January 01, 2003
by Emily Buckley
By Gavin Owens, Ph.D.
Moisture management is crucial to the successful operation of any milling or other cereal processing plant. The moisture content of the material being processed influences raw material characteristics, process behavior, product quality and economic viability. Thus the management of moisture within a process should be taken seriously and treated in a scientific manner. The results can be dramatic and rewarding.
Excess moisture at any stage of grain handling has serious quality implications. Moisture content above 15% makes water available for mold and mycotoxins to develop on grain, which is a well-documented phenomenon. Insects can also utilize moisture in grain above this moisture level. Activity by either of these elements can further release moisture and increased grain deterioration is possible. If this kind of deterioration is left to progress undetected or unchecked, the consequences can be catastrophic. The five main areas of concern are:
• Insect infestation: This can result in deposits of allergy causing material and grain damage (e.g. mites and weevils).
• Molds: Their growth can release toxins, such as mycotoxins.
• Germination activity: This releases alpha-amylase and converts starches to sugar. In extreme cases the sugar is used to produce seed leaves and roots.
• Excess moisture in raw materials, interim and final products causes handling problems, such as flowability, lumps and bridging in hoppers.
• If moisture content exceeds product specifications, the purchaser may reject consignments.
Low moistures create a different set of problems. Low raw material moisture causes excess dust to be produced during handling and the material is more abrasive on handling equipment. Low moisture content material entering the process can adversely affect product quality because the interim products are brittle and fracture easily. This point is of crucial importance in milling, where most separations are performed on the basis of size separation. Ultimately, if the moisture of the material in the mill is maintained at its optimum level, the process can be operated in an aggressive fashion to maximize product yield without sacrificing quality.
Where it is possible to manipulate product moistures after the process, further scope exists for managing the moisture content of interim mill products to maximize process performance and yield. This is because the targets of achieving specification moisture content and maximizing process performance may not be achievable simultaneously.
Moisture management can be broken down into three distinct areas, namely the moisture content in raw material, intermediate product and final product.
This is the first area to consider because it is unusual to receive grain at exactly the right moisture content for processing. In fact most plants are designed around the fact that raw materials have to be modified to enable processing.
There are many processing plants in existence that take grain straight from producers and store it on site until required. In damper climates, such as those of Northern Europe, this grain must be dried, because harvest moistures are too high for storage and processing. Two types of dryers are mainly used for drying, namely mixed flow tower dryers and cross flow dryers. Diesel fuel is the most common fuel used, but there are examples of driers using natural gas, coal, wood and other fuels. These choices are largely dictated by local supply and economic criteria.
In other areas of the world harvest moistures are low enough for safe storage, and the only post harvest treatment required is cooling, which inhibits insect activity.
The target moisture content for short term storage is 15 to 15.5% for feed and milling wheat, but the moisture target can be set at 12% or lower where longer term storage is required.
Product moisture is principally determined by raw material moisture content, but it may be manipulated in a number of ways. If the moisture content of incoming grain is low, water is added. This may be a one, two or three stage process and involves the addition of water or steam to the grain in a controlled fashion. This is followed immediately by mixing to ensure even addition. Interim storage of this wet grain is required in order to facilitate even absorption of moisture into the center of each kernel.
If incoming moisture contents are too high, many mills will reject the raw material, because they have no drying facilities available to them and milling processes cannot cope with high moisture content material. It is for this reason that maximum moistures are specified in many purchase contracts.
This must be carried out in a controlled fashion. There are two elements involved, namely moisture measurement and application. Moisture content can be measured with absolute accuracy using an oven, but this approach is slow and cannot be carried out online. All other moisture measurement techniques are indirect and, thus, are prone to error. However many of the indirect measurement techniques have progressed to the stage where errors are very low and reliability is very high. Equipment used for measurement has already been briefly discussed (see World Grain, August 2002, p28; E-Archive #55419).
Intermediate and Final Products
Moisture is lost during processing of grain in a mill. The severity of the milling process has a direct impact on the amount of moisture loss because this is proportional to the amount of heat generated in the process and transferred to the product. Pneumatic conveying also contributes significantly to this phenomenon because of the amount of air that comes into contact with the product.
Since moisture is lost throughout the milling process, the characteristics of intermediate mill products vary as they progress through the process. This is an area that could be addressed in the future to enhance mill productivity.
This moisture loss can leave the moisture content of mill products too low for consumers. Therefore corrective moisture addition may be necessary. Specialist addition equipment is required for this operation since moisture must be added to powder products. It is for this reason that this level of processing is not carried out in many mills and instead raw material moisture contents are manipulated in order to achieve the desired product moistures.
Ultimately operating a process is about making a profit from the activity. This can be maximized by sourcing raw materials at low moisture without paying a premium. The process should then be managed so that the moisture content of products leaving the process is as close as possible to the maximum allowable by the customer. The moisture loss during the process should be minimized, because this is a direct result of the energy input to the process as well as being a loss of revenue. If these variables are monitored during operation, profits will be maximized.
There is an interesting point to be made about maximizing moisture contents in products. Where significant tonnages are handled, the transport cost in moving extra moisture from mill to customer becomes significant. If the mill customer is an internal one (i.e. belongs to the same company), then there is a strong argument for sending the product ‘dry’ and encouraging the customer to complete any necessary moisture addition. The haulage savings can be considerable.
To conclude, moisture management is a crucial part of milling operation management. Moisture has a dramatic impact on process performance and product quality; therefore mill operators ignore moisture management at their peril.
This is the phenomenon of weight gain in a milling process where the moisture content of mill products is greater than that of the raw materials. This is a common occurrence in flour mills where the moisture content of the flour, wheatfeed and germ is greater than that of the wheat taken in. For example wheat being processed may arrive on site at 11% moisture, while the products leaving may be at 15% moisture. The difference is attributed solely to moisture addition.
This occurs where intake moisture content is close to or higher than product moisture content. This occurs principally where dried grains are being processed and the intake moisture thresholds are only just exceeded.