Proteins, Hardness and Allergens

摘要

Proteins are key constituents of wheat grain and flour. A number of specific proteins/classes of proteins are critical to the processing and end-use quality, and hence value and utility of wheat. The commercially important proteins of wheat may be broadly classified as glutenins and gliadins of various molecular weights and solubilities, enzymes, puroindolines and allergens. The gluten proteins are the quintessential components that provide the ascendancy of wheat as the world's number one cereal crop and food source. Gluten proteins are genetically and chemically complex and varied. They include high-molecular weight glutenins coded for by genes at the Glu-1 loci, low-molecular weight glutenins coded by Glu-3 loci, and gamma- and omega-gliadins coded for by the Gli-1 loci all on homoeologous group 1 chromosomes, and the alpha-gliadins coded for by the Gli-2 loci on group 6 chromosomes. Each Glu-1 locus in hexaploid wheat generally contains two genes, the products of which are described as 'x-type'and 'y-type' (Glu-1-1 and Glu-1-2, respectively). There are estimated to be 30-40 LMW glutenins and 150 alpha-gliadins in hexaploid wheat. The three most important enzymes associated with wheat quality include granule-bound starch synthase (GBSS or Waxy), alpha-amylase and polyphenol oxidase (PPO). GBSS is important due to its role in synthesizing amylose, and thus affecting starch amylose-to-amylopectin ratios and the physical behavior of starch. Amylases and PPO, when present, reduce quality throughstarch breakdown and discoloration, respectively. Amylases are synthesized de novo during pre-harvest sprouting; PPO levels vary due to allelic differences among varieties. Puroindolines are unique proteins in that they are associated with kernel softness. When absent or in a variant form, hard kernel texture results. A brief over-view of wheat proteins as allergens is provided.