Beef abomasum is the fourth and final stomach compartment of a cow, often called the “true stomach” because it functions most like a human stomach. Unlike the other three compartments, which ferment and mechanically break down plant material, the abomasum produces hydrochloric acid and digestive enzymes, working essentially the same way a single-stomached animal’s stomach does. In the kitchen, it’s known as reed tripe, and it has a long history as both a food and a source of enzymes used in cheesemaking.
Where It Sits in the Four-Stomach System
Cattle are ruminants, meaning their digestive system includes four distinct stomach compartments that process tough plant fibers in stages. Food first enters the rumen, a massive fermentation vat where bacteria begin breaking down cellulose. It then passes through the reticulum, which filters smaller particles forward, and the omasum, which absorbs water. The abomasum comes last, sitting toward the lower right side of the abdomen, just behind and beneath the rumen.
In a mature cow, the rumen dominates, holding about 80% of the total stomach volume. The abomasum accounts for roughly 8%, the omasum 7%, and the reticulum 5%. The total capacity of all four compartments together ranges from 95 to 230 liters. Interestingly, this ratio is completely reversed at birth. A newborn calf’s abomasum is the largest compartment, with twice the volume of the rumen and reticulum combined, because the calf relies on milk digestion rather than fermentation. By about 12 weeks of age, as the calf starts eating solid feed, the rumen overtakes the abomasum in size.
How It Digests Food
The abomasum is the only compartment that works through chemical digestion rather than microbial fermentation. Its inner lining is folded into ridges that produce gastric juices containing hydrochloric acid, maintaining a pH of about 2, which is highly acidic. This environment serves two purposes: it begins breaking down proteins from the cow’s diet, and it kills the enormous volume of bacteria that flow in from the rumen.
That second function is particularly notable. The rumen is packed with billions of microbes that help ferment plant material, and those microbes themselves become a major protein source for the cow. When they wash into the abomasum, the acid and an enzyme called pepsin break their proteins apart for absorption further down in the small intestine. The abomasum also secretes lysozyme, an antibacterial enzyme specifically adapted to dissolve bacterial cell walls. This is a specialization that non-ruminant stomachs don’t need, since they don’t receive the same flood of microorganisms.
A Common Veterinary Problem
The abomasum is the site of one of the most recognized health issues in dairy cattle: displaced abomasum. This happens when the organ loses muscle tone, fills with gas, and floats out of its normal position, either to the left side (behind the rumen) or high up on the right side beneath the last ribs. In both cases, the entrance and exit of the stomach become kinked, slowing the passage of food.
Displaced abomasum occurs almost exclusively in dairy breeds, particularly high-producing and older cows. Most left-sided displacements happen within the first month after calving, while right-sided cases spread out over the first three months. The condition is closely linked to feeding practices. Diets heavy in grain and low in roughage cause the abomasum to enlarge while the rumen shrinks, and sudden changes in grain levels around calving are frequently suspected as triggers. Affected cows typically go off their feed (especially concentrates), show a sharp drop in milk production, become dull, and pass less manure than normal. Veterinarians diagnose it by listening for characteristic sounds when the gas-filled stomach becomes trapped against the body wall.
Reed Tripe in the Kitchen
Each of the cow’s four stomach compartments produces a different type of tripe. The rumen yields blanket (flat) tripe, the reticulum produces honeycomb tripe, and the omasum gives book tripe. The abomasum is sold as reed tripe, named for the long, parallel folds of its inner lining. Its flavor ranges from strong to mild depending on preparation and sourcing, and its texture is distinct from the more commonly sold honeycomb variety.
The most famous dish built around beef abomasum is lampredotto, a traditional street food in Florence, Italy. Lampredotto is made by slow-cooking the abomasum in a vegetable broth with herbs, then chopping it and serving it on a bread roll, typically topped with a spicy sauce or salsa verde. It remains a staple of Florentine food culture and one of the city’s most recognizable sandwiches.
Nutritionally, tripe in general is a lean protein source. A three-ounce cooked serving contains about 80 calories, 10 grams of protein, and 3 grams of fat with essentially no carbohydrates. It provides meaningful amounts of iron, zinc, selenium, phosphorus, calcium, and B vitamins like niacin.
The Cheesemaking Connection
Beyond its role as food, the abomasum of young calves is the original source of rennet, the enzyme mixture that has been used to make cheese for thousands of years. The key enzyme is chymosin, which causes milk to separate into solid curds and liquid whey by breaking apart a specific protein in milk called kappa-casein. Chymosin accounts for about 95% of rennet’s clotting activity, with pepsin contributing the rest.
Calf rennet is extracted from the abomasum of suckling calves, specifically because young animals produce high concentrations of chymosin to digest their mother’s milk. The extraction process and its application in cheesemaking have remained fundamentally unchanged for millennia. Today, many cheesemakers use microbial or genetically engineered alternatives to calf rennet, but traditional animal rennet is still preferred for certain aged and artisanal cheeses. Companies in New Zealand and Australia, among others, continue to produce calf rennet from abomasums sourced at licensed meat production facilities.
Physical Appearance
Visually, the abomasum looks different from the other three compartments. Its outer surface has a slightly purple tint compared to the paler forestomachs. The inner lining is reddish to reddish-purple with a wet, glistening surface, reflecting the constant production of gastric secretions. The interior is lined with prominent rugal folds, the ridges that increase surface area for acid and enzyme secretion. These folds are what give reed tripe its characteristic lined texture once cleaned and prepared for cooking.