Wheat drying is not simply a matter of avoiding deterioration, and the processing methods used also affect its digestibility.
Very humid wheat harvested after bad weather is often unsuitable for human consumption. This harvesting condition often results in germination and an increase in amylase and protease levels. These enzyme content changes can affect the size of baked goods and reduce the strength of gluten. Therefore, such damaged wheat will be sold to the animal feed raw material market.
Wheat and other grain storage requires a moisture content of approximately 14% to ensure stability and retard mold growth. Therefore, if the grain is wet at the time of harvest, it is necessary to carry out drying treatment.
Mildew is a special problem. It not only consumes plant nutrients, but also reduces the nutritional value of the final product. More seriously, it can also cause mycotoxins to damage animal health and further increase animal production costs.
The moist wheat used in animal feed is usually subjected to extreme high temperature and rapid drying to reduce production costs, and the use temperature may be as high as 100 °C. Generally, heat treatment irreversibly changes the structure and composition of the grain. The temperature range and moisture content clearly affect this change.
1 Paste
In the presence of excess moisture, as the grain is heated past the critical temperature, the native starch granules begin to absorb moisture and swell, eventually causing the starch to be "gelatinized." This can result in dissolution of the starch granules and destruction of the semi-crystalline structure, but this is less likely to occur in a restricted water state. Under high moisture conditions, when the temperature is higher than 60 ° C, the crystal structure of wheat will be digested. When the water reduction is limited, the disintegration of the crystalline structure of wheat requires more energy.
When the grain starch is completely gelatinized, the digestibility in vitro is greatly increased. Starch dissolves and the amylase site of action increases. In some cases, starch gelatinization can increase the enzymatic rate by about a thousand times. Observation of starch granules under an optical microscope or polarized light can detect whether the starch is gelatinized.
So, after harvesting the moist grains and drying them, do we consider the changes in starch structure? Ideally, the dry summer harvested wheat has a natural moisture content of 14%. However, almost all grain foods require a certain degree of drying processing.
In fact, the higher the moisture content of the grain, the more difficult it is to harvest due to the tendency to clog the machine, so the wet grain is forced to be delayed into the bin until the moisture content is reduced and the grain can be harvested.
For example, in the UK, the moisture content of the grain during harvesting and heat treatment is unlikely to be higher than 25%. Generally, the moisture content of corn is recommended to be 15% to 19%. However, the moisture content is usually 30% to 40%. However, a typical starch granule diagram indicates that in order for the crystalline structure of the starch granules to digest (at its gelatinization temperature), the water content must be above 50%. Although the moisture content of the grain just harvested from the field is very high (above 30%), the moisture content is not sufficient when the heat treatment temperature reaches 100 ° C when dry. In this way, only the starch in the grain will lose its structural order and gelatinization when it is in excess water, and thus become more digestible. Therefore, the starch in the dried grain, if any, is rarely gelatinized.
It should be noted that most poultry feeds undergo a series of processing, including drying, such as granulation, tableting and micronization, which can cause a degree of gelatinization.
2 secondary processing
However, it is also necessary to consider secondary processing, and dry heat treatment of the grain affected by the bad harvest weather can actually improve the crystallinity of the starch structure, which is almost a gelatinization reverse crystallization treatment. This may result in starch being more resistant to digestion during hot processing and less susceptible to chick endogenous digestive enzymes. This "improved" process is called annealing.
Annealing is generally considered to be delayed heating at high moisture, just below the starch gelatinization temperature. However, it has also been described as a higher condition for dry heat processing. These indicate that the higher dry heat treatment effect is better than the slow drying at 55 °C. Because, under the same initial moisture conditions, long-term lower temperature drying is more likely to anneal the starch than short-term high temperatures.
Proteins associated with starch are also affected by heat treatment. Investigations have shown that temperatures do not cause some changes at temperatures between 30 ° C and 50 ° C. The starch structure did not change and most of the changes occurred on the protein components.
Can these in vitro studies be transferred to animal feeding?
Dry weathering of weather-affected grains should not be confused with grain processing used in animal feed because of the extreme heat, pressure and later steam treatment. It was found that heat treatment at 160 °C for corn with an initial water content of 33% to 38% did not affect its chemical composition, but the physicochemical properties were not determined. The drying temperature fluctuates depending on the corn sample.
3 heat treatment
The authors found that (unpublished), compared to the initial moisture content of 37% wheat, the initial moisture content of 27% of wheat has a high starch digestibility after drying at room temperature or 100 °C. Drying at 100 ° C can also improve starch digestibility compared to room temperature drying. However, microscopic imaging shows that changes in starch integrity do not result in an effect on digestibility, which means that other components may be involved.
The authors speculate that the other components mentioned above may be proteins associated with starch granules, which are susceptible to changes in heat treatment and moisture content.
The effect of high temperature drying on the nutritional quality of grains in poultry diets does not appear to be single and certain. Microscopic observation can determine whether the digestibility is improved based on the structural integrity of the starch. A potential risk is that annealing during drying may increase the subsequent gelatinization temperature. Therefore, the feed processed by this method may not be easily digested. High temperature drying reduces starch digestibility by increasing starch-related protein degradation. This effect can only be measured by the solubility/recovery experiment of the protein, not the gelatinization temperature. Thus, three quality parameters - microscopy analysis, gelatinization temperature and protein solubility - are useful for determining the nutritional value of cereals affected by drying techniques.
(Translated from feed international, 2011, 1/2, P22-23.)
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