Published online by Cambridge University Press: 27 February 2018
In vitro gas production techniques have become popular for characterizing the rate and extent of digestion of ruminant foods. In a typical gas production study, gas (predominantly carbon dioxide) is produced as particles of substrate are fermented by rumen micro-organisms in a bicarbonate buffered culture medium. Innovations in equipment design, including automated pressure recording systems and mathematical descriptions of the gas production profiles themselves, make the techniques both simple and precise and therefore good as laboratory procedure. The technique of measuring gas is of value in ruminant science because the kinetics of gas production and substrate degradation are very closely correlated. However, although it is relatively easy to measure gas volumes and to determine the kinetics of gas production, the underlying processes that give rise to the gas in the first place are complex and not well understood. Therefore, there is concern about what is being measured in gas production studies and how this relates to the digestion process in the ruminant animal. In this paper we review some of the fundamental properties of gases, describe their behaviour in liquids and consider some of the biological and chemical factors influencing gas production. From a knowledge of how gases behave in liquids and at gas-liquid interfaces, both at ambient and increased temperatures and pressures, it is possible to deduce what is happening in gas production studies. However, although the technique is invaluable for obtaining information about the digestion of particulate substrates in anaerobic ecosystems, we conclude that gas production should be used with caution in routine food evaluation studies.
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