Tripe is the stomach lining of cows and other animals. So how is tripe digested by humans? You would assume that it would not be broken down by the normal digestive process. If it is, why is it an effective stomach lining?
Our digestive juices can break down tripe because it has lost the protection it had when it was part of a living animal. Inside our own digestive tract it is broken down by gastric acid in the form of hydrochloric acid produced by parietal cells, and enzymes such as pepsin.
These substances could eat away at our own stomach lining too, were it not for the fact that it is covered in a layer of thick bicarbonate-rich mucus produced by so-called goblet cells. Bicarbonate is alkaline, so it neutralises the acid secreted by the parietal cells. Without the continuous bicarbonate supply your stomach would digest itself.
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鈥淕astric acid could eat away at our own stomach lining, were it not for the fact that it is covered in a layer of thick mucus鈥�
When, occasionally, gastric acid does reach the stomach lining, it causes a gastric or peptic ulcer. It was once believed that the majority of ulcers were caused by gastric acid, but we now know that the bacterium Helicobacter pylori is more often to blame, and that drugs such as aspirin can make matters worse. H. pylori weakens the protective mucous coating of the stomach and duodenum, allowing acid to get through to the lining beneath. The bacterium is able to survive in stomach acid because it secretes enzymes that neutralise it.
Leigh Farina, Cambridge, UK
Part of the answer lies in the fact that tripe is obtained from the ruminant fore-stomach, or reticulorumen, which does not secrete digestive juice in the way our own simple acid-secreting stomachs do. Cows eat grass and other fodder containing large quantities of cellulose. Mammalian enzymes cannot digest cellulose, so ruminants have evolved a complex stomach structure and a symbiotic arrangement to do the job.
The first three stomach chambers contain bacteria, protozoans and fungi. These organisms can ferment plant cells to yield volatile fatty acids (VFAs), which are converted by the liver to sugars and other substances needed for energy, growth and lactation. The reticulorumen must be able to circulate and pulverise ingested food before absorbing the VFAs, which is why it is a muscular organ with an epithelium that is both tough and able to allow VFAs to be absorbed. Tripe consists of this epithelium and the underlying supportive and muscle layers, making it a nutritious food.
That the 鈥渢rue鈥� stomach is prone to self-digestion is evidenced by the formation of ulcers. Its only protection from this type of attack is an unbroken layer of tenacious mucus and trapped bicarbonate which is continuously secreted. After death, and with the mucus stripped away, an acid stomach would itself become highly digestible.
Ian Jeffcoate, University of Glasgow Veterinary School, Glasgow, UK
Stomach lining forms only one side of tripe. The other side is easily attacked and digested. Chewing also disrupts tripe鈥檚 structure, leaving it open to enzymatic attack. Because it is dead tissue it has no effective repair processes. In addition, as with many structures that are resistant and self-repairing in a living organism, the process of cooking destroys tripe鈥檚 integrity.
Richard Lucas Hawley, Hampshire, UK