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1.1. Lipid metabolism in the ruminants and significance of hydrolysis and hydrogenation in the rumen. The digestion pattern of ruminants differs from other mammals in that the food of ruminants is subjected to a microbial fermentation in the rumen before passing into the true stomach. Carbohydrates, proteins, organic acids and many other food constituents are attacked by the micro organisms in the rumen and as a result short-chain fatty acids (VFA), CO2 , CH4 , NH3 etc. are produced as the end product of microbial metabolism. This ruminal fermentation has a considerable effect on the metabolic processes of the animal and moreover the functions of the rumen microorganisms are intimately associated with certain metabolic disorders of the ruminant (Bryant, 1959) e.g. Ketosis, bloat etc. It is now generally believed that the organisms of functional significance in the rumen are protozoa and bacteria which are capable of growth under the anaerobic conditions prevailing. The rumen provides an ideal anaerobic environment for a large and diverse microbial population at a temperature of 39°-40°C. The pH of the ingesta is slightly acid and the bacteria are adapted to live between pH 5.5 and 7.0 (Hungate, 1966). It is apparent from earlier studies that the lipids of ruminants differ in several respects from those of non-ruminant herbivorous animals in particular. Occurrence of unusually high proportions of stearic acid and the presence of trans acids and isomeric forms of oleic, linoleic and linolenic acid in depot fats and tissue lipids are peculiar to ruminants. [From Introduction]

The role of plant lipases in the hydrolysis of dietary lipids in the rumen of pasture-fed ruminants has been investigated by means of in vitro and in vivo experiments with rumen contents. Lipases present in the leaves of numerous pasture plants remained highly active for at least 5h in the presence of actively metabolising rumen microorganisms, leading to the hydrolysis of triglyceride. Parallel experiments showed that the lipase activity of actively metabolising rumen microorganisms in rumen fluid was very low. A slight increase in lipase activity attributable to microorganisms occurred after about a 4h incubation with autoclaved plant extract at which stage the metabolic activity of the microbial population had passed its peak. In vivo experiments showed that the lipolytic activity of rumen fluid obtained 0.5h to 5.0h after feeding fresh grass was about twice that of rumen fluid obtained after overnight fasting. Lipase activity was present in clarified rumen fluid. Pairedfeeding experiments with monozygotic twin cows demonstrated that the lipase activity of clarified rumen fluid prepared from the twin 0.5h after feeding fresh pasture was much higher than that of clarified rumen fluid from the 20h-fasted twin. Further paired-feeding experiments showed that lipase activity was higher in protozoa-free rumen fluid from the pasture-fed than from the hay-fed cow. In both treatments the highest levels of lipolytic activity were observed in the rumen samples removed 0.5h after feeding and there was a steady decline in activity over the sampling periods. Nevertheless, there was appreciable activity in the rumen samples obtained from the hay-fed animal which is consistent with the presence of lipase activity in the extracts of dried grass. Multiple forms of plant esterases and phosphatases were present in the soluble fraction of rumen fluid several hours after feeding. It is concluded that the rapid release of fatty acids which follows the ingestion of pasture is due mainly