Module 5. Oilseed processing
Lesson 18
PROCESSING OF SOYBEAN AND OTHER
OILSEEDS; DAIRY ANALOGUES
Most of the unit operations involved in processing of oil seeds are discussed in the lesson no. 17 (Fig.17.1). In this lesson special requirements for processing of soybean and other oilseeds are discussed.
18.1 Soybean
Soybean can be processed at 12% moisture if not dehulled to make 44% protein content meal. In earlier years, in order to make high protein content soybean meal, hulls were loosened by drying the seeds from 13% moisture to 10% moisture at 65°C, cooling and holding for 1-5 days before dehulling. The hot dehulling process was developed to eliminate double heating of soybean. Soybean at 13% storage moisture are heated to 60°C during a period of 20-30 minutes to allow the moisture to migrate to the surface. Then they are rapidly heated to surface temperature as high as 85°C to loosen the hulls. During this process moisture is reduced by 1-3%. The soybeans are then cracked into six to eight pieces and flaked at 60-65°C and 10-11% moisture.
The phospholipase enzyme activity can be minimized by the following: drying seeds adequately before storage, avoiding breakage in handling, moving seed rapidly from dehulling to extraction with minimum addition of water, operating outside the optimum activity temperature range of the enzyme.
Soybean oil is the main source of commercial lecithin harvested by a degumming operation before alkali neutralization.
18.2 Rapeseed/Canola
The majority of rapeseed/canola oil recovery is by press-solvent extraction, with about 60% of the total oil removed in the first operation. Seed often is flaked at 7.0-9.5% moisture in two stages: to 0.4-0.7 mm thickness in the first and to 0.2-0.3 mm in the second.
Cooking reduces moisture content to 5-6% for hard pressing, denatures the protein and enhances coalescing of minute oil droplets into larger ones, inactivates the enzyme myrosinase (thus preventing hydrolysis of glucosinolates and reducing sulphur content of the oil), inactivates phospholipases and prevents development of monohydratable phosphatides in the oil.
18.3 Corn germ
The distribution of oil in corn is approximately as follows: germ,83%; endosperm, 15%; bran, 1.3%; tip cap, 0.7%. Most corn germ is a by-product of starch and corn sweeteners production by wet milling or alcohol fermentation, and contains 44-50% oil when dried to 2-4% moisture content. Dried wet milled germ is moistened to 8%, heated to 90-105°C to soften its intracellular structure, flaked to 0.10-0.13mm in thickness, prepressed and solvent extracted to 0.5% residual oil.
18.4 Cottonseed
Cottonseed requires special handling techniques. Flaked cottonseeds are extruded through an expander at 13% moisture and 100°C. The objective of flaking is to rupture both the spheresomes containing the oil and glands containing gossypol. The flaked cottonseed material is held hot for 20 minutes, retextured at 113°C, dried/cooled to 11% moisture at 57°C before solvent extraction.
The residual oil content in cake from a press is in the range of 4.0-6.5%. Solvent extracted meal contains 0.5-0.7% residual oil.
18.5 Peanut
The processing of peanut poses special problems. They mature in soil, and additional efforts are needed to remove small stones and sand to reduce wear on machines. Aflatoxin contamination is a potential and must be monitored constantly.
Shelled peanuts contain 50% oil. Generally peanuts are processed by full press or prepress solvent extraction. To provide traction in the screw press, peanuts are cooked to 5% moisture content. If only a prepressing is given, the cake is crumbled and solvent extracted.
Direct solvent extraction process uses two stages:
18.6 Sunflower Seed
Sunflower seed is the only major raw crop oilseed that does not have identified antinutritional factors requiring inactivation during processing. Oil type seeds are black in colour and consist of 20% hull and 40-45% oil. The seed clings to hull, which is thin, flexible and difficult to remove. Typically, oil-type sunflower seed are dehulled to the desired fiber content, heated, flaked, prepressed and than solvent extracted.
18.7 Vegetable Protein Concentrates and Isolates
Due to high protein content and ease of extraction, soybean is widely used to obtain protein for special food uses. Soy protein is made from dehulled, defatted soybean meal. Dehulled and defatted soybeans are processed into high protein commercial products such as soy protein concentrates and soy protein isolates. Soy protein is used in a variety of foods such as salad dressings, soups, imitation meats, beverage powders, cheeses, non-dairy creamer, frozen desserts, whipped topping, infant formulas, breads, breakfast cereals, pastas, and pet foods. Soy protein is used for emulsification and texturization in various products. Specific applications include as adhesives, asphalts, resins, cleaning materials, in cosmetics, inks, leather, paints, paper coatings, pesticides/fungicides, plastics, polyesters, and textile fibres.
