Sourdough bakers will know that there are millions of natural micro-organisms within their leaven, and among them are the wild yeasts and many different strains of bacteria, the most common being the type that are called Lactic Acid Bacteria (LAB for short). Mostly they arrive on the feeding flour, but they also arrive from our skin, saliva, and so on, and even by more mysterious ways. The lactic acid bacteria create the sourness that gives “sourdough” bread its name. While they are busy fermenting the sugars in the flour to create the energy they need to sustain life, scientists have divided them into two camps based on their feeding patterns: the homofermentative LAB only produce lactic acid, while the heterofermentative LAB start out making lactic, but can swap to acetic acid production by switching to ferment a different class of sugar.
What is pH?
The pH is familiarly known as a measure of acidity. In science, when the pH scale is used to describe the intensity of an acid, it relates to the number of free hydrogen ions a particular type of acid donates to a solution. For the layman struggling with science, it helps to know that very acidic compounds are low on the scale, while alkaline compositions are high.
Pure water is regarded as “neutral” (neither acidic nor alkaline), corresponding to a pH of 7. A solution with a pH less than 7 is considered acidic; a solution with a pH greater than 7 is considered alkaline, or basic. Here are a few common things to act as pointers: the pH of battery acid is 0.8; lemon juice is 2; a tomato is 4.5; milk is 6.4; egg is 8.5; lye is 13-14).
The sour taste of the bread comes from the amount of acid in the bread, being acid content, rather than the pH of the bread, being its acidity level. A mature and productive leaven will feature a balance between both types of acid, lactic and acetic. Within that balance there are about one hundred times more bacteria than yeasts, and the volume of lactic acid cells will usually outnumber the acetic cells by about 4:1. Acetic acid is more volatile than lactic acid, thus it is associated with aroma as well as taste.
Members of the triticeae family – rye, spelt, bread wheat, durum wheat – all have a pH of a little above 6 when mixed with water.
Understanding Lactic Acid
During the gradual maturing of a leaven (taking months), and in the shorter term, whenever it is left to ripen after being fed, it gradually acidifies, causing the pH level to lower considerably. Lactic acid, being more capable of releasing hydrogen than acetic acid, is more acidic in flavour than acetic acid and thus it lowers the host’s pH more than would be the case if acetic acid was predominant. This comes as a surprise to the layman baker, since in a “common knowledge” way we know vinegar is acetic acid, and we consider that a particularly harsh flavour. (Vinegars are around 2 to 3 pH, slightly less acidic than lemon juice). At the same time, when we think of lactic acid in dairy products we do not think of harshness, and in observing a juvenile sourdough leaven we may place a tiny blob of it on our tongue, and we say it tastes “lactic” as though that is understood to be mild. The point is – of course it is mild since it is a juvenile and its lactic acid bacteria have not been long on the job.
There are bakers, like Chad Robinson in California, who are adamant that superior sourdough bread barely tastes sour at all.
To ensure that their bread has flavour that is regarded as gentle on the acidic scale they adopt two main tactics: they refresh the leaven on a short cycle (possibly twice or even thrice a day) before making their production leaven; and the production leaven itself must always be employed to leaven the bread when it has just peaked in its maturity, rather than being left to go beyond that peak, when it will naturally start to acidify itself. This practice – refreshment on a short cycle, more than once a day, thus always using a just-ripened leaven – utilises the knowledge that it is the wild yeasts that are the early feeders, and if the leaven is repeatedly refreshed when those yeasts have reached ripeness in the region of six to eight hours, such a short cycle ensures that it is the yeasts that always make hay, not the bacteria, which are prevented from ever being left long on the job.
Management of the leaven
Given that a healthy leaven has several times more lactic acid molecules than acetic, the most dramatic changes brought about by the baker’s management of the leaven will be those that affect the volume of acetic acid. To take an example, if the baker favours wetter leavens with whiter doughs, those choices will lower overall acid production and produce negligible amounts of acetic acid. On the other hand, favouring tighter leaven and stiffer dough made with coarser flour (meaning more bran, more minerals known as ash) will lead to a rise in both types of acid, with acetic acid being a higher proportion of total acid.
When the pH level is allowed to plunge so low (approaching a pH of 3) that the heterofermentative lactobacilli have died out, and only types of yeast that can cope with the most acidic environment are working, those are the circumstances when Type I sourdough (the type that is associated with small craft bakers and household bakers) is converted to a Type II (associated with industrial-scale craft bakers who keep it in vast tanks and use it as a dough conditioner, or are only making rye bread). The baker needs to be aware that none of this happens if the regime for the leaven is daily feeding, and the temperature range is always kept between 20°C and 30°C. For a juvenile leaven, a starter, such a routine will ensure healthy, active Type I sourdough somewhere between the 10th and 15th refreshments.
That’s it Panarians. Have a happy Christmas, and I wish you all good baking for 2020,