INTRODUCTION MICRONUTRIENT STABILITY IN FEED PROCESSING J. Broz, E. Schai and M. Gadient F. Hoffmann-La Roche Ltd, Basel, Switzerland Feed processing aims to improve the distribution of nutrients as well as the digestibility of carbohydrates in compound feeds (e.g. pelleting, extrusion). However, these processes may be harmful to labile nutrients, such as vitamins, that can be easily oxidized. Vitamins as biologically active micro-nutrients are generally quite sensitive to various physical and chemical factors. Sensitivity of individual vitamins is summarized in Table 1. Several factors can influence vitamin stability during feed processing and storage. Heat, pressure, humidity, friction and redox reaction vary drama-tically among the different ways feed can be processed. PELLETING Pelleting is a process carried out in most feed plants around the world. The fact that a high percentage of all compound feed is marketed as pellets indicates that this process is required to provide feeds of high quality. The basic characteristics of pelleting technology have not changed over the years. In pelleting, the most important adverse factors are friction (abrasion), pressure, heat, humidity and conditioning time. Feed additives most sensitive to this process are well known and include vitamin A, K 3 and C, but also carotenoids and feed antibiotics (Gadient, 1986; 1994). The most destructive aspects of the pelleting process have been identified as wet steam, fat addition and high energy input. Pelleting can affect the natural vitamin content of feed ingredients as well as the vitamins added in the form of commercial products. Modern forms of vitamins are usually less affected by the process, because they are especially developed for use in animal feeds. A common principle behind the formulation of current products is the pro-tection of a sensitive vitamin chemically with an antioxidant and physically with a protective coating. The coating is frequently a gelatinebased matrix, though the specific product design differs according to the supplier of the vitamin products. In recent years, the specific energy input in the pellet mill has increased from an average of 10 kwh/t feed to around 20 kwh/t (Gadient, 1994). This is due to harder pellets requested in the market and the use of cereal substitutes and by-products of low pelletability, such as tapioca or corn gluten. Additional processing, e.g. prolonged conditioning and double pelleting, further increase the aggressiveness of the pelleting process. Although not dramatic, these changes tend to increase losses at pelleting and even more during subsequent storage of compound feeds. The results presented in Figure 1 confirmed that under such conditions, the coating of advanced forms of vitamin A and AD 3 product markedly improved vitamin A stability. A high-quality coating has also been shown to be required for the stability of feed carotenoids (Gadient, 1994). Vitamin products currently marketed for feed application show generally good stability in pelleted feeds. However, it is 1
relatively difficult to predict vitamin losses occurring in a specific pelleted feed. As already mentioned, vitamins are influenced during the process by a multitude of factors which are variable and not completely standardized. As a general guideline, stability of vitamins after three months of storage at room temperature in a compound feed pelleted under typical conditions is indicated in Table 2. In contrast to all other vitamins, an effective and economic coating has not yet been developed for vitamin C, which is an essential nutrient for aquatic animals. Several attempts have been made to improve the stability of L-ascorbic acid. In recent years, very stable derivatives of L-ascorbic acid, e.g. L- ascorbyl polyphosphate, are increasingly used in complete diets for fish and crustaceans. EXTRUSION The extrusion process involves much higher energy inputs (50-100 kwh/t feed) and temperatures than pelleting. The dominant adverse factors are pressure, heat, humidity and redox reactions. It is, therefore, considered as the most aggressive process to vitamins and other sensitive feed additives. Vitamins A, E, B 1 and folic acid have been found to be strongly affected by this process and therefore, sizeable supplementation overages should be used in extruded feeds. The losses occur rather during the process itself and to a lesser extent during subsequent storage. Stability tests conducted with a pet food (Gadient, 1986) confirmed that vitamin A in a hardened coating pro-vides much better retention values than conventional sources because its coating has a higher physical stability. Other studies demonstrated that also for vitamin E and folic acid the use of coated forms can be recommended (see Figure 2). Besides pet foods, extrusion has also become the standard process for fish feeds. This technology has been shown to increase digestibility, to reduce nutrient leaching in water and to enhance the holding capacity for added fat. Stability of ascorbic acid and other vitamins in extruded fish feeds was studied recently by Gadient and Fenster (1994). Several sources of L-ascorbic acid were tested in main types of extruded fish feeds. In addition, the stability of other vitamins was investigated in a catfish feed. The results obtained in one trial are summarized in Table 3. In general, both ascorbyl monophosphate and ascorbyl polyphosphate were found to be quite stable, i.e. retention values were higher than 90% in the stored extruded feeds and above 85% in the stored semi-moist feeds. In contrast, coated forms of vitamin C were unstable, showing retention values around 50% after extrusion and below 16% in the stored fish feeds. In contrast, losses of the other vitamins in the catfish diet were only marginal. Despite the above mentioned results, it has to be kept in mind that extrusion is an aggressive process and even minor changes in processing parameters may influence the stability of feed additives. USE OF EXPANDER More recently, hydrothermal processes utilizing temperatures higher than 90 C are frequently used to produce more hygienic compound feeds, in parti-cular to reduce salmonella contamination. However, such processes, espe-cially the use of expander, are often regarded as potentially destructive to feed additives. An expander is a simplified low-cost extruder with a fixed ring-gap configuration. In the expander barrel, conveying is combined with shearing and pressure. Usually, in a pelleting system, the expander 2
