The extrusion of textured vegetable protein is a unique process
Abstract: One of the fastest growing global segments in the food industry is the production of TVP or texturized vegetable protein. This alternative protein source provides an economical and ecological option to traditional sources, such as meat and poultry.Textured vegetable protein (TVP) is a unique extruded product that can be produced from various specifications of raw materials while controlling functional characteristics such as density, rehydration speed and time, shape, product appearance and taste.
Keywords: textured vegetable protein, extrusion, soy textured protein
Extrusion cooking is a continuous process by which many foods are produced on an industrial basis. Texturized proteins, a unique product made by extrusion, can he produced from a wide range of raw ingredient specifications, while controlling the functional properties such as density, rate and time of rehydration, shape, product appearance and mouthfeel. Raw material specifications for extrusion of texturized proteins have increased and now include: PDI ranges from 20 to 80; fat levels from 0.5 to 6.5%; fiber levels up to 7%; and particle size up to 8 mesh (2360 micron). Additional benefits of extrusion cooking are denaturing of the proteins, deactivation of heat liable growth inhibitors, control of bitter flavors and the homogeneous bonding of ingredients that may include colors, chemicals and other additives which can have an effect on appearance or textural quality.
Extrusion of texturized proteins is one of many successful applications of this unique cooking process. There are other methods utilized to produce texturized proteins including spun soy protein isolates and formed meat analogs. Spun soy protein isolates involve redissolving precipitated vegetable proteins and passing them through a spinneret into a precipitating bath. The bundles of fibers resulting from this are compacted, shaped, flavored, cooked and/or dried and packaged. Formed meat analogs are blends of various protein sources such as isolates, glutens, albumin, extrusion-cooked vegetable proteins and others which are blended with oils, flavors and binders before forming them into sheets, patties, strips or disks.
Extrusion-cooked texturized proteins include meat extenders in the form of chunks or small granular pieces which are wet milled or produced directly off the extruder. Extruders also are able to produce a meat analog tha t has a remarkable similarity in appearance, texture and mouthfeel to meats. The utilization of extrusion cooking throughout the food industry has shown that a variety of products can be made on extrusion equipment. Some of these products include breakfast cereals, breadings, snacks, instant rice, instant pasta, starch modifications, animal and aquatic feeds.
Traditionally the most popular raw material for production of textured vegetable proteins in an extrusion system has been slightly toasted defatted soy flour. This defatted soy flour usually meets the following characteristics: 50% protein minimum, 3.5% fiber maximum, 1.5% fat maximum and PDI of 60 to 70.
Soy flour with these specifications allows controllable production of textured proteins in chunk and extended form on single screw extrusion cookers. Other vegetable protein sources also have been used as raw materials for texturizing, and these include: glandless cottonseed flour, rape seed or canola concentrates, defatted peanut flour, defatted sesame flour, as well as soybean grits, flakes, meal, concentrates and isolates.
The use and development of twin screw extrusion cookers in the field of texturized proteins has increased the raw material specification range to include raw materials that include: PDI ranges from 20 to 70, fat levels from 0.5 to 6.5%, fiber levels up to 6% and particle sizes up to 8 mesh.
Protein dispersibility index. The Protein Dispersibility Index (PDI) is the percentage of total protein that is dispersible in water under controlled conditions of extraction. The PDI is now the preferred measurement with regard to specification of raw ingredients, as it is a more reliable figure when compared with NSI. Textured soy products have been produced with raw materials ranging from 20 to 70 PDI.
Fat level. Raw materials have been texturized containing 0.5 to 6.5% fat levels. This higher range of fat (5.5%) allows mechanically extracted soybean meal to be texturized into meat extenders and meat analogs. When extruding material with higher fat levels, generally it can be said that increased shear energy input and higher temperatures are required to maintain a desired product integrity.
Fiber level. The presence of fiber in extruded soy proteins inhibits or blocks the interaction or cross-linking of protein molecules necessary for good textural integrity. Changing the extruder configuration to impart more shearing action into soy proteins containing higher levels of fiber may achieve a final product with textural properties similar to soy protein with lower levels of fiber.
Particle size. With regard to successful production using single screw extrusion cookers, the exact limitations of particle size requirements of defatted soy flour have never been determined and the range is very wide. The only limitations encountered are as follows: Very fine flour, below 400 mesh (38 micron), should be limited because of problems in wetting this very fine powder without lumping; also, very coarse product, over 80 mesh (180 micron), requires complicated pre- moistening, and sometimes, whole granules are seen in the finished product. We, therefore, recommend a product grind of 95% through 100 mesh (150 micron), with a maximum of 50% through 325 mesh (45 micron) for defatted soy flour.
Twin screw extruders can, sometimes use raw material with a particle size range up to 8 mesh (2360 micron) without affecting the textural properties of the final product.
It is also believed that proper preconditioning has a great effect on the ability of the extrusion cookers to utilize the larger particle size raw materials for textured protein production. Preconditioning is a time, temperature and moisture level relationship and controlling variables allows for the soy flour or grits to be evenly pre-moistened and pre-tempered.
