FOOD CHEMISTRY

Module 2. Food proteins

Lesson 7
THEORIES OF FORMATION OF TEXTURIZED PROTEINS

7.1 Introduction

Texturization (when applied to plant proteins) is the development of a physical structure which will provide, when eaten, a sensation of eating meat. Meat "texture" is a complex concept comprising visual aspect (visible fibers), chewiness, elasticity, tenderness and juiciness. The principal physical elements of meat which create the texture complex are: the muscle fibres and the connective tissue. Many plant proteins have globular structure. Texturization confers a fiber-like structure to globular proteins. Suitable processes give the protein chewiness and good water holding property, cooking strength and a meat-like structure. These products have an ability to retain these properties during subsequent hydration and heat treatment. These texturized proteins are often used as meat substitutes, extenders and meat analogues. Commercial textured vegetable protein products are manufactured almost exclusively from soy protein.

7.2 Process of Texturization

The more successful approaches to plant protein (soy protein) texturization can be classified in two categories. The first approach tries to assemble a heterogeneous structure comprising a certain amount of protein fibers within a matrix of binding material. The fibers are produced by a "spinning" process, similar to that used for the production of synthetic fibres for the textile industry. The second approach converts the soy material into a hydratable, laminar, chewy mass without true fibres. Two different processes can be used to produce such a mass: thermoplastic extrusion steam texturization. During texturization, globular proteins are unfolded by the breaking of intra-molecular binding forces. The resultant extended protein chains are stabilized through interactions within the neighbouring chains.

7.2.1 Spin process/fiber spinning

In this method the starting material should contain 90 % or more protein (protein isolate). The molecular weight of proteins should be in the range of 10-50 kdal. Proteins of less than 10 kdal are weak fiber builders, while those higher than 50 kdal are disadvantageous due to their viscosity and tendency to gel in the alkaline pH range. The major steps involved are

(1) A solution of high protein concentration (10-40%) is prepared. The viscous concentrated protein solution is technically known as dope. The protein is then solublized by addition of alkali by raising pH of the dope to about 10. The dope is aged at this high pH with continuous stirring. At such a high pH, the electrostatic repulsions promote complete dissociation of the proteins into sub units and also causes extensive unfolding of the individual polypeptide chain. All this results in high viscosity. However, prolonged exposure to the high pH should be avoided to minimize the loss of sulfur containing amino acids and to avoid formation of potentially toxic degradation products.

(2) The dope is then pressed through a die-plate containing a thousand or more holes each with a diameter of 50-150 μm. As the dope flows through these holes, streaming orientation of the unfolded protein molecules takes place. Thus the molecules tend to extend and align themselves in a parallel manner.

(3) The liquid filaments coming out of the die enter a coagulation bath at pH 2-3. This bath contains an acid (acetic, citric, phosphoric, lactic, or hydrochloric) and usually 10 % NaCl. Here the proteins are coagulated by the iso-electric pH and by salting-out effect. Because of their elongated, parallel orientation the protein molecules of each filament interacts strongly with each other through hydrogen, ionic and disulfide bonds, to form a hydrated protein fiber.

(4) The coagulated protein fibers are removed from the bath on rollers, in a winding-up state. The speed of the roller is such that the fibers on the rollers get stretched and as a result the individual polypeptide chains achieve still better alignment, associate more closely and form more intermolecular bonds. This increases the mechanical strength and chewiness of the fiber, but may decrease their water holding capacity.

(5) The fibers are then compressed with /without heating between rollers to remove some water, promote adhesion and increase toughness. The bundles are then placed in a neutralizing bath (NaHCO₃ and NaCl) at pH 5.5 to 6.0. Sometimes it may also be placed in a hardening bath of NaCl.

(6) Additional treatments involve passage of the fibers through a bath containing a binder and other additives such as aroma compounds and lipids. This improves the thermal stability and aroma.

(7) Finally, the soaked fiber bundles are heated, cut, assembled and compressed. These fibers are similar to those found in meat and the texture of products containing spun fibers resembles meat.

7.2.2 Extrusion method/thermoplastic extrusion

This is the major technique used at present for texturization of vegetable proteins and is also referred to as thermoplastic extrusion. It leads to the formation of dry, fibrous, porous granules or chunks, which possess a chewy texture upon rehydration. The starting material for these processes need not be a protein isolates. Thus, less costly protein concentrates or flours (containing 45-70% protein) can be used. The addition of small amounts of starch or amylose improves the final texture but a lipid content of above 5-10% is detrimental. Up to 3% NaCl, CaCl₂ may also be added to improve the texture. The major steps involved are-

• The moisture content of the starting material is adjusted to 30-40% and the additives are incorporated.
• The protein mixture is fed to the extruder where it is exposed to a high pressure(10,000 to 20,000 kPa)
• Over a period of 20-150 s, the mixture is elevated to a temperature of 150-200°C . Under these conditions, the mixture is transformed into a plastic viscous state, in which solids are dispersed. Hydration of the proteins takes place after partial unfolding of the globular proteins followed by stretching and rearrangement of the protein strands along the direction of mass transfer. The thermal coagulation of proteins may also occur.
• The mixture is then extruded through a small diameter orifice into normal pressure environment. This results in flash evaporation of the internal water with the formation of expanding steam bubbles leaving behind vacuoles in the protein chunks.
• After cooling, the protein polysaccharide matrix possesses a highly expanded dry structure. The porous material is able to absorb 2 to 4 times its weight of water giving a fibrous, spongy structure with chewiness like meat. These products are stable even under sterilization conditions.