Agriculture Reference
In-Depth Information
Figure 11.2. The vacuum oven method is one of the primary methods in seed moisture content determination.
100°C in a partial vacuum until a constant weight is attained (Fig. 11.2). Non-aqueous volatiles may still be
lost, however, because they are released at lower temperatures under vacuum. Some ovens use phosphorus
pentoxide to absorb the moisture released by the seed.
In some cases, moisture content should be expressed on the basis of
fresh weight (fw)
; in other cases,
moisture content may be expressed on the basis of
dry
weight (dw)
. For seed industry purposes, moisture
is normally expressed on the basis of fresh weight. The following equations show the relationship between
fresh and dry weight:
% moisture (dw basis) =
wt before drying (fw) - wt after drying (dw)
wt after drying (dw)
% moisture (fw basis) =
wt before drying (fw) - wt after drying (dw)
x 100
wt before drying (fw)
Depending on the procedure used to determine water loss from seeds, oven-drying can be considered
either a direct or indirect method. When water lost by vaporization is collected and quantiied, as in the case
when using phosphorus pentoxide to absorb water, the oven-drying method represents a direct method of
moisture content determination. When loss in moisture is determined by weighing seeds before and after
drying, as in the case in routine seed moisture testing, the oven drying method is considered an indirect
method of moisture content determination.
dISTILLATIon METHodS
Distillation methods avoid some of the disadvantages of the oven methods, particularly the removal of vola-
tiles other than water. However, the main disadvantage of distillation methods is that the amount of water
distilled depends on the boiling point of the solution used. The toluene distillation method uses toluene with
a boiling point of 111°C. The condensing lask is connected to a Leibig condenser by a distilling trap of the
Bidwell and Sterling type with 0.1-ml graduations. The weighed sample is placed in the lask with toluene,
brought to a boil and the water distilled until no more is given off. The weight of the collected water is
determined by knowing the weight at the temperature at which it is measured. This test usually is completed
between 1 and 5 hours, although Hart and Golumbic (1962) recommended longer periods.
