Separating things and then putting them back together is a common principle in industrial food production. Agricultural raw materials are broken down into their individual constituents, such as proteins, starches and fats, and the food industry then reassembles these building blocks into specific products. In many cases, not all of the harvested crop ends up in the food we eat. In some instances, by-products are formed that must be reused outside of the food industry. These processing steps can also lead to the loss of dietary fibres, vitamins and minerals for the finished product.
To reduce food waste and retain as much of a food's nutritional value as possible, the research group led by Patrick Rühs, Professor of Food Structure Engineering, is looking for ways to use the harvested crop in its whole form wherever possible. For legumes, they have now demonstrated an astonishingly simple approach to turning whole peas, beans or lentils into a product whose fibrous structure makes it particularly appealing to the human palate.
"It's a texture people are familiar with from meat or fish"
"During chewing, a directional, fibrous structure provides a distinctive bite that is familiar to – and enjoyed by – many people from meat or fish," explains Rühs. "In meat and fish, this structure is formed by the directional arrangement of muscle fibres."
Water and a freezer are all that the researchers need to also turn peas, beans, or lentils into a fibrous product. They call their method "freeze-structuring."
First, the researchers soak the legumes and blend them finely to create a uniform distribution of starch, proteins and cell wall fragments. Heating the purée for 30 minutes at 90°C, causes the starch to gelatinise together with the other constituents, and the purée becomes a gel.
The researchers freeze this gel from one side in a targeted manner using a mould that is insulated on all sides except one. This causes ice crystals to form in parallel and inwards from the uninsulated side. As they grow, the crystals squeeze and compress the legume purée into thin parallel layers. When the gel is thawed, the ice melts, but the directional, fibrous structure remains intact.
Different legumes produce different textures
The researchers tested a whole series of different types of beans, peas, and lentils and were able to create a directional fibrous structure with all of them. "Our method works with legume species that together account for 96 percent of legume production worldwide," says Andrea Bach, a doctoral student in Rühs's group. Bach is one of the two first authors of the study, which was recently published in the journal NPJ Science of Food .
However, the researchers also identified certain differences: the fibres were particularly strong and stable with red and black lentils, as well as with mung beans, while they were slightly softer with soybeans or black beans. The final strength of the product also depended on how much water the researchers added to the legumes. "It's an advantage to have a range of consistencies," explains Elin Perler, who is also a doctoral student in Rühs's group and one of the first authors of the study. "It allows more flexibility in developing recipes, and it's even possible to combine different legumes."
"The new method is so simple that it can be performed easily by anyone in a household kitchen," says Bach.
Which legume-based products consumers prefer remains an open research question. The researchers focus is on understanding how the freeze-structuring process creates the desired texture. Developing specific recipes with spices or marinades is something they prefer to leave to professional chefs, and the research group is already collaborating with suitable partners.
The ETH Zurich researchers now want to delve deeper into why different legumes produce different consistencies during the freeze-structuring process. A better understanding of these mechanisms could help them design food products with specific textural properties in the future.
