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Rainfall just prior to wheat harvest sprouts grain, reduces flour qualityRainfall just prior to wheat harvest sprouts grain, reduces flour quality

Roy Roberson 2

July 18, 2007

4 Min Read

Recent tests by researchers at Virginia Tech University confirm that rains just prior to wheat harvest may cause grain to germinate (or sprout) in the head. Germination causes the production of alpha amylase, an enzyme that breaks down starch and proteins. Sprouted grain produces lower quality flour and products made from the flour.

As sprouting increases, so does damage to starch quality. In general, the longer wheats differ in susceptibility to amylase increases. The Falling Number values for the sprouted white wheats were higher than those for the hard red winter wheats at the same level of amylase. Both genetic differences and growth conditions were responsible for the differences in susceptibility.

Speaking at a recent Virginia Small Grains Field Day, Virginia Tech Graduate Student Kelly Burt explained that wheat is ultimately purchased on milling quality. Therefore, the degree of harvest time sprouting has a direct affect on the value of the crop.

In general, values below 300 seconds are indicative of poor quality for milling and baking purposes. Sprout damaged grain can affect mixability, crumb strength, sliceablity, and volume.

Wheat that does not meet the minimum Falling Number standard is unsuitable for milling and is usually sold to the livestock feed market. Flour quality wheat generally receives a premium of $.40 or more per bushel over feed wheat.

According to the U.S. Wheat Associates report, “Relatively high damaged kernel values and low Falling Numbers in the East reflect the difficult harvest conditions there, while very good values were recorded in other states.”

Last year's flour data reflected a decreased Falling Number average.

In 2006 the Falling Number average for East Coast wheat was 260 seconds a 25 percent decrease from the five year average of 347 seconds.

The Falling Number test is the industry standard for measuring pre-harvest sprouting of wheat. Though it is sometimes impossible for growers to see pre-harvest damage in wheat, often there is some starch degradation taking place in the seed, causing problems in milling.

Falling Numbers are determined by taking samples of wheat flour and water and mixing into a slurry. The slurry is placed into a hot water bath and it begins to gel. The starch from the wheat seed makes the slurry thicker. By dropping a plunger into a test tube and measuring the time it takes for the plunger to fall to the bottom, an estimate of starch quality and usefulness can be made.

The thicker the mixture, the less starch damage and the longer it takes the plunger to move through the slurry. The thinner the slurry, the lower the Falling Number and greater opportunities for price dockage.

Flour with Falling Number values below 250 to 300 is generally not suited to milling.

Flour milled from pre-harvest sprouted wheat produces bread that is porous and sticky and has a low loaf volume. The grain has little value to the milling and baking industries and is discounted heavily. More than 4 percent damaged kernels, including sprouted kernels, causes grain to be rated Grade 3 or lower and unacceptable for bread making.

Grain that is slightly sprouted might be blended with sound grain for making flour, but grain that is severely sprouted usually is used for livestock feed.

As part of her graduate studies, Burt will be screening over 80 released and experimental wheat lines for their inherent dormancy as well as evaluating several environmental characteristics that affect pre-harvest sprouting (PHS), and thus lower Falling Numbers.

“We will look at head angles, length of awns, and other factors that affect moisture retention in wheat. We will also be looking at color of the grain because we suspect lighter colored wheat kernels will have more sprouting and lower Falling Numbers,” she says.

The end-result will add management information for growers and millers to make better handling decisions for wheat. It should also provide information so plant breeders can incorporate physiological factors into future varieties to reduce Falling Number problems in wheat.

Tolerance to PHS is a highly desirable but complex trait sought by plant breeders. The overall resistance to sprouting damage results from a combination of factors influencing water uptake and drying rate of the ear, grain dormancy and the availability of storage reserves to support germination.

These processes are regulated by a number of key resistance genes that interact strongly with environmental conditions that Burt and other Virginia Tech researchers will be studying.

Previous research has shown the degree of embryo dormancy at grain harvest to be directly correlated with genetic variation, which is known to be a major factor in sprouting of grains within the ear.

Environmental conditions during grain development, the temperature used to test germination, and the stage of grain maturity, all affect the degree of embryo dormancy. In general, more dormant grain is produced under cool growing conditions, whereas high temperatures during the later stages of grain growth break embryo dormancy allowing grains to germinate in the ear if rain occurs around harvest time.

Only a portion of the grain within an ear tends to exhibit the pre-harvest sprouting gene. By determining more precisely which environmental and morphological factors cause individual seed within a head to sprout, Virginia Tech researchers are hoping to better understand this phenomenon.

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