A number of methods for determining the baking quality of flours have
been developed and standardized throughout the world. In particular they have
been elaborated by the scientific bodies of the ICC (Vienna, Austria, www.icc.or.at)
and AACC International (St. Paul, MN, USA, www.aaccnet.org). They are regularly
checked and optimized in special study groups. So when making comparisons it is
important always to use the latest issues of the collections of methods.
The following chapter describes the methods for analyzing kernels and
the products ground from wheat or rye. Before the procedure is described
(usually the ICC method) the standards and principles of the ICC and AACC International
are compared in the form of a table. For a discussion of the results it is important
always to state the method by which the analysis was carried out.
1. Determining the Moisture Content
of Flours
The water content or moisture of the flours is very important not only
for their shelf life but also for determining the solids content. For many
tests it is necessary to know the dry weight of the material in order to
determine the amount of the substances it contains. The weight of the flour
sample therefore has to be adjusted to the moisture content of the flour. There
are tables to show that at a moisture weighed into the Farinograph instead of
300 g, since the solids content decreases as the moisture content increases.
Most analytical methods are designed for flour with a moisture content of 14%,
so at 12.5% only 294.9 g of flour are needed. Bakers have to think on the same
lines, since they need flours with a high solids content.
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| Fig. 37: Device for determining the moisture content (source:
Brabender OHG) |
With the exception of maize and brewer's barley it is possible to
determine the water content of all cereals and cereal products by drying them
at 130 °C for 90 min (ICC Standard 110/1). Fig 37 shows a device permitting
virtually continuous determination of the moisture content of several samples
at the same time. By using NIR spectroscopy (ICC Recommendation 202) it is
possible to obtain the result within seconds (Fig. 38). This method makes use
of the fact that the light rays in the nearinfrared (NIR) range are absorbed or
reflected by the sample at different wavelengths (see Spectrometry). The
calibration of the equipment enables the computer to determine the values for
moisture and protein very quickly. The moisture content of whole kernels can be
determined as well as that of flour. The near-infrared transmission technology
(NIT; Fig. 38) necessary for this is the more convenient method for mills,
since the kernels do not have to be crushed.
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Fig. 38: Determining the moisture content of whole kernels (left) and flour
(right) using near-infrared transmission technology (NIT) or near-infrared
reflection spectroscopy (NIR) (sources: Brabender OHG / Mühlenchemie GmbH)
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The water content of flour depends to a large extent on climatic
conditions during harvesting and ambient moisture during storage. Moisture values
well over 15% restrict the shelf life. Both storage time and storage temperature
have to be monitored to ensure that the cereal products do not become inedible
through enzyme or microbiological degradation and that their processing
properties do not suffer. In wholemeal products, especially, a stale, mouldy
taste can develop very quickly.
2. Determining the Mineral Content
of Flours
In most regions, flours are traded according to their mineral content
(which also used to be called the ash content). Since the baking properties of
the flours change with the degree of extraction, the mineral content should be
stated in concrete terms.
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Fig. 39: Muffle furnace for ashing flour, with porcelain crucibles (source:
University of Hanover)
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The mineral content is determined by ashing a flour sample in a muffle
furnace (Fig. 39) at 900 °C. The organic material initially burns quite
fiercely; after this it is necessary to continue heating the sample for up to
120 minutes. Only pure white ash is then to be seen in the porcelain crucible.
After cooling, the crucible is weighed and the quantity of ash stated as a percentage
of the dry matter. In Germany the type designation is obtained by multiplying
this value by 1,000. The mineral content of flours may fluctuate between a minimum
and a maximum, depending on the regulations in individual countries.
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Fig. 40: Changes in the flour values when the flour yield is increased
beyond the endosperm. red brown: aleuron cells; blue: cell walls; dark brown:
starch granules (modified from Bolling, 1986 by L. Popper; wheat micrograph
courtesy of VTT, Helsinki, Finland)
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In some cases the variability is very wide, and this may result in differences
in baking properties. Especially in the production of baked goods with a high
volume yield, e.g. bread rolls, it makes a difference whether the flour used
has a mineral content of 0.51 or 0.63%. If the level of extraction includes
little more than the endosperm of the grain, the protein content increases considerably
in addition to the mineral content (Fig. 40), but at the same time there is likely
to be a sharp fall in the volume yield.
In the case of the rye flour types, full use of the outer layers also
changes the colour of the flour. This may result in bread with a noticeably darker
crumb in spite of using the same flour type. The dough yield will also
fluctuate if the flour has a mineral content of 1.11 g on one occasion and 1.30
g per 100 g of dry matter on the next.
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