Bicarbonate of soda

One ingredient, several names

Sodium bicarbonate is the fine white powder commonly called baking soda or bicarbonate of soda. It dissolves in water to make a mildly alkaline solution and has a slightly bitter, alkaline taste. In two published South African green-fig konfyt recipes it appears in the preliminary boil, before the figs meet their syrup. One recipe measures it; the other makes it optional. Neither book says what the powder is meant to do to the fruit.

The chemical name is sodium hydrogen carbonate, NaHCO₃. Sodium bicarbonate, bicarbonate of soda, and baking soda are familiar names for this same compound. It is distinct from sodium carbonate, Na₂CO₃, also called soda ash and, in some older English-language kitchen writing, carbonate of soda. Both are sodium salts associated with carbonate chemistry, but they are not the same ingredient and should not be exchanged by name alone.

This distinction matters in older recipes. Hildagonda J. Duckitt’s cookbook gives a green-fig preserve “From a Very Old Cape Recipe Book. Dutch.” Its cooking water contains one teaspoonful of “carbonate of soda” along with salt. The printed wording does not identify a brand, grade or modern synonym, so the phrase cannot be read with certainty as sodium bicarbonate. In contrast, later sources name their ingredient directly: Cass Abrahams’s recipe uses “bicarbonate of soda,” while J. de V. Lötter’s book says “baking soda.”

In dry form, food-grade sodium bicarbonate is a white, odorless crystalline powder. It dissolves in water, although the character of the solution changes with the water and the conditions around it. It is not baking powder. Baking powder is a prepared leavening mixture that combines bicarbonate with an acid or acid salts. Plain bicarbonate reacts with available acid to release carbon dioxide. That reaction is useful in baking, but it is not the role described in the fig-preserve instructions: there is no batter or dough to hold gas, and the bicarbonate is used in a separate cooking-water stage.

Several properties make the powder useful in a kitchen. The US Food and Drug Administration’s list of substances added to food names bicarbonate of soda among them and records its technical effects as leavening agent and pH control agent, among others. PubChem describes the compound as odorless white crystals or lumps with a slightly alkaline taste, and notes that heat decomposes it; the American Chemical Society’s account of baking soda makes the same point in kitchen terms, that carbon dioxide can come from the powder on heating even without an acid, while an acid releases it far more readily at room temperature. The Australian Drinking Water Guidelines also list sodium bicarbonate among treatment chemicals and give the pH of a 10 g/L solution as 8.4.

Soda ash, factories and the kitchen tin

The story of sodium bicarbonate sits beside the older industrial history of soda ash, sodium carbonate. Before chemical manufacture expanded, soda-rich plant and seaweed ashes supplied alkali for soap, glass, bleaching and paper. Demand grew faster than those sources could reliably provide. In late eighteenth-century France, Nicolas Leblanc developed a process for making sodium carbonate from common salt. It helped turn alkali production into a large chemical industry, but soda ash was not the same product as bicarbonate.

The Solvay process, important at industrial scale from the 1860s, linked the two compounds more directly in manufacture. Even so, “baking soda” and “soda ash” are not interchangeable: they have different formulas and distinct culinary identities.

Bicarbonate also occurs as the mineral nahcolite, but common household supplies are products of industrial processing. The distinction between mined and manufactured material belongs to its supply history; it does not tell a cook which grade to use in a recipe. A modern packet labelled baking soda or sodium bicarbonate is distinct from baking powder, washing soda or a product labelled only with a historical term such as “carbonate of soda.”

In the nineteenth century, bakers put bicarbonate into dough with sour milk. Its acid reacted with the powder to release carbon dioxide, and bubbles held in the dough lightened cakes and biscuits. Sour milk varied in acidity, so the rise was hard to predict. Cream of tartar later provided a more uniform acid partner and helped make baking more regular. That history explains the modern shelf distinction. Bicarbonate releases carbon dioxide fastest when it meets an acid in the mixture, although heating alone also drives off some gas as the powder decomposes; prepared baking powder carries its own acid so that the reaction does not depend on whatever else is in the bowl.

What alkalinity does in food

In water, bicarbonate contributes alkalinity: it can accept hydrogen ions, and when it encounters acid, hydrogen carbonate can form carbon dioxide and water. A freshly prepared 10 g/L solution has a pH around 8.4 in the Australian Drinking Water Guidelines. That is mildly alkaline. A cooking pot does not have a fixed pH simply because a spoon of powder was added: dilution, water minerals, food acids, air exposure, heating and time all matter. The two preserve recipes provide no pH measurement.

Plant tissues respond to their cooking environment. Pectins occur in and between cell walls and help hold plant cells together. Food-science accounts describe alkaline cooking conditions as capable of softening plant tissue, while alkaline water may also slow the acid-driven colour shift that turns green chlorophyll toward olive pheophytin. The vivid green appearance can come with soft or mushy texture. Controlled work on vegetables shows why simple rules fail. In one study of snap beans and carrots, tissue soaked cold at pH 12.5 before cooking softened more slowly afterwards, while high pH or added salt during the cooking itself had little effect on softening in the tissues tested; pretreatment and alkaline cooking are different treatments.

These findings offer general context, not a proven fig mechanism. The food-science work discussed here does not examine sodium bicarbonate in Ficus carica, so it would be too confident to say that the soda “locks in” fig firmness or colour. The recipe texts establish placement and, in one case, an effect on cooking time; they do not quantify changes to pectin, bitterness, skin, aroma or fruit shape.

