1. INTRODUCTION
2. HEAT TREATMENT
3. GRINDING
4. PELLETING AND CRUMBLING
5. PROCESSING TO DESTROY NATURALLY OCCURRING TOXINS AND INHIBITORS
6. EFFECT OF PROCESSING ON MOULDS, SALMONELLA, AND OTHER HARMFUL SUBSTANCES
7. EFFECT OF PROCESSING ON THE AVAILABILITY AND NUTRITIONAL VALUE OF VITAMINS
8. EFFECT OF PROCESSING ON THE AVAILABILITY AND NUTRITIONAL VALUE OF TRACE MINERAL ELEMENTS
9. REFERENCES
G. Walker
Western Farmers Association
Seattle, Washington
Feeds are processed to facilitate handling and pelleting, and to increase feeding value by increasing digestibility or by inactivating specific growth inhibitors. Many processes that facilitate handling and pelleting increase the nutritional value of feeds as well, but the nutritional value of some feeds can be lowered by certain processes.
Heat treatment is employed to dry certain plant products and to improve the quality of pelleted feeds. Heat is sometimes employed in the extraction processes to remove oil from some oilseeds. Heat treatment improves the nutritional value of soybean meal by destroying the trypsin inhibitor which is naturally present, and by increasing the utilization of proteins and amino acids, fats, and carbohydrates present in the meal. Digestibility is enhanced by partial "cooking" and thus the ME value is increased.
Heat treatment increases the nutritional value of cereal grains by gelatinizing starches and improving digestibility. This change occurs during steam pelleting, when dry steam is added to the mixture of feed ingredients just prior to pelleting to condition the feed so that better quality pellets are produced. Proper conditioning of feeds before pelleting results in improved pellet durability and a reduced amount of fines in the finished product.
Grinding may increase the nutritional value of feeds by reducing the particle size of the ingredients and thereby increasing the surface area of the ingested feed and facilitating digestion. In addition, the grinding process adds metals to feeds from the grinding machinery and can prevent a micromineral deficiency. The metals that are added to feed are Fe, Zn, Cu, Mn, and Na.
The pelleting and crumbling process compacts the mixed feed ingredients and increases nutrient density and bulk density. For some species, pelleting improves palatability or acceptability of feeds. As mentioned above, the steam conditioning that occurs just before pelleting may improve digestibility. The heat generated during compaction of the pellet may also improve digestibility and destroy thermolabile toxic factors that naturally occur in some plant products. Pelleting enhances the availability of phosphorus in wheat bran and also permits the use of low density, bulky, unpalatable feeds that might not otherwise be practical to use.
Cottonseed meal contains gossypol, a compound with undesirable nutritional qualities. Free gossypol is associated with the pigment gland. Puncturing the pigment gland causes free gossypol to be mixed with other compounds in cottonseed meal and becomes bound. Bound gossypol is less harmful than is free gossypol.
The method used to remove the cottonseed oil affects the gossypol content of the residue which becomes cottonseed meal. Hydraulic pressed and solvent-extracted meals contain higher levels of gossypol than does prepressed solvent meal. Glandless meal has lower levels of gossypol. Treatment of the meal with iron salts eliminates the toxicity of gossypol, but also produces a darker meal which is unacceptable to the feed industry (see Table 1).
Treatment of cottonseed meal with phytase increases the availability of phosphorus, reduces gossypol toxicity, and increases the availability of some proteins.
Table 1. Toxins and Inhibitor Destruction by Processing
|
Feed Stuff |
Inhibitor |
Deactivation Process |
|
Cottonseed Meal |
Gossypol: Cyclopropane fatty acids; Phytate |
Add iron salts; rupture pigment gland |
|
Soybean Meal |
Trypsin inhibitor |
Heat, autoclaving |
|
Linseed Meal |
Crystalline water-soluble substance |
Water treatment |
|
Raw Fish |
Thiaminase |
Heat |
|
Alfalfa Meal |
Saponins; pectin methyl esterase |
Limit amount fed |
|
Rye |
5-N-alkyl resorcinols |
Limit amount fed |
|
Sweet Clover |
Dicoumarol |
|
|
Wheat Germ |
Unidentified |
Heat |
|
Rapeseed |
Isothiocyanate; thyroactive materials |
|
Heat destroys the trypsin inhibitor. Wet heat is more effective than dry heat.
The toxic factor for poultry can be removed by soaking the meal in water for 12-18 hours.
Aflatoxins, produced on some feeds by Aspergillus flavus, are not inactivated by normal pelleting procedures. Prevention of mould growth is the best means to avoid contamination. Removal and disposal of mould-damaged material is essential. Certain materials that are sometimes contaminated by aflatoxins, such as corn, peanut meal, cottonseed meal, copra, and fish meal, should be monitored routinely to check for the presence of aflatoxins.
In contrast to aflatoxins, salmonella is killed by pelleting. Meat meal is a feed ingredient that should be routinely monitored for the presence of salmonella; proper sanitation procedures will reduce the incidence of contamination.
Prolonged heat treatment can destroy the fat soluble vitamins and the water soluble vitamins thiamine, pantothenic acid, folic acid, and biotin. Such heat treatment can occur during the drying of protein supplements of plant and animal origin. For example, meat and bone meal contains little or no thiamine as a result of processing. The water soluble B vitamins are removed to some extent during the processing of fish into fish meal. The B vitamin content of fish-stick - water is higher than that of fish meal.
The pelleting process can alter the vitamin content of feeds. If insufficient anti-oxidants are present in the feed, pelleting may destroy Vitamins A, E, and K. On the other hand, pelleting may increase the availability of nicotinic acid and biotin, which are often present in feeds in a bound form.
Certain trace minerals act as pro-oxidants in feeds and can hasten the oxidative destruction of Vitamins A, D, and E. Manganese and iron are examples of such minerals. Non-nutritive feed additives, such as bentonite, which are added to feeds as binding agents, may interfere with the availability of riboflavin and certain divalent cations, such as zinc.
Some commercial methods for processing grains and oilseed meals result in decreased levels of certain vitamins in the final product. For example, coarse wheat flour grinders lose less Vitamin E than fine grinders. Germinated wheat may have a Vitamin E level as low as 30 percent of ungerminated wheat. Solvent extraction of soybean meal results in the removal of some of the Vitamins E and K and, thus, lower levels in the product.
Generally, the availability of minerals is not affected by processing. The availability of zinc is sometimes enhanced by addition of the chelate EDTA. Pelleting and crumbling do not affect trace mineral levels in feed.
National Research Council, 1973 Committee on Animal Nutrition; Center for Tropical Agriculture, University of Florida; and. Department of Animal Science, University of Florida, Effect of processing on the nutritional value of feeds. Proceedings of a Symposium, Gainesville, Florida, January 11-13, 1972. Washington, D.C., National Academy of Sciences, 491 p.
