Nutrient stability as affected by processing of feeds
Nutrient stability under normal conditions of storage and processing is one of the important factors determining the effectiveness of micronutrients or feed additives such as vitamin A. Nutrient stability is affected by a wide range of physical and chemical factors. Although many factors may cause serious nutrient degradation, measures can be developed to minimize losses by applying proper technology, which includes application of a protective coating for an individual nutrient; addition of antioxidants, control of temperature, moisture and pH and protection from air, light, and incompatible metals during processing and storage. Effect of various processes on micronutrients stability has been discussed in the following paragraphs.
Mixing
Because of the cost, nutritional importance and small quantity of micro-nutrients, more attention is needed for mixing them with other ingredients of the diet so that it can be mixed homoge nously. Particle size, p article shap e, specific weight, hygroscopicity, susceptibility to electrostatic charges, and adhesiveness of the particles due to physical properties such as rough surface are the physical properties of the supplement which may influence the mixing. Particle size of the feed additive is an important factor which influences its mixing with other ingredients of the mixture. Fine particles mix more homogenously than the coarse particles. Similarly, spherical particles mix more homogenously than those having rectangular shape or irregular shape. Hygroscopicity of feed additives is another feature which determines its mixing with other ingredients of concentrate mixture. Some ingredients such as common salt, cobalt chloride etc. are hygroscopic in nature and due to this nature there is a problem for their accurate weighing and mixing. Due to added moisture content, such ingredients tend to agglomerate with their neighboring particles and do not disperse smoothly in whole of the mass. Many micro substances are not uniformly distributed in their pure form or occur in a concentrate state. Therefore, these are mixed with another inert, non-active and homogenous material, prior to the mixing with feed mixture. Such inactive or inert materials may or may not change the physical characteristics of the micro substances but their activity is not influenced and this arrangement gives better mixing. To achieve the homogeneity of active micro substances in whole mass, premix is prepared usually with other micro ingredients using a small mixture to avoid the contamination of one premix with the other. The other problem in supplementing the concentrate with feed additives is that many times the concentrate mixture is subjected to the high temperature and moisture as in pelleting, which may be harmful to the added substance(s). The storage period also influences the activity of some feed additives due to the interaction of several factors such as minerals (which some times act as oxidizing agent), pH of the concentrate mixture, relative humidity and the ambient temperature. Concentrate mixture takes appreciable time, may be few weeks, before reaching the consumers from the manufacturing units. Keeping these facts in view, one has to choose the feed additive very carefully based on the type of processing of the ingredient in which it has to be supplemented in order to get maximum response from the animals.
Pelleting
During pelleting, the most critical factors to be considered are friction (abrasion), pressure, heat, humidity and conditioning time. Feed additives, most sensitive to these conditions include vitamins A, K, C and carotenoids. The most destructive aspects of the pelleting process have been identified as wet steam, fat addition and high energy input.
Extrusion
The extrusion process involves much higher energy inputs (50-100 kWh/t feed) and temperatures than the pelleting process. The dominant adverse factors are pressure, heat, humidity and redox reactions. It is, therefore, considered as the most destructive process to vitamins and other sensitive feed additives. Vitamin A, E, B1 and folic acid have been found to be strongly affected by this process and therefore, over dose of these micronutrients should be supplemented in extruded feeds. The losses occur during the process itself and to a lesser extent during subsequent storage. Stability tests confirmed that vitamin A in a hardened coating provides much better retention values than in the conventional sources because its coating has a higher physical stability. Coated vitamin E and folic acid have also been recommended.
Processing of forages
Vitamins present in forages also undergo changes when they are subjected to the processing such as drying during hay making and ensiling. But these processes cause losses of ß carotene depending upon the period of storage. The losses after one and four months storage of berseem and oat hays were 33, 28 and 71, 75 % , respectively. Similarly, loss of vitamin E in berseem and oat hay after one month of storage hay was 52 and 46%, which increased to 86 and 82 % , respectively, after 4 months of storage.
Processing of concentrate
To protect the vitamin A and E contents in concentrate mixture, it is generally recommended to mix the butylated hydroxyl toluene (BHT) or butylated hydroxyanisole (BHA) as an antioxidant to check the rancidity of fat of concentrate ingredients. The level of BHT or BHA normally varies from 125 to 200g/ tonne. Addition of BHT to the concentrate mixture enhances the loss of vitamin A, but not the concentration of vitamin E, which increased with the storage period and maximum loss of both the vitamins was recorded at sixth month of storage.
Vitamin retention
Stabilized vitamin A is the best source for the supplementation of compounded feed. Both vitamin A and carotene are unstable to oxidation; and fish liver oil is subject to oxidation unless protected by antioxidants. Trace minerals also catalyze the breakdown of vitamin A. The stability of naturally occurring tocopherols is poor and substantial losses of vitamin E occur in feeds and fodders on processing and preservation, as well as during manufacturing and storage of finished feeds. Vitamin E in feed ingredients is unstable under conditions that promote oxidation of feedstuffs such as heat, oxygen, moisture, oxidizing fats and trace minerals. Hence, there is a need to monitor changes in á- tocopherol and ß-carotene contents of fodders upon processing and preservation. Stability of added microencapsulated vitamins A, B1, B2 and D has been assayed after short period/ high temperature extrusion cooking in an accelerated storage test at 24°C and 37°C after 1, 3 and 6 months storage. The stability of vitamin A palmitate in these mixtures was extremely good. The average loss in vitamin A was recorded to be 1.4% per month. This stability of vitamin was attributed to the short period at maximum temperature.