Wheat. Peter R. Shewry

Чтение книги онлайн.

Figure 1.23 Gluten produced by hand washing, stretched to show the cohesive properties.

      Source: Taken from Shewry et al. (1995).

      Grain protein concentration is not usually determined directly but calculated based on the N concentration determined by chemical analysis (Kjeldahl wet chemistry or Dumas oxidative combustion) or by near‐infrared spectroscopy (NIRS) calibrated based on N analysis (Carson and Edwards 2009). It is often assumed that there is a constant relationship between the amount of N and the amount of protein in biological samples and that the crude protein concentration can be calculated by multiplying the N concentration by a constant factor, with N × 6.25 being most widely used. However, this is not the case because it wrongly assumes that all proteins have a similar N content. Gluten proteins, for example, have higher N contents than most other proteins due to the presence of between 30 and 50% of glutamine, an amino acid which contains two N atoms (as opposed to one in most other amino acids). Consequently, N × 5.7 is widely accepted as a conversion factor for wheat grain and flour (Draper and Stewart 1980). However, this factor is still imprecise as it will vary with the proportion of gluten proteins in the sample. Hence, it will be lower for high protein grain. Similarly, it will be lower for white flour than for wholemeal (with N × 5.83 having been suggested [Kent and Evers 1994]) and vary between mill streams. Nevertheless, it is clearly impractical to use a range of values and a conversion factor of N × 5.7 is almost universally used to calculate protein concentration for marketing and utilization of wheat grain.

Although protein concentration is not the same as protein quality, a minimum protein concentration is required for breadmaking and increasing protein concentration can, to some extent, compensate for lower quality. Hence, higher protein concentrations are required for pasta and breadmaking than for other uses (Figure 1.22). The minimum requirement for the CBP is around 13% DM. Products requiring a weak, extensible dough, such as biscuits, cakes, and pastries, usually require a protein content of less than 10.5% DM and levels down to 9% can be tolerated for some products (Carson and Edwards 2009). Low protein concentrations, and therefore high starch contents, are associated with higher alcohol yields in distilling and bioethanol industries (Taylor et al. 1993).

      1.5.4.5 Importance of Protein Quality

      In addition to protein concentration, the relative proportions and subunit compositions of the gliadin and glutenin fractions also have significant effects on dough rheology and functionality for different end uses. In broad terms, gliadins contribute to the viscosity and extensibility and glutenins to the elasticity and strength of doughs (Rustgi et al. 2019). The proportions of gliadins and glutenins may vary between genotypes (Wieser 2000) and there are also significant differences in quality associated with allelic variation in the protein compositions of the two fractions. In particular, one group of gluten proteins, the high molecular weight subunits of glutenin, are widely used as biochemical markers for dough strength (see Chapter 8 and Payne et al. 1987) based on analysis by sodium dodecylsulphate polyacrylamide gel electrophoresis or by capillary or reversed‐phase high‐performance liquid‐chromatography (RP‐HPLC) (Rustgi et al. 2019).

      The strong genetic control of protein quality means that cultivars are often grouped based on their intended end use. For example, in the UK cultivars are grouped depending on whether they have potential for breadmaking (Groups 1 and 2), biscuit making (Group 3), or neither end use (Group 4).

      The proportions and compositions of the gliadin and glutenin fractions are also affected by environmental factors, which can lead to effects on processing quality. For example, the availability of S is particularly important as it affects the ratio of N : S in the grain and, therefore the proportions of proteins which are rich in cysteine (a suphur‐containing amino acid required for the formation of disulphide bonds in glutenin polymers) (Wieser et al. 2004). Indeed, several studies showed that the grain S concentration was a better predictor of breadmaking quality than N concentration (Zhao et al. 1999a, b; Ruske et al. 2004).

      1.5.4.6 Measurement of Dough Rheology and Quality

      The properties of dough and gluten can be measured using rheology, a branch of physics that deals with the deformation and flow of matter. Several mechanical systems have been developed to measure the rheological properties of doughs in research and grain utilization.

These fall broadly into two types. Recording dough mixers, such as the Farinograph, Mixograph, and Reomixer, measure the mechanical energy required to mix a dough to its maximum (peak) resistance. They thus provide data on the peak resistance, the time required to mix to the peak (both measures of dough strength), and the breakdown of the dough when overmixed (a measure of stability). The second type of system, such as the Extensograph, Alveograph, and Kieffer Rig, measures the properties (resistance and extensibility) of the dough after it has been mixed to peak resistance and allowed to rest, by extending the dough until it breaks. However, these mechanical systems are low throughput and clearly not appropriate for the analysis of large numbers of samples in breeding programmes.

      Several simpler systems are therefore widely used, by breeders as well as in the food industry. For example, in the SDS‐sedimentation volume test, wholemeal flour is mixed with weak lactic acid and sodium dodecyl sulphate (SDS) in a measuring cylinder. This solution dissolves much of the protein but not the high molecular weight proteins, particularly the glutenins, which are a major contributor to the volume of suspended sediment recorded after a period of settling (Fullington et al. 1987).

      Solvent retention capacity (SRC) determines the ability of flour to retain a set of four solvents (water, 50% sucrose, 5% sodium carbonate, 5% lactic acid) (Slade and Levine 1994). Because these solvents are preferentially absorbed by one or more of the major grain components (glutenin [lactic acid], starch [sodium carbonate and sucrose], and pentosans [sucrose]), their relative retention can be used to predict aspects of grain composition and quality. The relative retention is expressed as the gluten performance index (GPI), which is calculated as:

      (1.1)

      The GPI has been reported to explain the performance of glutenin in dough development (Kweon et al. 2011).

      1.5.5 Other Factors Affecting the Acceptability of Wheat for Different End‐Uses

СКАЧАТЬ