Protein levels. The percentage of protein is normally inversely related to the remaining constituents of a raw material such as fat and fiber. For example, soybean protein content goes up as the oil and hulls are removed. Therefore, as a protein level of a raw material decreases, the textural integrity and water holding capacity decrease and the bulk density of the final products increases.
Adjustments in pH. Increasing the pH of vegetable proteins before or during the extrusion process will aid in texturization of the protein. Extreme increases in pH will increase the solubility and decrease the final product (1) Modifying pH above 8.0 also may result in the production of harmful lysinoalanines; (2) Lowering the pH has the opposite effect and will decrease protein solubility, making the protein more difficult to process. Undesirable sour flavors in the texturized vegetable protein products may be evident, if the pH is adjusted below 5.0.
Modifying the pH to the alkaline side will increase the water absorption. This is generally done by using calcium hydroxide or the more widely used sodium hydroxide at about 0.1% or as required.
Calcium chloride. Calcium chloride (CaCl2) is very effective in increasing the textural integrity of extruded vegetable proteins and also aids in smoothing its surface. Dosing levels for CaCl2 range between 0.5 and 2.0%. With the addition of CaCl2 and small amounts of sulfur, soybean meal containing 7.0% fiber may be texturized, retorted for one hour at 110°C and still maintain a strong meat- like texture. Sodium chloride (NaC1) does not appear to add any benefit to the texture of extruded vegetable proteins. In fact, it tends to weaken the textural strength.
The addition of sodium alginate will increase chewiness, water- holding capacity and density of extruded protein products. Sugar will also disrupt the textural development of soy proteins (3).
Soy lecithin. When added to formulations of vegetable proteins at levels up to 0.4%, lecithin tends to assist smooth laminar flow in the extruder barrel and die configuration, which allows the production of increased density soy products. The ability to make dense vegetable protein products is related to the higher degree of cross-linking that occurs during the extrusion process.
Sulfur and sulfur containing ingredients. Sulfur is known for its ability to aid in the cleavage of disulfide bonding, which assists the unraveling of long twisted protein molecules. This reaction with the protein molecules causes increased expansion, smooth product surface and adds stability to the extrusion process. These benefits, however, are not without some undesirable side effects that include off- flavors and aroma.
Sodium metabisulfite, sodium bisulfite, as well as cystine, can be used with effects similar to those from using sulfur.
The normal dosing levels for sulfur or sulfur derivatives – are in the 0.1 to 0.2% range. Cystine is used at approximately 0.5 to 1% level.
Color enhancers. When supplementing light colored meats with meat extenders made from textured vegetable proteins, it is desirable to bleach or lighten the color of the extruded meat extender. Bleaching age nts such as hydrogen peroxide are often used for this purpose. Dosing levels for the hydrogen peroxide range from 0.25 to 0.5%. Pigments such as titanium dioxide are also used at levels between 0.5 and 0.75% to lighten color, but at increased levels will weaken the textural properties of extruded vegetable proteins.
The above raw materials or combinations are generally mixed prior to the extrusion cooker, except in the cases where small liquid additions can be added directly to the base raw materials in the extrusion cooker itself.
Definition and functions of extrusion of textured proteins. Extrusion cooking has been defined as “the process by which moistened, expansile, starchy and/or proteinaceous materials are plasticized in a tube by a combination of moisture, pressure, heat and mechanical shear. This results in elevated product temperatures within the tube, gelatinization of starchy components, denaturization of proteins, the stretching or restructuring of tractile components and the exothermic expansion of the extruder” (4). Extrusion is widely used to accomplish this restructuring of protein-based foodstuffs to manufacture a variety of textured convenience foods. When mechanical and thermal energy are applied during the extrusion process, the macromolecules in the proteinaceous ingredients lose their native, organized structure and form a continuous, visco-elastic mass. The extruder barrel, screws and die align the molecules in the direction of flow. This realignment “exposes bonding sites that lead to crosslinking and a reformed, expandable structure’ that creates the chewy texture in fabricated foods (5).
In addition to retexturing and restructuring vegetable food proteins, the extrusion cooking system performs several other important functions: It denatures proteins. Proteins are effectively denatured during the moist, thermal process of extrusion. Denaturation of protein “lowers solubility, renders it digestible and destroys the biological activity of enzymes and toxic proteins” (6). It causes deactivation of residual heat- labile growth inhibitors native to many vegetable proteins in a raw or partially processed state. Growth inhibitors exert a deleterious physiological effect on man or animals, as revealed by growth or metabolism studies. It controls raw or bitter flavors commonly associated with many vegetable food protein sources. Many of these undesirable flavors are volatile in nature and are eliminated through the extrusion and decompression of the protein at the extruder die. The use of preconditioning and atmospheric-venting devices in the design of the extrusion system also assists in volatilization and removal of off- flavors. It provides a homogeneous, irreversible, bonded dispersion of all microingredients throughout a protein matrix. This not only insures uniformity of all ingredients such as dyes throughout the product, but provides a means whereby minor ingredients can be intimately associated with potential reaction sites promoting cross-linking or other desirable che mical and physical modifications. And, it controls the shape and size of the final extruded protein in convenient and transportable portions for packaging in the retail or institutional marketplace.