By Theresa Siebert, Petrus Britz, Pr Eng and Agrelek

Soya ice cream is a great substitute for people who suffer from lactose intolerance. It is classified as Paravine, in other words, a non-dairy frozen dessert. It resembles traditional ice cream, but contains soya protein products instead of milk protein. It is thus a dairy analogue with several health advantages including zero cholesterol, zero lactose, low calories and saturated fats.

We thank the ARC Institute for Agricultural Engineering in South Africa who made the information available to the readers of ProAgri Zambia.

Product description: Soya ice cream

Soya ice cream is prepared in the same way as dairy ice cream, but replaces the dairy cream with tofu, soya milk and/or soya isolates. Other typical ingredients include water, high fructose corn sweeteners, soya oil or vegetable oil, fruits, flavouring, soya lecithin, vegetable gums, salt and colourants. It may or may not contain egg. All other standards of identity are similar to ice cream. The quantity and quality of each ingredient used in the formulation of an ice cream mix is a crucial factor in the quality of the end product. The processing conditions play an equally important role in the success of the product.

Soya proteins:

Soya milk is the main ingredient of soya ice cream. Fortification and formulation of the soya milk is necessary to obtain 10% total solids and 4,5% protein content. A good quality soya milk is required to produce good quality ice cream. Any off flavours that may be present in the milk will also be noticeable in the ice cream. (See report on “soya milk” for preparation details.)

Sweeteners:

Generally, a mixture of dextrose, maltose and glucose is used to sweeten soya milk and to increase the solids content. Cane sugar and corn starch syrups are used. Sweeteners reduce the freezing point while it improves the volume, texture and shelf life of products.

Vegetable oils:

Vegetable oils contribute to the taste, texture and colour of the product. It is also a source of fat. Hydrogenated cocoa oil is highly saturated and resembles dairy fat very closely. Therefore, it acts in a similar way as dairy fat regarding freezing properties, mouth feel and flavour. Other common vegetable oils that can be used include sunflower oil, maize oil and soya oil. These are polyunsaturated fats and act differently in the ice cream formulation.

Stabilisers and emulsifiers:

The primary function of a stabiliser in ice cream is to reduce the formation of ice crystals during storage, thus ensuring a smooth texture during storage and product uniformity. Stabilisers also increase the yield of the ice cream by increasing and stabilising the volume of air whipped into the mix. The percentage of volume increase in the mixture is referred to as the “overrun” and varies between 70 and 100%. Examples of stabilisers used in soya ice cream include gelatine, sodium alginate, cellulose, carrageenan, locust bean gum, guar gum, and pectin. The level of stabilisers added to the ice cream mix varies between 0,2 and 0,3%. Soya lecithin is also added as an emulsifier to ensure that the oil and water (milk) soluble ingredients are mixed into a stable, homogenous dispersion (emulsion). Approximately 0,1 to 0,2% emulsifier is added to the ice cream mix. Other emulsifiers such as mono and diglycerides can also be used together with lecithin.

Flavours:

The addition of flavours of good quality is an important step. Soya ice cream comes in a wide range of flavours, including vanilla, chocolate and strawberry in the USA. A cream or butter flavour is also typical of soya ice cream.

Other ingredients:

Salt (sodium chloride) may be added to improve certain properties of the final product.

Preparation of ice cream formula

Soya milk is mixed with water and heated to 82°C before the oil and other pre-mixed ingredients are added. The high temperature aids dispersion and hydration of the various ingredients. Heating also pasteurises the mixture.

Homogenisation of ice cream formula

Homogenisation is the process whereby a fluid is forced through a very narrow opening under high pressure. It aims at breaking up fat globules, reducing them in size and thus ensuring their suspension throughout the liquid. Homogenisation prevents the separation of fat and aqueous phases and produces a stable emulsion. The ice cream mixture is homogenised at 82°C and in two phases, at 3 000 psi and 1 500 psi respectively.

Cooling and ageing of the mixture

The homogenised mixture is cooled to 4°C in a plate heat exchanger and pumped to an insulated, temperature-controlled tank. Here it is kept at 4°C for 3 to 24 hours to age prior to freezing. Various reactions take place during ageing, including the hydration of stabilisers and proteins, crystallisation of fat droplets and increased viscosity.

Flavouring of the ice cream mix

Although some of the flavours can be added to the mix during blending, volatile flavours should only be done after ageing to best preserve their aroma. Permitted colours, flavours, fruit concentrates, cocoa et cetera are mixed into the aged ice cream mix. Mixers with a gentle stirring action are used to blend all added ingredients into the ice cream mix.

Freezing of the ice cream mix

The main purpose of the first freezing process is the formation of small ice crystals and the incorporation of air into the mix. This is achieved by special scraped surface freezers with capacities of 100 to 3 800 litres per hour, which
allows for rapid freezing and the formation of small ice crystals. Rapid freezing is desirable for the texture of the final product, which is correlated to the size of the ice crystals. A large number of small crystals are desirable since they cannot be detected in the mouth and create the impression of a smooth, creamy texture.

The ice cream mix enters the freezer and is chilled by cold, compressed air that is incorporated into the product while it is being frozen to –6°C. The freezer agitates the mixture continuously with scraper blades that scrape the thin layer of frozen product off the side, while beaters mix the ice crystals with the unfrozen mixture to seed the mixture with small ice crystals. The residence time in the freezer is only about 30 seconds. The compressed air that is incorporated into the mixture increases the volume of the final product with 70 to 100%.

Packaging and hard freezing of soya ice cream

The ice cream is immediately filled into the retail containers, sealed and taken to freezer rooms. Here it is hard frozen at –20°C for six hours. Retail containers are generally plastic tubs of various shapes and sizes with resealable plastic lids and paper labels. It is very important that the packaged product should be kept at –20°C or below throughout storage, transport and display to maintain the aerated and fine crystallised texture.

Labelling of soya ice cream

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 displayed on the container.

Next month we shall look at soya milk.

Published with the acknowledgement to the ARC Institute for Agricultural Engineering for the use of their manuals. Visit www.arc.agric.za for more information.

LITERATURE SOURCES

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.
5. Marshall, RT & Arbuckle, WS. 1996. Ice Cream, 5th ed. New York: Chapman & Hall