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Terms relating to mouthfeel characteristics

Mouthfeel

Watery

Creamy

Getaway

Juicy

Mushy

Body

Oil

Astringent

Dry

Greasy

Hot

Moist

Waxy

Cold

Wet

Slimy

Cooling

3. Viscosity of extrudate paste is measured using amy-lographs.

4. The extrudate dimension, particularly for snack foods, is another important characteristic. It is often referred to as the expansion ratio or the puff ratio and is defined as the ratio of the area of cross section of the extrudate to the area of cross section of the die. In this article, the swelling of extrudate due to elastic and moisture effects is referred to as extrudate expansion. Extrudate swell is referred to as swelling from all effects other than moisture. Puffing is referred to as the swelling believed to be predominantly from the moisture effect.

5. Water absorption index (WAI) is the weight of sediment formed per unit mass of sample, after the sample is centrifuged and the supernatant is removed. Water absorption capacity (WAC) is the ratio of the weight gained over the original weight after a known mass of powdered extrudate is soaked in water for a fixed period of time at a given temperature. Water solubility index (WSI) is the percentage of the initial sample present in the supernatant obtained from the WAI test.

The methods used to obtain these properties are not standardized. Because different methods have been used to obtain a property by the same name, procedures used to determine the properties should be reviewed before drawing conclusions.

The textural properties of the extrudate are affected by the process variables, including temperature, screw speed, screw and barrel dimensions, and product variables (eg. moisture content, composition, and particle size). The properties are evaluated as a function of different parameters, the most common of which includes product moisture, process temperature, and extruder screw speed. Other parameters, some of which may be interrelated, include residence time, die dimensions, screw dimensions, shear rate or shear stress, and product composition. When a number of process conditions are to be evaluated at different levels, the experimental size could be very large. To reduce the experiments to a manageable scale without increasing the experimental error, the response surface methodology (RSM) has been used to develop relationship between dependent and independent variables. According to Meyers (8), "response surface procedures are a collection involving experimental strategy, mathematical methods, and statistical inference which, when combined, enable the experimenter to make an efficient empirical exploration of the system in which he is interested." The RSM helps in finding an approximate function to enable one to predict future response and to determine conditions of optimum response.

In the RSM approach, the entire system is modeled as a black box. The mathematical functions that are developed to correlate the dependent and independent variables are empirical in nature. An advantage of this approach is that a detailed understanding of the changes that occur during the process is not necessary. The disadvantage is that a general relationship is not available, which requires studies for different machines and different formulations of raw material.

Proteins

The basic constituents of all proteins are amino acids. The linear protein chain consists of amino acids linked together by peptide bonds between amino and carboxylic sites. Proteins can have different levels of structure—primary, secondary, and tertiary—that result in a three-dimensional shape. Texturization of proteins can be used to produce two types of products (9). The first are meat analogues that are sold in place of meat. Such products have found acceptance in some countries, such as Japan (10). The second type of textured plant proteins (TPP) is as a meat extender. Several reviews on protein texturization using extruders can be found (9,11-15). During the extrusion process in the presence of water, globular proteins and aluerone granules unravel and align themselves in the presence of the shear field that exists in the extruder (14). Thermal denaturation (loss of native structure) is also expected to occur and is thought to be necessary for texturization (9), although some investigators contend that denaturation is not a necessary step in the texturization process (16).

The mechanism of texturization has been the subject of intense research. It is generally agreed that intermolecu-

lar disulfide bonding of proteins is responsible for the structure formation of spun soy fiber (17). It has been reported that the presence of heat and pressure cause the protein molecules in defatted soy meal to disassociate into subunits and become insoluble (18). The mechanism of bond formation has been investigated during extrusion (19). Results indicated that disulfide bonding had no role in extrusion texturization, suggesting that intermolecular amide bonds are instead responsible for texture formation during extrusion processing. The importance of amide bonds in protein texturization was also reported (20).

However, other researchers disagree with these findings. One study reported the formation of intermolecular disulfide bonding when defatted soy meal was extruded at temperatures between 110-150°C and found no evidence of intermolecular peptide bond formation (21). Another study used a single-screw extruder without a die and reported the formation of intermolecular disulfide bond for blends of defatted corn gluten and defatted soy flour extruded at 145°C (22). Others have also refuted the idea that intermolecular peptide bonds are responsible for texturization of extruded soy protein (23). The reason for this disagreement is not clear. The temperatures used by the first investigators (20) were higher than 175°C whereas the latter investigators (21,23) used lower temperatures. It was reported that thermal polymerization of peptide bond formation requires a minimum temperature of 180°C (24). This could be one possible reason for the conflicting results obtained by these researchers.

Most of the work evaluating product quality has used soy proteins. One study extruded defatted soy meal of moisture content 30% (dry basis) at a constant screw speed of 100 rpm with temperatures ranging from 107 to 200°C (25). The product density decreased with temperature (Fig. 1). Shear force and work increased with increasing temperature, but breaking strength increased as temperature increased to 160°C and then decreased with further increase in temperature. Another study used a three-

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