I often get strange reactions when I tell people that legumes – such as beans, lentils, and cowpeas – are at the top of my list of favourite foods. The conversations that follow are guaranteed to elicit strong opinions, with most people saying they don’t like the effects of legumes on their digestive system! People also express frustration about the long cooking time, with many avoiding cooking dry legumes from scratch.
Legumes are a cost-effective source of nutrition and have, historically, played an important role in the diets of people in sub-Saharan Africa, Latin America, and Asia. Legumes contribute to improved household nutrition, especially where women have greater influence over household food choices, child nutrition, and ultimately, health, because these aspects are generally linked to the household tasks traditionally performed by women.
Legumes are a good source of protein, fibre, and micronutrients. They have a low glycaemic index (GI), which means that they take long to digest, and they may safeguard against cardiovascular diseases (CVDs), and type 2 diabetes mellitus (T2DM). This makes them an ideal dietary component both for preventing and for those living with these conditions. However, these benefits are often overshadowed by the tendency of legumes to have long cooking times. This is the result of legumes developing a hard-to-cook (HTC) defect due to the storage methods used to keep them dry for long periods of time under sub-optimal conditions. These longer cooking times are a major constraint to a wider acceptance and use of legumes.
To soak or not to soak? That is the question.
Legumes contain oligosaccharides, which are complex carbohydrates found in some plant products. The human digestive system lacks the enzymes necessary to digest oligosaccharides, which leads to their fermentation by the bacteria found in the gut. This causes excessive gas (flatulence) as a by-product of the bacteria fermentation.
Soaking legumes helps to soften the texture of the final product and reduce oligosaccharide content, as the oligosaccharides dissolve/leach into the soaking water, which is thrown away. The longer the soaking time, the more oligosaccharides are dissolved in the soaking water, thereby reducing the gas/flatulence after digestion.
While soaking legumes reduces the effects of the HTC defect and improves the cooking quality, it can also leach important nutrients such as water-soluble vitamins and minerals and can affect the flavour. Other processing methods that can be considered include fermentation, repeated boiling, autoclaving, and enzyme treatment. These methods have also been found to impart unique sensory characteristics, reduce the oligosaccharides, improve digestibility, reduce anti-nutritional factors (compounds in foods that reduce the availability of nutrients), and boost the nutritional and biological value of the legumes.
The general rule with cooking legumes is to increase the moisture content, which softens the grain to reduce the cooking time. The answer to the question of whether to soak legumes is not either soak or don’t soak; rather, a combination of soaking, sprouting, and removal of the seed coat followed by cooking leads to the greatest increase in nutrient digestibility and bioavailability – that is, nutrients are better absorbed and more available for the body to use – as well as a reduction in oligosaccharides. Thus, a win-win situation!
Understanding the primary factors that affect food choice or the method of preparation is very important in promoting the use of less popular, yet nutritious products. The choice of food processing or preparation methods depends largely on the underlying chemistry of these methods and their impact on the intended use, nutrition, or consumer. However, while there is rising interest in and concern about nutrition among consumers, they may not know how to prepare foods to maintain their nutritional value and enhance their palatability. There is thus a knowledge gap in terms of readily available, relevant information to help consumers make healthier decisions. Research such as that which I am conducting with my FSNet-Africa research team – which aims to offer implementable solutions to food systems challenges – is what can be applied to fill such knowledge gaps.