Advantages of organic acids for feed and feed raw materials
Prevention of disease transmission and enhancement of growth and feed efficiency are critical factors in modern animal production (Mroz, 2005). When pathogenic bacteria contaminate feed, it becomes a potential route of transmission of disease to animal and human populations, and is consequently of great concern to producers and consumers (Crump et al., 2002; WHO, 2006). Food-producing animals (e.g. cattle, chickens, pigs, and turkeys) are the main reservoirs for many of these microorganisms, which include non-Typhi serotypes of Salmonella enterica, Campylobacter species, Shiga toxin producing strains of Escherichia coli, and Yersenia enterocolitica (Mead et al., 1999). The microflora found in feed materials comes from a variety of ecological niches e.g. soil and the animals' gastrointestinal (GI) tract. The GI tract pathogens can be introduced into food chain by animals defecating in the farm environment or by fertilisation of crops with manures (Maciorowski et al., 2007), consequentially making feed a carrier for animal and human pathogens.
Feed materials may be inoculated with microorganisms, mostly bacteria and fungi, at any time during growing, harvesting, processing and storage. Counts of microorganisms vary depending on the function of materials, location of its origin and climatic conditions (D'Mello 2006). It is known that microfloral growth is dependent on moisture, pH value, temperature and composition of feed materials (Maciorowski et al., 2007). For example, the optimal temperature for E. coli O157:H7 is 37°C, with a minimum of 7-8°C and a maximum of 46°C. The optimal pH is between 6 and 7, however it might stand a pH range between 4.4 to 9.0. The E. coli O157:H7 doubles in number approximately every 0.4 h at the optimal temperature and pH value. Some microorganisms including E. coli may adapt to conditions without water and can actively grow in stored feed. Some authors reported that grains and oilseed crops possess a diverse microflora, with populations ranging from 5x103 to 1.6x108 colony-forming units (CFU)/g that are highly resistant to low moisture conditions (Multon, 1988; Richard-Molard, 1988). The orientation values of mesophilic bacteria, including Listeria monocytogenes, Pesudomonas maltophilia, Thiobacillus novellus, Staphylococcus aureus, Streptococcus pyrogenes, Streptococcus pneumoniae, E. coli and Clostridium kluyveri etc. for swine compound feed are shown in Table 1 (VDLUFA, 2007).
Table 1. Orientation values for the contamination levels of mesophilic bacteria in swine feed.
|Normal Physiologic Discharge||Mesophilic bacteria x 106|
|Diet||Very good, ?||Good, >||Poor, >||Very poor, >|
|Breeders, fattening pigs||6||6||25||60|
|Breeders, fattening pigs||1||1||5||10|
Experimentally, very low doses of E. coli O157:H7 may result in colonization of some piglets. Once some piglets are colonized, they may amplify E. coli O157:H7 and transmit it to other piglets via contact. Enterotoxigenic E. coli strains are a major cause of diarrhea and death in neonatal and newly weaned pigs. Enterotoxigenic E. coli adhere to the small intestinal microvilli and produce enterotoxins that act locally on enterocytes. This action results in hypersecretion of water and electrolytes, and reduced absorption (Amezcua et al., 2002).
Heat treatment, usually during conditioning, pelleting or extrusion has been shown to be an effective way to reduce microbial loads in feed materials and compound feed. Reduction of the bacterial contamination by heat is dependent on the temperature and treatment time. However, these methods do not prevent a recontamination of feed materials and compound feed afterwards (Çelik et al. 2003; Maciorowski et al., 2007, WHO, 2006). Dietary acidification with organic acids has been shown to contribute to environmental hygiene preventing feed raw materials and compound feed from microbial and fungal deterioration. Moreover, constant treatment with organic acids has a residual protective effect in feed, which helps reduce recontamination and also reduce the contamination of milling and feeding equipment as well. Supplementation of organic acids in feed tends to decrease the feed pH, buffer capacity and to prevent undesirable microbes' growth. However, for each acid, its specific inhibiting effect on bacteria, yeast and mould has to be considered when recommendations for feed supplementation are made. For example, some organic acids, like formic and propionic acids, have broader antimicrobial activities and can be effective against bacteria and fungi, including yeast.
