Whisky Raw Materials: Wheat Specifications
One of the advantages of wheat is that there are a large number of different
varieties to choose from, although only a few soft wheats (such as Riband,
Consort and Claire) are currently regarded as suitable for distilling (Brosnan,
2001). Some of the main factors used to distinguish wheat varieties are hardness or softness of the grain, winter or spring habit, and (more recently)
protein content. Within these classes, wheat can further be described according to specific weight, contamination with foreign materials (including other cereal grains), the level of screenings, the absence of sprouted grains (degree of soundness), the moisture content, and measures of properties of their doughs as well as other measures of processing quality. Some of these properties are influenced primarily by genetic factors and are inherent in the varieties of wheat produced, while others are governed by environmental factors such as soil fertility, rainfall, and temperature, both during the growing season and at harvest (Orth and Schellenberger, 1988).
In the past cereal buyers in distilling companies tended to avoid specifying
too many parameters for wheat, since this would incur a price premium to
suppliers as more parameters were specified (Brown, 1990). However, nowadays it is normal in purchasing wheat suitable for distilling to specify not only moisture content, specific weight, hardness and nitrogen content, but also, in some cases, wheat variety (Brown, 1990; Nicol, 1990).
The types of wheat best suited for alcohol production are soft white or red
winter wheat varieties. Durum and hard red spring wheats are generally not
suitable for alcohol production, owing to the lower starch content and the
resultant low yield of alcohol (Stark et al., 1943).
Winter wheat is the highest yielding cereal in Britain, and comprises the
largest acreage sown. In Scotland, winter wheat is sown in late September to
November and is harvested the following September. The optimum seed rate
depends on when the crop is sown; a low seed rate (300 seeds per square
metre) is suitable for early sowing, whereas a high seed rate (exceeding 450
seeds per square metre) is suitable for November (winter) sowing. Nitrogen
applications are carefully controlled during crop growth, as nitrogen applied
to crops nearer harvest accumulates in the grain and gives an increased final
nitrogen content, which is not a favourable trait in wheat used for distilling
although it is desirable in bread wheat (David Cranstoun, Scottish
Agricultural College, personal communication).
Distillers prefer to use soft wheat varieties because they tend to give fewer
processing problems in distilleries. Hard wheats are associated with higher
viscosity worts than soft wheats, irrespective of protein content (Brown 1990),
and tend to increase problems in important rate-determining areas of the
process, such as the transfer of worts and centrifugation and evaporation of
Specific weight is the weight of a known volume of grain (expressed in
kilograms per hectolitre, kg/hl). This was originally referred to as the bushel
weight and is broadly similar in concept to thousand corn weight, which is
used for barley. The higher the specific weight the more starch (and protein)
it contains, and grain distilleries usually set specifications at no lower than
72 kg/hl (Brown, 1990). The size of kernels or screenings may also be specified as this may be important in some distillery processes; if the grains are too small they can pass through mills and enter the process whole, which might mean that not all the available starch is extracted, leading to decreased alcohol yields.
The differences in hardness of wheat appear to relate to adhesion between
starch granules and storage proteins (Wrigley et al., 1988). Hardness of grains
is often measured by a simple milling test, with samples that take the longest
to mill being classified as the hardest and vice versa. The hardness or softness
of grain texture is linked to the way in which the starch is bound up within the
protein matrix, and is thus related to the nitrogen content. The inverse relationship between grain nitrogen level and alcohol yield is well known, and simply relates to the relatively low starch content of higher nitrogen wheats (Brosnan et al., 1998). This has been confirmed by distilling trials, where it has been found that there is a direct link between nitrogen content and spirit yield.
[Source: Whisky: Technology, Production and Marketing 2003