Improving bread with alpha-amylase

Late maturity alpha-amylase (LMA) and preharvest sprouting (PHS) are considered two key challenges in the Australian wheat industry, however, recent research suggests there is more to the amylase story.

Wheat is a major source of carbohydrate and protein in human diets. Carbohydrate is stored in the wheat grain as starch which makes up more than 70 per cent of total grain dry weight. The plant uses this stored energy during germination to fuel the first stages of seedling growth.

α-amylase is considered to be one of the primary enzymes responsible for initiating the breakdown of starch during germination. To date, three isoforms (functional versions) of α-amylase have been identified in wheat – TaAMY1, TaAMY2 and TaAMY3. These three isoforms perform similar functions but act within different parts of the grain and at different stages of grain development and/or germination.

Current thinking - α-amylase reduces grain quality

Late maturity α-amylase (LMA) and preharvest sprouting (PHS) are genetic defects in wheat. They both lead to a high level of α-amylase in the grain prior to harvest. The enhanced expression of α-amylase in both LMA and PHS results in a reduction in Falling Number (FN) (a test of gel viscosity ) and subsequent downgrading of the grain in quality rankings, resulting in a reduced price for growers.

Preharvest sprouting (PHS) is a genetic defect in which the grain germinates prior to harvest. This defect is usually triggered by heavy rains occurring at grain maturity and results in severe losses for the farmers. α-amylase is one of the enzymes responsible for this early germination. PHS also results in loss of functionality for end uses such as baked products and noodles as the proteins and starch are digested in the grain.

High levels of alpha-amylase in wheat grain are thought to be detrimental to end uses such as baked goods.

Late maturity α-amylase (LMA) leads to the increase of a single α-amylase family during grain development that remains at grain maturity. This condition is seen in some wheat varieties in response to environmental triggers. No studies have been conducted to determine whether LMA is detrimental to end product quality.

As the FN test is assessing α-amylase activity, it is unable to distinguish between LMA and PHS, thus, both defects are treated similarly when grain is traded. It is assumed that high levels of α-amylase in the grain, regardless of the form or source, lower baking quality. Therefore, samples with a low FN score are downgraded, often to feed grade which reduces the farmers’ income.

Australian wheat breeders are required to screen for LMA across two cycles before selection of desirable wheat lines for release to growers. This mandatory requirement has two consequences:

  1. it limits the breeders’ ability to test and release new lines in a timely manner
  2. as testing for LMA occurs towards the end of the breeding cycle, any line that is classified as susceptible to LMA is very unlikely to be released.

Even a one year delay in releasing a higher yielding wheat line can have significant financial impacts for breeders and farmers alike and limits the release of enhanced elite varieties.

Is it the whole story?

Despite grain with low FN being downgraded and excluded from milling grists, bakers have long added α-amylases either in the form of malt flour or modern commercial enzyme preparations to improve the quality of their baked goods. Grain with insufficient α-amylase will yield bread with poor loaf volume unless α-amylase is added to the formulation to compensate.

In fact, the addition of α-amylases helps the generation of fermentable sugars, stimulates yeast fermentation and therefore increases bread loaf volume and shelf life. Common best baking practices highlight the necessity of adding baking improver (which contains amylases) into the flour and water mixture to help improve baking quality.

The truth about α-amylase rises to the surface

We applied our expertise in wheat genetics to investigate the impact of specific α-amylase isoforms on grain quality. For example, we conducted small loaf baking studies on engineered high amylase lines, specifically expressing the wheat alpha-amylase isoform TaAMY3, with apparent low FN. Wheat lines with a low FN would normally be classified as severely sprouted and sold as feed wheat, however, these experimental lines showed enhanced loaf volume and colour in our small scale baking tests.

We conducted these small-scale trials using genetically modified plants, a great tool to gain knowledge. The trial wheat plants were grown in a controlled glasshouse experiment (approved by the Office of the Gene Technology Regulator) to get enough grain to evaluate the suitability for bread-making.

Our findings raise questions as to the validity of the assumptions that:

  • LMA is detrimental to end product quality
  • a low Falling Number is always indicative of a reduction in quality.

This suggests the need for a better understanding of the impact of elevated expression of specific α-amylase on end product quality in order to develop the appropriate response to solve LMA and PHS.

If agricultural production is to increase in an environment of growing pressures on land use, reduced availability of inputs, and increasing climate uncertainty, there is need for a thorough investigation into the effect of LMA on grain quality. At present large numbers of otherwise excellent advanced wheat lines are potentially discarded as LMA susceptible despite there being no scientific evidence to show that LMA is detrimental to end product quality.

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