Dietary acidification is important to create unfavorable conditions for microorganisms and for reduction of pH and stimulation of GI tract enzymes. Optima pH is needed for enzyme activation, for example, pepsinogens are rapidly activated at pH 2, but very slow at pH 4. Pepsin has optima between 2 and 3.6, and remains inactive at pH 6 (Kidder and Manners, 1978). Due to insufficient production of HCl and pancreatic enzymes, and sudden changes in feed consistency and intake, piglets have limited digestive capacity and absorption at weaning. Moreover, the stress associated with weaning is known to disturb the intestinal microflora. Various studies show that acidification of the diets decreased pH-value in feed and consequentially reduced the coliform and E. coli counts along the intestinal tract, decreasing scouring and mortality of piglets. It has been shown that acid conditions favor the growth of lactobacilli in the stomach, which possibly inhibits the proliferation of E. coli and produces lactic acid and other metabolites which lower the pH and inhibit E. coli. The reported pH levels of the swine diets in these studies range from 4.36 to 5.79 (Kluge et al., 2006; Partanen et al. 2007; Mroz 2008). Dietary acidification by a mixture of organic acids decreased the pH value in these swine diets by 0.15 to 0.98 pH units. The decrease in the pH values was dependent on the inclusion levels of organic acids, which varied from 0.5 to 3%, and composition of the diet. This was in agreement with a recent study, where a blend of formic and propionic acids (Biotronic® SE forte, BIOMIN, Austria) at an inclusion level of 0.3% reduced the pH by 0.11 pH units in starter and grower diets. A higher inclusion level of 0.5% of the same acid blend reduced pH of the diet by 0.23 and 0.21 pH units in the starter and grower diets, respectively. The pH and B-values of starter and grower feed are shown in Table 2.
Table 2. Efficacy of the blend of formic and propionic acids on decrease in pH and B-value in swine feed.
|pH Value||B-value *, mEq/kg|
|Inclusion of acids blend kg/t feed||0||3||5||0||3||5|
*B-value was determined by titrating 10% of feed slurry with 0.1 M HCl to obtain a pH value of 5.
Another factor affecting the response of acidifier would be the buffering capacity of the diets. By definition, the buffering capacity (B-value) is the change in the pH value of a defined volume or mass after the addition of a strong acid. A more practical definition in the feed industry is that the B-value is the amount of 1 M hydrochloric acid (HCl) solution which needs to be added to a 10% slurry of feed or a feed ingredient in 100 ml of water in order to obtain a pH-value of 5, in some cases a pH value of 4 or 3. This definition is the reason why we find different values for the same expression in practical applications. For example, various studies reported B-values ranging from 380 to 700 mEq per kg feed (Bolduan et al., 1988; Blank, 1999; Mroz et al. 2008). Acid-buffering capacity is lowest in cereals and cereal by-products, intermediate or high in protein feedstuffs and very high in minerals. It might be reasonable to assume that the buffering capacity of pig feed can be considerably influenced by selection of feed ingredients, and it may in part result in differences in the effectiveness of acidifiers. It is recommended that swine diets should not exceed 700 mEq/kg of feed B-value. High protein and mineral content of feed ensures rapid animal growth, but generates high buffering capacity, thus reducing levels of HCl in the stomach. Results of some studies demonstrated that high B-value of the diet increased gastric pH and resulted in decreased amino acids digestibility (Jung and Bolduan, 1986; Lawlor et al. 1994; Blank et al., 1999). Lowering dietary buffering capacity, via acidification with organic acids, has been shown to inhibit luminal growth of enterotoxigenic microflora and to enhance swine performance (Ravindran and Kornegay, 1993; Gabert and Sauer, 1994). The results of an in vitro study showed that an acidifier consisting of a blend of formic and propionic acids (Biotronic® SE forte, BIOMIN, Austria) at an inclusion level of 0.3% decreased B-value by 16 and 17% in starter and grower pig diets, respectively. Moreover, the inclusion level of 0.5% of the same acid blend decreased B-value by 18 and 19% in starter and grower pig diets, respectively. The decrease in B-value was directly related to the inclusion level of acidifier and the diet composition.
Acidifiers are powerful tools to maintain animal health and improve their performance, as well as to control feed and environmental hygiene. Consistent beneficial effects on productivity in weaned pigs have been reported in numerous scientific studies with results showing the decreased microbial counts in feed and improved animal growth performance, reduced diarrhoea, morbidity and mortality rates. Furthermore, an overwhelming portion of livestock producers consider acidifiers as an outstanding solution to enhance performance and, therefore, profitability.
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