Soy milk has been part of the Asian diet for centuries and is becoming increasingly popular in other parts of the world too as it is being perceived as a healthy alternative to cow’s milk. However, as soy milk has traditionally been produced in small batches in small shops and has a short shelf-life, manufacturers need to look at new production methods if they want to match the needs of today’s convenience seeking consumers.
The use of ultra-high temperature (UHT) technology can help create products with shelf-lives ranging from several months up to a year. In order to manufacture homogenous products with the right mouthfeel and creaminess, the right emulsifiers and stabilisers must be used.
Soy beverages have a history of over 5,000 years and are an important source of protein for many people, especially those who are lactose intolerant. Soy milk contains the same amount of protein as cow’s milk and therefore, is often perceived as an alternative to dairy milk. Besides being a good source of protein, soy based foods are also known to provide numerous health benefits, such as:
- Lower fat content, mostly unsaturated fat with zero cholesterol
- Naturally lower sugar content compared to regular milk
- Only contain vegetable protein
- Fewer people allergic to soy than cow’s milk
- Promote bone health
- Promote weight loss/control
- Prevent cancer
- Prevent cardiovascular diseases
Countries such as China, Japan, Brazil, the US and Thailand consume vast amounts of soy-based products and are likely to continue to do so. Market research firm Euromonitor International has projected very promising growth for soy milk and drinks from 2013-2018. In 2013 alone, soy drinks had a market size of US$2.6 billion, of which, the market size of soy milk was US$4.81 billion.
Hung Chieh Tsai, Taipei, Taiwan
Soy milk is the water extract from whole soy beans. It is an emulsion of fat in water containing water soluble proteins, carbohydrate and oil droplets. Soy milk has traditionally been produced by small shops daily in small batches. These are sold daily and have a short shelf-life. The soy beans are usually soaked, grinded, filtered and cooked to produce soy milk.
Nowadays, a large proportion of industrially produced soy milk is made by means of the UHT process and aseptic filling which will provide a longer shelf-life at room temperature. This puts a great demand on the stability of the product to remain homogeneous throughout its entire shelf-life to gain consumers’ acceptance.
This can be done through direct as well as indirect UHT systems. From a stability as well as sensory point of view, direct systems can provide better results. Direct systems result in less chemical changes in the product due to lower total heat load and extraction of oxygen from the product in the vacuum chamber. However, indirect systems are usually used due to lower investment and operating costs.
Soy drinks are popular beverages in countries such as China, Brazil, Mexico, Malaysia and Argentina. They contain no less than 1.5 per cent protein, less than 0.5 percent fat and 5-15 percent sugar. Unlike soy milk (3 percent protein or more, 1 percent fat or more) which is known as a non-dairy milk alternative, soy beverages are recognised as sweet and nutritious drinks that are consumed as soft drink.
Whichever UHT system is used, the application of emulsifiers and stabilisers in soy milk and soy drinks is necessary. These products are emulsions of oil in water, and to ensure optimal emulsion stability, ie: reduce creaming during the entire shelf-life, proper homogenisation and addition of emulsifiers and stabilisers are necessary. Furthermore, stabilisers can improve the stability of the proteins, thereby minimising sedimentation during the storage of the soy products.
Emulsifiers For Soy Beverages
Mono- and di-glycerides of fatty acids are commonly used in soy beverages. Mono- and di-glycerides are emulsifiers produced by the reaction of vegetable fats and oil and glycerol. The end result is a molecule with ambiphilic properties, meaning part of the structure is hydrophilic while other moieties have a lipophilic nature and consequently, it is placed at the interface between the fat/protein and water during homogenisation, which helps to reduce fat separation.
When used in multiphase systems, the emulsifier will take a position which is favourable with respect to energy and minimise coalescence of the fat globules.
Stabilisers For Soy Beverages
Stabilisers are water –soluble polysaccharides extracted from land or marine plants or from microorganisms. Adding stabilisers helps create the network required to suspend particles, increase viscosity, and improve mouthfeel. They are used for their thickening and stabilising properties.
There are many types of stabilisers available which are unique in their own way:
Carrageenan, which is derived from red seaweed, is commonly used as a stabiliser in soy milk. Carrageenan forms a helix with the negatively charged sulphate group. Carrageenan can be classified into kappa, lambda and Iota carrageenan based on their chemical composition.
In order to achieve the best results through carrageenan application, the filling temperature has to be less than 25 degree C in order to form a carrageenan network. This network will lose its gel strength if it is subjected to high temperature.
Microcrystalline cellulose (MCC) is becoming increasingly popular as a stabiliser in chocolate beverages. MCC is derived from plant fibres and is often used in combination with carrageenan for synergistic effects. With shear, MCC forms a three-dimensional matrix and is not affected by temperature.
When suspension of particles is required, gellan gum is often used. Gellan gum is produced through fermentation of algae. There are two types of gellan gum, high acyl gellan gum and low acyl gellan gum. Gellan gum form gels in which particles are trapped.
Pectin, mainly extracted from citrus peels, is often used in low pH beverages. They are used as a gelling agent, thickening agent and stabiliser in food.
Challenge Of Chocolate Soy Drinks
The challenge in getting a stable and uniform soy product lies in choosing the right emulsifier and stabiliser blend (E/S), especially in chocolate soy beverages.
Underdosing/unsuitable E/S will cause sedimentation whereas overdosing will cause separation and/or gelation. Numerous factors have to be taken into consideration when using any E/S blends: the percentage of total solid, percentage of fat, percentage of protein, and percentage and type of cocoa powder used all have some effects on the end product.
Besides, the type of heat treatment plays a part too. Pasteurisation, UHT and retort are the three common types of heat treatments used on soy beverages. The lower the amount of heat used in pasteurisation, the shorter the shelf-life of the product. Retort is the harshest heat treatment to soy beverage. The dosage of E/S needs to be adjusted according to the type of heat treatment used.
Creating a stable chocolate soy beverage is more difficult than chocolate milk as it contains more fibres. By using a standard carrageenan system, the network will not be good enough to hold the cocoa particulates. A microcrystalline cellulose or gellan gum system will help in creating the network required for such application.
Other factors such as filling temperature will also have an effect on the stability of the end product. There are E/S solutions available in the market for filling temperatures below and above 30 degree C. In addition to neutral pH soy beverages, acidified soy drinks with a low pH of around four can also be created.
A refreshing soy drink can be flavoured with a juice concentrate such as orange, apple and mango depending on local taste and preferences. As it is a low pH beverage, the heat treatment must be set to 110 degree C/4s, as low pH and high heat treatment will cause the protein to be precipitated out.