Introduction

Soaking and rehydration are two culinary techniques applied to legumes, mushrooms, and fish.

A previously dehydrated food is immersed in a liquid so it can absorb and regain its original water content.

The process involves submerging the food in water to restore the lost moisture. Water is absorbed by osmosis, and carbohydrates regain their texture as they absorb water. There is a noticeable increase in volume.

Legumes are dried seeds removed from their pods and include lentils, chickpeas, broad beans, beans, and peas.

A useful tip to assist with legume rehydration is the addition of sodium bicarbonate to the soaking water. This helps achieve a nearly neutral pH.

Normally, in acidic conditions, cell walls harden, while in basic conditions, cell walls disintegrate and proteins and vitamins leach into the cooking liquid.

Hard and calcareous water results in tough legumes because the cellulosic fibres bind to soluble calcium. The pectins combine with calcium, producing calcium pectates that toughen the skin.

Types of Soaking and Rehydration

Beans and lentils are soaked in cold water to rehydrate them, whereas chickpeas are soaked in warm water to reduce the hydration time. Rehydration usually requires long periods, which also helps reduce cooking time. Chickpeas and beans are typically soaked overnight. Small lentils barely need soaking to cook quickly.

A short soak or rinse can also be used, particularly for rice varieties like Basmati, which is submerged in cold water, stirred, and drained. This process is repeated several times until the water runs clear. This prehydration helps remove loose starch grains, preventing the rice grains from sticking together during cooking. A similar process can be used for boiled potatoes.

Physico-Chemical Effects of Soaking

Enzymatic changes are initiated in the legume, inducing germination.

In fact, if the rehydration process is prolonged too much, fermentation by microorganisms may begin.

Minerals and vitamins are often lost due to soaking. This happens through osmosis, with substances moving from areas of higher concentration (the legume) to areas of lower concentration (the water).

The content of some anti-nutrients is also reduced.

Certain oligosaccharides like raffinose dissolve, which helps reduce flatulence in the colon.

It is important to change the soaking water.

After soaking, oligosaccharides like raffinose are reduced by 39%, stachyose by 48%, and verbascose by 40%.

Trypsin inhibitors, which are protease inhibitors, are also diluted.

Rehydration of peas for 18 hours reduces haemagglutinating activity by up to 65%. After 24 hours, this activity can decrease by 66–98%.

Adding sodium bicarbonate to the soaking water prevents the formation of calcium and magnesium pectates, which toughen the skin.

Demineralised water can also be used to avoid the formation of pectates with calcium and magnesium from the water.

The addition of sodium chloride (NaCl) can prevent the formation of calcium pectate by producing sodium pectate, which is soluble.

As mentioned at the beginning of this article, prolonged soaking may lead to germination of the legume.

Germination is the set of phenomena by which the embryo, in a dormant state within the seed, resumes growth and develops into a seedling. It involves hydrating the seed until sprouting begins.

Sprouts are increasingly being sold commercially. The most common are soya bean sprouts, but there are also sprouts from alfalfa, mung beans, sunflower, lentils, chickpeas, onion, rice, fennel, mustard, wheat, and more.

The process is simple—just extend the soaking time to allow the seedling to begin sprouting.

Physico-Chemical Effects During Germination

The fermentation process enhances nutritional qualities.

Proteins are hydrolysed into amino acids.

Carbohydrates are converted into simple sugars.

Fats are converted into fatty acids.

Vitamin content increases significantly during the germination process.

For example, in soya beans, vitamin A content doubles in 48 hours, increases by 280% in 54 hours, and by 370% in 72 hours.

Vitamin C in sprouted wheat increases by 600% in the first few days of germination, and vitamin E triples within four days.

Some sprouts contain chlorophyll, which is very similar to haemoglobin. Chlorophyll carries oxygen to the cells and is a good detoxifying and regenerating agent.

Effects on Anti-Nutrients

During germination, trypsin inhibitors decrease to negligible levels. (Trypsin inhibitors prevent the breakdown of proteins into amino acids.)

Germination is less effective than cooking in reducing trypsin inhibitor levels but more effective at reducing phytic acid, stachyose, and raffinose.

In all cases, proper cooking of legumes and sprouts is important to eliminate as many residual anti-nutrients as possible.