Lactic Acid Fermentation in Sourdough
Submitted by Debra Wink | The Fresh Loaf — A few years ago, I was asked to explain lactic acid fermentation in sourdough, and the difference between homo- and heterofermentation. Not an easy task, partly because I wasn’t satisfied that I knew enough, or that I could reconcile what I was reading in bread-baking books with what I had learned in school.
To sort it out, I had to dig deeper into the scientific literature. Answers are there in bits and pieces, although not in a context that is easy to make sense of. As I plugged away at deciphering current microbiology textbooks and scientific research, I started to see things in a new light. And so now, I want to share what I’ve learned with those who wish to know more.
First, I’d like to introduce the concept of a metabolic pathway. On paper, a metabolic pathway can be illustrated in a flow diagram that represents a sequence of enzyme-controlled chemical transformations. While the pathways in this discussion start with sugar and finish as various end-products, there are several intermediate compounds formed along the way as one thing is converted to the next.
The names may be intimidating at first glance, but don’t let them scare you. Knowing their chemical reactions and what all the compounds are is not as important here as understanding their overall purpose, which is to produce energy for the organism. Like all living things, microbes need energy to perform the tasks that enable them to live, grow and multiply.
Some pathways generate more energy than others. Through respiration, glucose and oxygen are turned into carbon dioxide and water via the Krebs cycle, also known as the tricarboxylic acid or TCA cycle. You may have seen it before if you’ve studied biology, because it’s the same pathway we humans use. It is aerobic, meaning that oxygen (O2) is involved, and it generates far more energy than any fermentation pathway.
Whenever oxygen is available, respiration is favored by facultative anaerobes like yeasts, because they will always take the path that generates the most energy under the prevailing conditions. For the most part though, bread dough is anaerobic (without oxygen), and fermentation is an alternative pathway that doesn’t require oxygen. When yeasts ferment sugars, they produce alcohol (ethanol) in addition to carbon dioxide. Fermentation produces much less energy than respiration, but it allows microorganisms to carry on when no oxygen is available, or they lack the ability to respire as is the case with lactobacilli.
Bacterial fermentation is more varied than fermentation by yeast. Bacteria produce organic acids that contribute, for good and bad, to the quality of bread. Controlling acid balance and degree of sourness is something that artisan bakers strive to do, so it may be useful to understand where the acids come from and how their production can be influenced by things that are within the baker’s control.
In yeasted breads, acids come in small doses from naturally occurring bacteria present in flour and commercial yeast. (Fresh yeast generally has more bacterial inhabitants than dried, and whole grain flours more than refined.) In sourdough breads, acid-producing bacteria are supplied in much greater numbers from starter. There are many different species and strains of bacteria found in various types of starters, and because they produce lactic acid while fermenting sugar, they fall under the heading of Lactic Acid Bacteria (LAB).
Lactic acid bacteria common to sourdoughs include members of Leuconostoc, Pediococcus, Weissella and other genera. But by far, the most prevalent species belong to the very large and diverse genus, Lactobacillus. Based upon how they ferment sugars, lactic acid bacteria can be sorted into three categories. Please bear with me now, because while these terms may look impossibly long and technical, they are actually self-descriptive. Take homofermentative LAB for example. Homo-, meaning “all the same,” refers to the end product of fermentation (by lactic acid bacteria), which is only, or “all” lactic acid. Heterofermentative then, means “different” or mixed end products. As lactic acid bacteria, heterofermentative LAB produce lactic acid, but they also produce carbon dioxide gas, alcohol or acetic acid as well.
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