Physicochemical and enzymatic hydrolysis characteristics of glycogen granules isolated from ruminal protozoa that are relevant to their analysis and digestion

Authors: Hall, Mary Beth

Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/30/2019
Publication Date: 3/14/2019
Citation: Hall, M. 2019. Physicochemical and enzymatic hydrolysis characteristics of glycogen granules isolated from ruminal protozoa that are relevant to their analysis and digestion. Journal of Dairy Science. https://doi.org/10.3168/jds.2018-15758.
DOI: https://doi.org/10.3168/jds.2018-15758

Interpretive Summary: Rumen microbes digest much of the feed a cow consumes. In turn, they provide nutrients the cow uses to grow and make milk. Understanding the digestion characteristics of microbial products and factors that affect their analysis is essential to improving our predictions of nutrients available to the cow. Rumen microbes such as protozoa can ferment carbohydrates or store them as "glycogen", which is similar to starch. Evaluation of protozoal glycogen showed that it needed to be gelatinized for analysis, but even ungelatinized, it was more rapidly digested by enzymes than were corn or wheat starch. Accordingly, for most accurate analysis, protozoal glycogen should be gelatinized. For predictions of digestibility, it may be estimated as more digestible than plant starch. These findings will be useful to dairy nutritionists in more accurately formulating diets to meet the needs of dairy cows.

Technical Abstract: Digestion and analysis of rumen microbial products are influenced by the physicochemical and enzymatic hydrolysis characteristics of those products. The objective of this study was to describe those characteristics for isolated protozoal glycogen granules. The protozoal glycogen granules were determined to be 98.3% alpha-glucan by enzymatic-colorimetric starch analysis. The granules underwent gelatinization, which commenced at 65°C, as compared to purified wheat and corn starches which initiated gelatinization at 50°C and 65°C, respectively. Treatment of ungelatinized samples with amyloglucosidase for 2 h at 39°C showed approximately 3-fold greater hydrolysis to glucose for protozoal glycogen (24.7% of dry matter; DM), than for wheat (9.0% of DM) or corn (7.4% of DM) starches. After gelatinization at 85°C, protozoal glycogen again showed a greater extent of enzymatic hydrolysis (95.3% of DM) than the starch samples (89.4% of DM for both wheat and corn). Thus, it appears that native glycogen may be more readily digested by small intestinal enzymes than corn or wheat starch, and that protozoal glycogen samples should be gelatinized to allow accurate enzymatic analysis of the alpha-glucan it represents.