Review: Improving feed digestibility through processing

Evaluation of the effects of processing technologies on digestion of NSP is hampered by the potential shift of polysaccharides recovered in the fiber fractions of common, gravimetric, fiber analysis methods. Results from in vivo studies describing effects of processing technologies or enzyme treatments on crude fiber, neutral detergent fiber, or acid detergent fiber digestibility, instead of NSP digestibility, should therefore, be interpreted with care. Detailed information on the composition of the NSP-fraction and digestibility of its components will help to identify and understand modifications that occur during processing.

Processes based on mechanical modification of feedstuffs that are commonly used in the feed industry, such as hammer and roller milling increase solubility of the NSP-fraction resulting in a 6–7% unit increase in coefficient of apparent total tract digestibility (CATTD) of the fiber fraction in both pigs and poultry. Dry thermal processes have a minor impact on physicochemical properties of feedstuffs and consequently, the effects on the coefficient of apparent ileal digestibility (CAID) and CATTD of the fiber fractions in pigs and poultry are limited. Hydrothermal processes that include high shear forces such as expander processing and extrusion cooking are more effective and increase solubility but also viscosity.

The CATTD of fiber fractions in pigs can be increased on average 3% units by hydrothermal processing of feeds and feed ingredients, although some studies have reported unchanged or even decreased digestibility values. In poultry, CATTD of fiber fractions can be increased 4–16% units by hydrothermal processing. Increased digesta viscosity resulting from technological processing of feed and feed ingredients can be counteracted by the addition of specific enzymes. Enzyme addition to heat processed diets and diets containing heat processed ingredients results in a 3- to 4-fold reduction in viscosity compared with enzyme addition to unprocessed diets, or diets containing unprocessed ingredients. In addition, modifications in cell wall architecture obtained by processing technologies will improve the accessibility of NSP to enzymes.

As a result, the effects of enzyme addition on digestibility of the fiber fraction are 1.5–6 times larger, when applied to heat processed diets compared with unprocessed diets.

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