Gibson fermentation vessels to mimic fermentation in the colon at the Quadram Institute (credit Glória Máté-Koncz).

Beans are a type of pulse. Pulses are nutritious seeds from legume plants including lentils, chickpeas and beans.

We begin digesting beans in our mouth and they pass through to our digestive system, to our gut microbes, which are key to their digestion. The cell structure of beans affects how we digest them.

Understanding how we digest pulses helps us make informed decisions about how we can prepare and eat pulses to achieve specific health benefits, such as keeping us fuller for longer, maintaining a stable blood sugar level or nourishing our gut microbiome.

Before diving into how digestion works, it’s useful to understand the structure of pulses.

What are pulses made of?

Pulses are made up of many tiny cells. Inside each cell, you’ll find starch, protein and a small amount of fat. These nutrients are locked in by an outer layer called the cell wall, which is a type of dietary fibre.

What makes pulse cell walls unique is their high pectin content. Pectin is a soluble fibre found in both the cell wall and between the cells, where it acts like a natural cement. When pulses are cooked in water, the pectin softens, allowing the cells to separate easily without rupturing. As a result, macronutrients such as starch and protein mostly remain locked within the intact cells even after chewing.

In contrast, when pulses are ground into flour through a process called dry milling, the cell walls rupture, releasing the nutrients.

This cellular difference plays a big role in how we digest pulses.

Digestion of beans begins in the upper gut

Digestion of pulses begins in the mouth, where chewing reduces food size.

If you eat pulses cooked in water, they break apart into cells during chewing and mix with saliva, which contains the enzyme amylase. Enzymes in our digestive system reduce the size of larger molecules found in food to help with their absorption.

Salivary amylase starts breaking down starch in the mouth. However, if the pulse cell walls remain intact, digestion in the mouth by amylase is limited.

Once swallowed, the chewed beans travel to the stomach. In the stomach, the enzyme pepsin begins to digest proteins.

Fat digestion would also start in the stomach, although pulses are naturally low in fat.

Absorbing nutrients from beans in the small intestine

In the small intestine, digestion of beans continues:

• The enzyme α-amylase breaks starch into smaller sugars, which are reduced into glucose for absorption and use by the body.
• Proteins (made of long chains of amino acids) are reduced to smaller peptides and amino acids, which serve as building blocks for tissues, but also for enzymes and hormones.
• Vitamins and minerals (like B vitamins, potassium, and iron) in pulses are primarily absorbed in the small intestine.

The role of pulse cell structure in digestion

The cellular structure of pulses determines how quickly starch is digested in the small intestine.

Research has shown that starch from whole cooked pulses is digested more slowly than starch from dry-milled pulses. This slower digestion is beneficial for blood sugar regulation, as it leads to a slower rise in glucose levels after eating. It also means more starch remains undigested in the small intestine. This undigested starch is a natural type of resistant starch and reaches the colon, where the microbiota can ferment it.

Recent work in our group led by Dr Cathrina Edwards and Imperial College London found that gut hormone responses to food are effected by the cellular structure of pulse meals. Intact cell structures triggered greater satiety hormones compared to milled or broken cells. This suggests that the way pulses are processed could impact how full we feel after eating and how our body regulates appetite, which might be important for those who want to lose weight.

Digestion in the colon

After digestion in the upper gut, the remaining components of pulses, travel to the large intestine, also called the colon. Here the gut microbiota ferment dietary fibre and resistant starch.

During this fermentation, gut microbes produce beneficial compounds called short-chain fatty acids, notably acetate, propionate, and butyrate. These short chain fatty acids have multiple health benefits, especially butyrate, which fuels the cells lining our colon and supports gut health.

Boiled pulses deliver more resistant starch to the colon than conventional pulse flours, which enhances short chain fatty acid production.

How do we study the digestion of beans in the laboratory?

We typically study upper gastrointestinal digestion of beans in laboratory models. These involve incubators that mimic body temperature and rotors that simulate the physical mixing of food.

Each stage of digestion requires specific conditions and enzymes, which must be carefully prepared. By collecting samples at different time points and measuring the release of target molecules, we can assess the rate of digestion.

We can study fermentation in the large intestine in many ways. One example is the Gibson vessel, which contains trillions of gut microbes from a stool sample. The stool is mixed with a nutrient-rich medium that mimics the pH and environment of the colon, then combined with the food sample, in this case, beans.

The vessels must be kept in oxygen-free conditions to reflect the colon’s natural environment. Gibson fermentation vessels, are jacketed and connected to a temperature-controlled water bath to maintain the correct temperature. Microbial fermentation takes longer than enzymatic digestion, but as with the upper gastrointestinal model, we can monitor the process by sampling and analysing the molecules produced over time such as short chain fatty acids

By extracting microbial DNA from these samples and sequencing, we can also study how different cellular structures in beans influence the gut microbiome.

In my PhD, I investigated the digestion of fava beans, one of the most widely grown pulse crops in the UK, yet still underused in local diets.

Research to boost the potential of pulses

During my PhD, I have also worked with my industry partner AB Mauri, a bakery business, to incorporate pulses into baked goods, helping to boost their fibre content.

During my placement, I developed muffins made with PulseON®, a patented pulse flour processed in a way that preserves the natural cellular structure of pulses.

More work is needed to incorporate PulseON into food, but over the years, there will be food products in the market that contain whole pulse cells.

Through our research into how beans are digested, we can bring the health benefits of pulses to more people.