The virtues of sodium bicarbonate.
CONSIDER THIS PASSAGE from an anonymous work published in 1838 under the title Le cuisinier parisien: “Beans, peas, and lentils, and many other vegetables cook well only in very pure and light water; [water] from rivers and streams is always the best; that from wells is worthless. In places where only well water is to be had, it can be made suitable for cooking vegetables by adding to it a little carbonate of soda, dissolved in water to the point that it no longer whitens the water. It leaves a small deposit; one takes the clear liquid and uses it to cook the vegetables.” Why should the quality of the water determine the tenderness of vegetables? And why should carbonate of soda (or wood ash, also recommended by the same author) be useful?
Basic Softening
Ash and bicarbonate have in common the property of making water basic, or alkaline. To determine whether this alkalinization acts on vegetables, we may begin by cooking lentils in three identical pans over the same heat. In the first pan let’s use distilled water as the cooking medium; in the second, water made basic by the addition of sodium bicarbonate; and in the third, water that has been made acidic by adding a bit of vinegar.
The difference in tenderness between the three samples is so clear that no laboratory instrument is needed to measure it: When the lentils in pure water are just cooked, the ones in acidified water are still as hard as pebbles, whereas the others in water enriched by sodium bicarbonate are falling apart. Thus the relative acidity of the cooking medium determines the rate at which the vegetables are softened.
Why should this be? Vegetables are composed of cells held together by parietal tissue, which is composed of pectin and cellulose. To soften this tissue, one must therefore modify its pectic “cement.” In an acid environment, pectin molecules are neutralized: Their –COO- carboxylate groups capture the hydrogen atoms, yielding electrically neutral –COOH groups. Because the pectin molecules are no longer subject to electrostatic repulsion, the lentils remain hard. By contrast, sodium bicarbonate triggers the ionization of the –COOH carboxylic acid groups into –COO- groups, with the result that electrostatic repulsions between the pectin molecules cause them to separate, breaking down the parietal walls and thus softening the lentils.
The Hardness of Water
Is the change in the acidity of the water used to cook the lentils the only effect of sodium bicarbonate? Le cuisinier parisien indirectly suggests that the hardness of the water, caused by the presence of calcium ions, plays a part as well. To test the effects of these ions, let’s once again use two identical pans, adding lentils and putting them over the same heat, then pour distilled water into the first pan and water that has been artificially hardened by the addition of calcium carbonate into the other one. After about 45 minutes the lentils cooked in pure water are done, but the lentils cooked in the hard water are still as hard as wood. This time, the effect results from the calcium ions, whose two positive charges bind them with phytic acid molecules and pectin molecules, reinforcing molecular cohesion rather than weakening it. Monovalent ions, such as sodium, do not establish such bonds.
These phenomena are of particular interest to lentil producers, who are looking for ways to make their products easier to prepare. Can lentils be precooked, for example? At the Institut National de la Recherche Agronomique station in Montfavet, Patrick Varoquaux, Pierre Offant, and Françoise Varoquaux have studied this question with a view to identifying the optimal conditions for softening such seeds without releasing either the starch they contain or, by rupturing the molecular structure of the integument, cellular fragments. Steam cooking would be a good way to achieve this result if it did not take so long.
The Montfavet researchers cooked lentils at different temperatures for different lengths of time and measured the firmness of the seeds. They observed first that the firmness diminished with the length of cooking time, in inverse ratio to temperature. This came as no surprise, but further quantitative analysis proved to be instructive. The curves charting the firmness of the seeds as a function of time, for each temperature, display an elbow shape, which indicates that the softening consists of two phenomena. The first is rapid and probably is associated with the diffusion of the water toward the interior of the lentils; the second is slower and seems to be associated with the gelling of the starch in the hot water, which causes a starchy paste to form.
The researchers also observed that the percentage of lentils that burst open during cooking increases exponentially as a function of time as the temperature rises above 80°C (176°F). Temperature thus affects both the integrity of the lentils and their firmness: At temperatures above 86°C (187°F), the proportion of lentils that fall apart exceeds the proportion of lentils that become soft while retaining their form—hence the culinary rule suggested by these studies, namely that lentils should be cooked at a temperature lower than 80°C (187°F). Of course, this takes time.