is positioned between the conditioner and the pellet mill. Processing of feed with an expander prior to pelleting is widely used in Europe. Expanders have several advantages over traditional mash condi-tioners (Pipa and Frank, 1989; Coelho, 1994). In an expander, more of the starch is gelatinized and this factor maximizes pellet durability and nutrient digestibility. In addition, expanded feed can be pelleted easily and therefore pellet output is higher. High temperatures and pressures in an expander also destroy salmonellae and other pathogenic microbes. Feed manufacturers, however, expressed concern about the destruction of vitamins and other feed additives in compound feeds during expander condi-tioning. In fact, in the expander-pellet mill system, mash feed encounters moisture, high temperatures, friction and other processing stress factors. This concern prompted various investigations into the extent of losses during the expansion process and subsequent pelleting. Pipa and Frank (1989) exa-mined the stability of major vitamins in various feed types expanded around 120 C. They found no effects of the process on the recovery of vitamin B1 and E. For vitamin A, losses varied according to the feed type and reached 20% in some cases. In similar tests, Moulois (1991) found a decrease for vitamin A content in all tested diets. Schai et al. (1991) evaluated the effects of the expansion process on retention of several vitamins. They used a broiler feed expanded in a commercial feed mill at 106 C and then pelleted at 95 C. The results of this trial are presented in Figure 3 and indicated that these vitamins were not affected by the process to any significant extent. exception of vitamins A, K 3 and C, which are also susceptible to more traditional processes such as pelleting and extrusion. CONCLUSIONS Current forms of vitamins developed for use in animal feeds are relatively stable during feed processing. Compounds most sensitive to the pelleting process are vitamins A, K 3 and C. Extrusion of feeds is considered as the most aggressive process and in addition to the above mentioned compounds also vitamins E, B 1 and folic acid may be strongly affected. Advanced feed forms of vitamin A markedly improved its stability during both the pelleting and extrusion processes. The use of coated forms of vitamin E and folic acid result in an improved stability even during the extrusion. Nevertheless, supplementation overages are recommended for extruded feeds. No reasonable coating has been developed for vitamin C, but very stable derivatives (e.g. L-ascorbyl polyphosphate) are now available for feed appli-cation. Additional use of an expander does not affect the stability of most vitamins, with the exception of vitamins A, K 3 and C, that are also susceptible to other kinds of feed processing. Summarizing these results, it is obvious that expansion of feed does not adversely affect most vitamins to a greater extent, with the 3
REFERENCES Coelho, M.B., 1994. Practical vitamin stability with expanders. Proc. Arkansas Nutr. Conf., Fayetteville, pp. 52-61. Gadient, M., 1986. Effect of pelleting on nutritional quality of feed. Proc. Maryland Nutr. Conf., pp. 73-79. Gadient, M., 1994. New technological aspects in the use of feed additives. Zootecnica Intern., January 1994: 58-63. Gadient, M. and R. Fenster, 1994. Stability of ascorbic acid and other vita-mins in extruded fish feeds. Aquaculture 124: 207-211. Pipa, F. and G. Frank, 1989. High-pressure conditioning with annular gap expander: a new way of feed processing. Advances in Feed Technology 2: 22-30. Moulois, G., 1991. Trials on use of expanders in feed industry. Intercoop Workshop, Copenhagen, Denmark; November 1991. Schai, E., J. Broz and M, Gadient, 1991. Stabilität von Futterzusatzstoffen in expandiertem Mischfutter /On the stability of feed additives in expanded compound feeds/. Kraftfutter 74(11): 526-528. Table 1. Sensitivity of vitamins to external factors Vitamin Heat Oxygen Humidity Light A + + 0 + D 3 0 + 0 0 E - 0-0 K 3 0 0 + - B 1 0 0 0 - B 2 - - 0 0 B 6 + - 0 0 B 12 0 0 0 0 Pantothenic acid 0 - + - Niacin - - - - Biotin 0 - - - Folic acid 0-0 + C - + + 0 - Insensitive to hardly sensitive 0 Slightly sensitive to sensitive + Very sensitive 4
Table 2. Pelleting stability of vitamins Vitamin Retention after 3 months of storage at RT (%) A - conventional form 60-90 A - advanced form 80-90 D 3 - conventional form 70-90 D 3 - advanced form 80-100 E 95-100 K 3 (MSB) 50-70 B 1 60-80 B 2 95-100 B 6 80-100 B 12 50-80 Pantothenic acid 80-100 Niacin 95-100 Folic acid 50-80 Biotin 95-100 C 30-60 Table 3. Stability of vitamin C sources and other vitamins in an extruded catfish feed (retention, %) Vitamin Feed stored (months) 0 1 3 Vitamin C source AASC 51 31 16 AAPP 100 100 96 Vitamin A 100 100 Vitamin E 94 100 Vitamin B1 100 100 Vitamin B2 100 100 Vitamin B6 100 100 Pantothenic acid 100 92 Folic acid 96 88 Biotin 84 87 AASC = L-ascorbic acid AAPP = L-ascorbyl polyphosphate Adapted from Gadient & Fenster (1994) 5
Figure 1. Vitamin A stability in a pelleted pig protein concentrate Retention (% of mash) 100 90 80 70 60 50 40 30 20 10 0 93 82 After pelleting 94 ROVIMIX A T 8 6
Figure 2. Extrusion losses of vitamin E and folic acid forms Retention (% of mash) 100 90 80 70 60 50 40 30 20 10 0 78 87 Vitamin E 7
Figure 3. Retention of vitamins in an expanded broiler feed (Schai et al., 1991) 100 90 80 70 60 50 40 30 20 10 0 A D3 E B1 B2 Ca Pant Retention (%) After process Stored 8