The purpose of the screenroom is to supply grain that is free from impurities, and in optimum condition for milling.
The final stage of the screenroom after cleaning, is to "condition" the wheat. To allow for the maximum extraction of flour and to ensure the quality parameters can be met, water is added to the grain. "Conditioning", "Tempering" or "Damping" are all terms used to describe this part of the process.
The addition of water toughens the bran to reduce powdering ( and subsequent darkening of the flour ) during the milling process. The bran layers are also loosened to ease the separation from the endosperm. It also helps to "mellow" the grain by softening the starchy structures of the endosperm.
- The amount of water added at this stage will be dependant on several factors:
- The variety of grain to be milled
- The natural moisture content
- If the mill is an elevator mill or pneumatic
- The prevailing climate
- Specification of the finished flour
Normal moisture levels in wheat vary from around 9% upto 14% dependant on variety, and these will normally need to be "conditioned" to between 15-17% prior to milling.
In cases where extremes of the above exist, the conditioning is normally done in two stages. The maximum percentage that can be added in one pass is around 5% due to the rate at which the grain can absorb the water.
After damping the grain is normally left to stand for anywhere between 6 and 24 hrs to allow the moisture to penetrate evenly through the grain (by osmosis). This time is dictated by the grain type and is shorter for soft wheats and longer for hard wheats. The theoretical rate of water addition as described above, is easy to calculate when setting up the water flow to a suitable mixer or agitator. However, in reality, the natural moisture content of grain is not uniform throughout a given parcel of wheat and these fluctuations will be carried through to the mill if a constant rate of water is added. This will lead to imbalance in the milling process causing inconsistency of quality and/or processing problems. To get round this, most modern flour mills will have automatic moisture measuring devices which control the water addition system. The nett result is a consistent moisture across the batch. These devices can be broadly split into two types - Feed Forward and Feed Back systems. Feed Forward systems measure the moisture of the dry grain and calculate the addition required and control the water flow accordingly. Feed Back systems measure the damped wheat and control the water addition accordingly.
The advantage of the Feed Back system is that it requires less instrumentation as the flow rate is irrelevant, but due to the added water being mainly on the surface of the grain immediately following damping, the accuracy is questionable. The Feed Forward system is far more accurate as it measures the dry grain both for moisture and density, combined with measuring the flow rate to calculate the required water addition rate. The disadvantage of this, apart from the cost of the system, is that if there is a malfunction and too much water is added there is nothing to check the resultant output.