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Bleaching grain

Fall rain can mean bring lower quality wheat and fewer hard vitreous kernels

An untimely rain just before harvest can cause wheat kernels to “bleach” or whiten and can lead to some loss in quality that can cause downgrading at the elevator.

Bleaching of grain kernels is caused by wet conditions at or near maturity when alternate wetting and drying causes tiny fissures because the grain expands when wet then doesn’t dry back to the same size.

Hard vitreous kernels (HVK)

The Canadian Grain Commission makes the important distinction that “bleaching” is not a grading factor in wheat; rather it is the percentage of hard vitreous kernels (HVK) in the sample that determines some wheat grades.

Vitreousness is the natural translucence of a kernel that is a visible sign of kernel hardness. HVK are a grade determinant for the amber durum wheat class in Canada and the red spring wheat classes in Western Canada. Both wheat classes have different tolerances for the allowable percentage of HVK. Canada Western Red Spring (CWRS) wheat must have a minimum of 65 per cent HVK to be graded as No. 1 and anything lower than that will be graded as No. 2. The tolerance levels are much lower in durum wheat, which must have a minimum of 80 per cent HVK to make No. 1, 60 per cent minimum for No. 2 and 40 per cent minimum to be rated as No. 3.

HVK content is related to protein content and milling quality, which is particularly important in durum wheat. Non-vitreous kernels are produced under cool, wet conditions at or just before the grain matures and can also be caused by insufficient nitrogen fertilization. Flours milled from non-vitrous wheat will have reduced protein content and will not produce good bread. Non-vitreous kernels are not a grading factor in soft wheats because they have no impact on end use for most products, like cookie, pastries and oriental noodles that are made from soft wheat flour.

“The reason for the impact on amber durum is that when you get a non-vitreous kernel it starts to get starchy white inside instead of the usual amber colour,” says Daryl Beswitherick, program manager of quality assurance at the Canadian Grain Commission. “What happens then is that in the processing of that durum, the aim is to produce semolina and if you have the starch in there, the particle sizes will be smaller and it will turn out more like flour than semolina.”

Beswitherick emphasizes that HVK is not a major down grading factor for CWRS and not all bleaching indicates non-vitreous kernels. “It’s possible to get some samples that have a very whitish look to them but once you look more carefully at the kernels they are still hard and it’s just whitish on the bran and they are still hard vitreous,” he says. “When they become very starchy they become more orange and are in sharp contrast to the natural sample colour and are obvious non-vitreous kernels.”

More from the Grainews website: Protein lower than last year

Determining the percentage of HVK is still a manual, visual test and is done by separating a 25-gram sample from a sieved 250 gram sample. It is then inspected for the number of kernels that have natural translucence. There has been research in Canada and Australia into development of machine vision systems to classify individual wheat kernels as either HVK or non-vitreous but to date no system has been commercially employed.

HVK was originally implemented as an indicator of protein content for wheat, but once elevators were equipped to do their own protein testing it was no longer needed and was eliminated as a grading factor in other classes of spring and winter wheat because it doesn’t generally affect the end use quality for these classes.

Late rainfall

The larger concern with late rainfall on wheat is related to other problems that can result such as pre-harvest sprouting, which can cause lower test weights, lower 1,000 kernel weights and other quality issues. “When there is an indication that sprouting has occurred in the kernel, even if there is no actual sprout visible, there will be enzyme changes that negatively affect seed quality and viability,” says Mitchell Japp, provincial cereal crop specialist with the Saskatchewan Ministry of Agriculture.

Japp says that there are sporadic reports this season of problems with some bleached grain in the main durum growing areas of south central and east central Saskatchewan due to fall rains, but doesn’t think it’s likely to be a widespread problem.

There really isn’t much that can be done once the rain stops except trying to get the grain harvested and dried down as soon as possible to prevent further bleaching or other problems such as mold or sprouting, says Japp. He does suggest that farmers, especially those growing durum wheat in regions prone to late summer or early fall rains, should consider a variety that offers some resistance to pre-harvest sprouting.

Whether it’s referred to as bleaching or not, the fact remains that HVK is still a grading factor for certain classes of wheat. “They still have to meet end use specifications so there are grading specifications that they have to meet and this is one of them,” says Beswitherick.

Angela Lovell is a freelance writer, editor and communications specialist living and working in Manitoba. Find her online at

About the author


Angela Lovell

Angela Lovell is a freelance writer based in Manitou, Manitoba. Visit her website at or follow her on Twitter @angelalovell10.



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