18.7.1 Production methods
Edible soy protein isolate is derived from defatted soy flour with a higher solubility in water. The aqueous extraction is carried out at a pH below 9. The extract is clarified to remove the insoluble material and the supernatant is acidified to a pH range of 4-5. The precipitated protein-curd is collected and separated from the whey by centrifugation. The curd is usually neutralized with alkali to form the sodium proteinate salt before drying.
Soy protein concentrate is produced by immobilizing the soy globulin proteins while allowing the soluble carbohydrates, soy whey proteins, and salts to be leached from the defatted flakes or flour. The protein is retained by one or more of several treatments:
All of these processes result in a product that contained ≥70% protein, 20% carbohydrates (2.7 to 5% crude fiber), 6% ash and about 1% residual fat, but the solubility may differ. One tonne of defatted soybean flakes will yield approximately 750 kg of soybean protein concentrate
18.7.2 Product types
18.7.2.1 Isolates
Soy protein isolate is a highly refined or purified form of soy protein with a minimum protein content of 90% on a moisture-free basis. It is made from defatted soy flour which has had most of the non-protein components, fats and carbohydrates removed. Because of this, it has a neutral flavour and will cause less flatulence due to bacterial fermentation.
Soy isolates are mainly used to improve the texture of meat products, but are also used to increase protein content, enhance moisture retention, and as an emulsifier. Pure soy protein isolate is used mainly by the food industry. It is available in health stores or in supermarket as pharma food. It is usually used in combination with other food ingredients.
18.7.2.2 Concentrates
Soy protein concentrate must contain not less than 70% soy protein and is basically defatted soy flour without the water soluble carbohydrates. Soy protein concentrate retains most of the fiber of the original soybean. It is widely used as functional or nutritional ingredient in a wide variety of food products, mainly in bakery products, breakfast cereals, and in meat products. Soy protein concentrate is used in meat and poultry products to increase water and fat emulsification and to improve nutritional values (more protein, less fat).
Soy protein concentrates are available in different forms: granules, flour and spray-dried. Because of their higher digestibility, they are well-suited for children, pregnant and lactating women, and the elderly. They are also used in pet foods, as milk replacer for infants (human and livestock), and even in some non-food applications.
18.8 Dairy Analogues
The rising demand by the consumers for healthy food has led to the alternative demand and development of dairy-like products, such as soy-based analogues. Soybean can be processed into many soy products which are analogous to dairy products such as soy beverage (soy milk), tofu (soy paneer), soy yoghurt.
18.8.1 Soy milk
Soy milk/soy beverage is a water soluble extract from whole soy beans. It is an off-white emulsion or suspension containing water soluble proteins, carbohydrates, and lipids. It resembles dairy milk in appearance. However, it is lactose free and represents an alternative to dairy milk.
Commercial soy beverages can be classified according to their composition such as: high solids soy milk (bean to water ratio of 1:5), dairy like soy extract (bean to water ratio of 1:7) and lower solids soy beverage (bean to water ratio of 1:20). Depending upon the processing parameters and water to soybean ratio, soymilk would have a typical solids content around 8-10%. Within this, protein is 3.6%, fat 2.0%, carbohydrates 19.9% and ash 0.5%.
Soy milk is healthy drink as it is cholesterol and lactose free. It also contains phytochemicals, which has proven health benefits.
18.8.2 Tofu
Tofu is a high protein food made from soybeans. It is used as meat or cheese substitute. It is sold as ready to eat cakes that resemble paneer or soft white cheese. The preparation of tofu involves extraction of soymilk and then coagulation of this extract to form curd. The curd is then pressed to form tofu cakes.
Typical wet composition of tofu is 85% moisture, 7.8% protein, 4.2% lipid. The remaining constituents are carbohydrates and minerals. The typical dry composition is made up of 50% protein, 27% fat and 23% carbohydrates and minerals.
Tofu can be categorised as silken or pressed tofu. Silken tofu production involves soy extract being finely filtered and heated before cooling to a temperature of 65-70°C. Calcium sulphate/magnesium chloride of low concentration is added to the extract. A fine, smooth and firm curd forms after 30-60 minutes. This curd is left unbroken.
In case of pressed tofu, coagulant is vigorously stirred into hot soy extract. The curd is broken and pressed. Pressed tofu contains about 22% protein and 61.6% moisture.
18.8.3 Soy yoghurt
While milk based yoghurt has long been consumed in many countries, soy yoghurt, also known as soghurt, is a relatively new product. It is produced through the fermrntation of soymilk by different cultures of bacteria to form soft, fragile, custard like texture, generally containing 12-14% total solids and possessing a clean tart flavour.
There are several types of soghurts. Soghurt can be produced in the form of a highly viscous texture, a softer gel or in frozen form as a dessert or drink. Generally they can be classified as: set type soy yoghurt, stirred type and drink type.