Beans have important health properties that are related to their slow rate of digestion. Understanding the digestibility of beans is therefore important in the nutritional sciences. Beans and other legumes have their starch contained within strong parenchymal cells that are resistant to digestion. Biologically this relates to the need for the plant to protect the seed before germination. Availability of legume starch to human and gut bacterial digestive enzymes is therefore hindered, thus reducing the digestibility of beans. This provides beans and other legumes with beneficial effects on postprandial glycaemia, and this may explain the inverse association of legume consumption with bodyweight, type 2 diabetes and cardiovascular disease. Evidence from studies suggests that the digestibility of beans is affected by a number of factors including the degree of heating they are subjected to during cooking. Long duration cooking with increased pressures as might occur during canning can significantly increase the starch hydrolysis rate in legumes.
The digestibility of beans has been extensively studied in the nutritional literature. Generally it has been shown that the canning process increases the rate of starch hydrolysis from legumes including beans. For example, in one study1, researchers fed 50 grams of carbohydrate to healthy volunteers after an overnight fast. The carbohydrate was administered either as a glucose drink, home cooked haricot beans, or canned haricot beans. The results of the study showed that the postabsorptive glycaemic response was highest for the glucose drink and lowest for the home cooked beans. The glucose drink and the canned beans also produced a glucose response that peaked at 30 minutes postprandially, compared to the home cooked beans that had a later peak glucose at around 45 minutes. The glucose curves also fell significantly below baseline levels for the glucose and canned bean groups. However, the home cooked beans maintained a shallower glucose curve that remained higher for longer, suggesting the digestibility of beans is affected by the canning.
Canning may increase the rate of digestion because the prolonged application of heat and pressure during the process breaks down the cell walls of the parenchymal cells. This then increases the availability of the starch contained within to the digestive process. In the same study, when the authors assessed the insulin responses to the three meals, they found that the insulin curves mirrored the glucose curves. In this regard, the home cooked beans produced the lowest peak insulin with the glucose drink producing the highest peak insulin. The insulin curve also peaked later and remained higher for longer in the home cooked beans compared to the canned beans and glucose drink. Assessment of the rate of starch hydrolysis using in vitro methods revealed that the digestibility of beans was eight times faster in canned beans compared to the home cooked beans. An eightfold more rapid rate of digestion was also found in canned lentils compared to home cooked lentils suggesting the effects were produced by the canning process.
The beans in this study were sourced from the same batch as those used in the canning and so the authors were sure the beans were similar prior to treatment. Treatment of the canned beans involved heating for 2 hours in a sauce at at 15 PSI, 120°C and pH 5. The authors previous studies showed that heating under pressure causes an increases the digestibility of beans, and that this effect is attenuated somewhat by a lower pH. However, work by other authors suggest that it might be the heating alone that is responsible for altering the digestibility of beans during canning. These results are consistent with other research in cereal that show that the structure of the outer bran layer is the determining factor in the resistance of the starch to digestion, and the likely reason that whole grains are more slowly digested than refined counterparts. This work also questions the relevance to fibre in the digestive process and show that it might not be as important in postprandial glycaemia compared to the containment of the starch within the seed or grain.
Although canned beans may therefore be nutritionally inferior to home cooked beans with respect to postprandial glycaemia, canned legumes may still confer beneficial health effects to humans. One study2 investigating the benefits of canned beans reported on the effects of canned beans on plasma lipoprotein levels in hypercholesterolaemic subjects over 21 days. The results showed significant reduction in serum triglycerides and total cholesterol with canned bean consumption. In addition the subjects had body weight reductions of between 1.0 and 1.5 kg over the course of the 21 day study, without forced calorie restriction. Changes to the digestibility of beans during the canning process therefore does not attenuate all of the health benefits of legumes. Another consideration with canned beans is the presence of sugar within the tomato sauce they often contain. Although sugar is detrimental to the health, it appears that the deleterious effects of the sugar within baked beans does not nullify completely the benefits derived from eating the beans.
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