Two published fig methods

Wines of South Africa publishes Cass Abrahams’s “Green Fig Konfyt,” credited to Cass Abrahams Cooks Cape Malay: Food from Africa. For 500 g green figs, it lists water to cover, 75 ml slaked lime and 5 ml bicarbonate of soda. The figs are peeled by grating away the skin, scored with a cross at the blossom end and soaked overnight in water and lime. The cook washes them thoroughly, then boils them in water and bicarbonate until tender and drains them. The cooking-water volume is left to the cook, and the recipe offers no reason for the bicarbonate.

That treatment belongs to the first boil. Abrahams’s figs meet the syrup only after draining. The recipe’s basic syrup for each 500 g fruit uses 500 g sugar, 750 ml water and 30 ml lemon juice, dissolved over low heat without boiling; cinnamon and dried ginger are added with the fruit, which cooks slowly until the syrup is thick and the figs shiny. Since lemon enters later, the printed sequence does not show the lemon and bicarbonate reacting together. The recipe gives no basis for treating bicarbonate as the syrup’s acid balancer or as a storage safeguard.

Lötter’s “Whole green fig preserve,” in The Fig in South Africa, begins with a different condition and sequence. It specifies 1.5 kg hard green figs of similar size, a small cross at the blossom end and a 10–20-minute preliminary boil in salted water. A pinch of baking soda is optional; the book says it may shorten the cooking. The figs should remain slightly firm, not collapse, yet allow a match to pierce the skin easily. The cook then drains them into a separate syrup. The pinch, salt and boiling water are not quantified. Lime is not part of the default hard-fruit treatment; Lötter mentions an overnight lime-water soak only as an option for large fruit that has begun to soften.

These methods share a place for bicarbonate in the early water boil, but their quantities cannot be compared. Abrahams gives 5 ml for 500 g fruit, with no volume for that cooking water; Lötter offers an unmeasured pinch for 1.5 kg figs and links it to a shorter boil. The formulas differ in fruit preparation, lime treatment and syrup seasoning as well. One is not a conversion of the other. Their authors preserve distinct decisions for particular batches, not a universal dose or a complete scientific explanation.

Reading the two methods side by side shows how much a short instruction leaves to the cook. Neither author gives the volume of the boiling water for the soda, the fig size beyond “similar” and “hard,” or the minutes beyond a range, so the cues are the ones the fruit supplies: skin that a match or fork pierces easily, flesh that is tender but still holds its shape. Duckitt’s older Cape recipe adds one more variation on the same pattern. Its cooking water holds one teaspoonful of “carbonate of soda” and one tablespoonful of salt in about three quarts of water, the pan is left open, and the figs are boiled until soft enough to pierce before they are drained and cooked in syrup. Three authors writing over more than a century thus place an alkaline salt in the water of a green fig’s first boil, and the stage that follows it is the same in all three: draining, then a long slow cook in sugar.

Where bicarbonate appears beyond konfyt

Baking remains the best-known kitchen use. In a formula containing an acid such as sour milk, yogurt, lemon juice, vinegar or cream of tartar, bicarbonate releases carbon dioxide. The gas expands as the mixture heats, leaving small pockets in a baked food. Baking powder combines a bicarbonate with acid components so that leavening does not rely only on the acidity of another ingredient. The distinction is practical: when a recipe names bicarbonate, it expects an acid source or another function; it is not automatically interchangeable with baking powder.

A separate food example makes the tissue effect easier to see. In a study of Canadian Wonder common beans, fresh and aged beans cooked faster in a 0.1 M sodium bicarbonate solution than in demineralised water. The authors attribute the progressive loss of texture mainly to the dissolving of middle-lamella pectin and the separation of cells; starch had gelatinised before complete cell separation, so starch swelling alone could not explain the softening. The experiment belongs to those beans and that solution; a fig skin and the unmeasured water in a Cape preserve are different materials. It shows how an alkali can alter the bite of a plant food without prescribing an addition to a different recipe.

In a preserve, this powder enters water before the fruit meets its syrup. Washing after a lime soak, boiling in the separate bicarbonate water, and draining keep Abrahams’s early treatment distinct from the lemon-scented syrup. Lötter’s fruit also moves from a preliminary pot to syrup, although his bicarbonate is only an option. The cook follows changes in the fruit—tenderness at the boil, then a shiny syrup coating—rather than looking for the trapped gas that matters in a cake. The two dishes share an ingredient but give it different measures and different cues.

Buying and keeping it dry

A modern packet for these recipes should identify sodium bicarbonate or bicarbonate of soda. Baking powder includes additional components; washing soda and soda ash are different substances. Old recipes that call for “carbonate of soda” use a name that can mean sodium carbonate as well as bicarbonate, so the printed wording is best read as the author wrote it.

Sodium bicarbonate can absorb moisture. The USDA technical report recommends keeping it dry and airtight. That is a storage-quality recommendation, not a use-by period. How long an opened packet stays effective depends on the product, so follow the package’s own storage and date guidance. The chemical identity of the powder also says nothing about how much to use in a particular fig batch; that remains recipe-specific.

The bicarbonate does not acidify the fruit. Neither recipe’s soda amount establishes a shelf life or a tested room-temperature process; those questions belong to the complete recipe and jar treatment rather than to this single ingredient.