C in the same jacketed kettle or plate exchanger by replacing the steam with cold water. The milk is then pumped into fermentation tanks.
Inoculation of milk
The active starter culture is added to the milk at a dosage level of approximately 3% and mixed well without incorporating air that could lead to foaming.
Flavouring of milk for set yoghurt
A wide variety of flavoured soya yoghurts can be manufactured. Permitted flavouring, colouring and sweeteners may be added and incorporated with batch blenders. A cream flavour (0,01%) is a very popular variety for soghurt. A layer of pasteurised fruit may be added to the packaging container prior to filling with soghurt. Care must be taken to ensure that the acids in the fruit do not destroy the yoghurt texture at the interface.
Packaging of soghurt
Set yoghurt is fermented in the final retail container and therefore packaging takes place immediately after inoculation and flavouring. Suitable packaging containers include preformed plastic tubes sealed with foil films or plastic snap-on caps. Snap-on lids require a safety collar to prevent tampering and spilling during transport and distribution.
Fermentation of set soghurt
Fermentation is the conversion of the sugar in the milk to simpler substances such as acids by the action of the starter culture. The sealed containers are incubated for three to four hours at 40 to 44°C (or until the pH reaches 4,5). Small processors can use water baths to achieve fermentation of the product. The water level should be maintained to just below the neck of the container. Small, insulated fermentation chambers with forced air circulation that can accommodate 250 to 750 litres of yoghurt are also used for batch fermentation.
Cooling and storage of set yoghurt
Set yoghurt is cooled to below 15°C to inhibit further culture activity. Batch cooling is performed by simply placing the containers in blast air cooler tunnels. The cooler capacity is of great importance. The set yoghurt should be cooled to 35°C within 30 minutes and further cooling to 15°C should
be accomplished within another 30 to 40 minutes. The containers are then placed in cold storage rooms for storage at 4 to 5°C.
Labelling of yoghurt
The containers should be correctly labelled according to the Labelling and Advertising Regulations. All added ingredients must be listed. The storage requirements and expiry date of the product should be clearly printed.
Next month we shall introduce the processing of sunflower seeds and specifically the production of sunflower meal.
Published with acknowledgement to the ARC Institute for Agricultural Engineering for the use of their manuals. Visit www.arc.agric.za for more information.
1. Applewhite, TH. 1989. Vegetable Protein Utilization in Human Food and Animal Food Stuffs.
2. Lui, KeShun. 1999. Soybeans: Chemistry, Technology and Utilization. Gaithersburg: Aspen Publishers, Inc.
3. Snyder, HE & Kwan, TW. 1987. Soybean Utilization.
4. Tanteeratarm, K. 1992. Soybean Processing for Food Uses.