Feed science constantly strives to optimize functional and nutritional properties of feeds aiming to meet the needs of future health, performance, and environmental demands. The main objective of this research was to examine if a treatment of barley grain with lactic acid (LA) and citric acid (CA), alone or together, with or without an additional heat treatment, and with the enzyme pullulanase (PUL) has beneficial effects on the content of key nutrients of the cereal with health-promoting properties such as resistant starch (RS) and dietary fibers. The second aim was to evaluate whether treatment of barley grain with LA and CA improves the phosphorus (P) availability for rumen microbiota in vitro. An increase of the P availability in cereals is a substantial opportunity to reduce P losses to the environment and to improve the utilization of organically-bound P mostly found in cereals. To answer these questions three studies were conducted. In the first study, several varieties of barley grain were treated for 24h in 5% (vol/vol) of LA, CA and PUL. For RS, total starch (TS), crude protein (CP), neutral detergent fiber (NDF) and ash, interactions between acid treatments and genotypes were observed. The organic residue (OR), a fraction that includes (1-3, 1-4)-ß-D-glucan (ß-glucan), was increased from 8.93 to 14.76% by CA treatment. An increasing acid concentration from 1 to 5% elevated the amount of RS linearly. The PUL treatment increased the RS content from 0.26 (untreated control barley) to 1.29% and therefore to the greatest extent of all treatments. In the second study, the potential additive effects of an additional thermal treatment on the nutrients were evaluated. Therefore, after steeping barley in 5% solutions of LA and CA for 24h at 22°C, the samples were oven-heated at 100°C for 2h with and without a rapid cooling (4°C for 2h). The combined acid and heat treatments with additional rapid cooling doubled the RS content compared to the acid treatments, whereas heat and cooling conditions showed limited effect on ash, protein and insoluble fibers.
In the third study, the effects of processing of barley with LA and CA (5% (vol/vol) for 24h) on the microbial composition, metabolic fermentation profile, and nutrient degradation at high or low dietary P supply by using the semi-continuous rumen simulation technique (Rusitec) were evaluated. Results showed a drop of total bacteria, but not of total protozoa and short-chain fatty acid (SCFA) concentration with the low P diet. Under low P but not under high P conditions the LA and CA treatments substantially enhanced the concentration of SCFA. The LA treatment increased the abundance of total bacteria in the low P diet.
Interestingly, in the low P diet, the CA treatment increased the most prevalent bacterial group, genus Prevotella, in the rumen fluid and NDF degradation at the same extent as inorganic P supplementation did in the high P diet. Treatments with CA and LA enhanced NDF degradation and reduced the degradation of ammonia concentration and CP compared to the control diet at both P levels.
In conclusion, data indicated that treating barley with LA, CA or PUL might contribute to an enhanced health value of the treated cereal. For ruminant nutrition, the beneficial effects of CA and LA treatments on specific ruminal microbes, fermentation profile and fiber degradation in the low P diet suggest a potential for the treatments in vitro, to enhance the availability of organic P. Further research is warranted to understand the mechanisms behind the effects observed herein with the respective treatments and to evaluate to which extent these treatments may enhance the availability of organic P in barley grain under in vivo conditions