So then, what is the precise difference between commercial yeast and a starter? If both came packaged, with microbial ingredients labels on the side, you might not think there was much at all, for both contain many different kinds of yeast and lactic acid bacteria. Yet as in all things relating to microbial leavening, the issue is one of proportion. Whereas in a home-grown starter the bacteria outnumber the yeast by about 100-1 microbe-for-microbe, in a packet of comercial yeast the reverse is true. There, the yeast outnumber the bacteria by about the same ratio.

The upshot of that of course is much faster rising, since there are so many more yeast critters consuming sugars and giving off CO2 and alcohol. The down side however, is there's less flavor. Or should I say there's less of a certain kind of flavor, since yeasts produce a heck of a lot of flavor on their own. One of the sources of that flavor is glutamic acid also known as glutamate, which the fermentation process produces in abundance. Glutamate is a molecule known to stimulate the so-called "fifth taste" or umami receptors on the tongue. The flavor it produces can probably be best described as "meaty" (here I should point out that glutamate is not the same thing as monosodium glutamate or MSG, which is a salt of glutamic acid). And while nobody will likely ever confuse the flavor of bread with that of meat, it's a common misconception among bread bakers that yeast is not a flavor-producer. Yeats may not create the "tang" that lactic acid bacteria do, but their contribution to bread's overall flavor profile is no less important.

So packaged yeast isn't one-dimensional like most people think. Since it contains the same variety of microbes, it too can be manipulated to create different effects. Making sponges and preferments (like poolishes and bigas) for instance. All are fairly wet mixtures that call for mixing flour, water and packaged yeast, then leaving it to sit for a time...often half an hour to an hour at room temperature, overnight if the sponge is refrigerated. What does that do? The answer is that is allows the lactic acid bacteria that are in with the yeast to catch up a bit and produce some of their own flavor. Granted half an hour isn't that much (though it can be noticeable), but 8-12 hours can do quite a lot. Many types of lactic acid bacteria can keep working even while they're in the refrigerator you see, which means they keep producing acid while the yeasts go to sleep (and sleep they must for that long, otherwise they'd consume every last molecule of sugar).

So, lowly packaged yeast has a few surprises in it after all, eh?

So how do you get from sourdoughs, starters and barms to packaged yeast...and what's the difference anyway? Both good questions, the answers to which, pleasantly enough, start with beer.

Brewing, as many of you already know, is an extremely ancient art. It dates back at least as far as the Egyptians (likely much further, though they're the ones with the written records). Like bread making, brewing depends on the action of yeast (fermentation), only instead of a mostly dry medium, the yeast work in a liquid medium. The upshot of this is the yeast "breathe" less, produce less carbon dioxide, but much more alcohol (to the delight of all concerned). Essentially beer is made by mashing malted (sprouted) grain and leaving it to soak. The soaking encourages the grain's enzymes to break down the starch (just as in bread) and convert it into simple sugars. Strained, the resulting sweet liquid is known as "wort". The next step is boiling the wort with hops to deactivate the enzymes and remove any rank flavors (that's what hops are mostly for). The resulting brew is then left to ferment into beer.

This can take many days, over which time the beer is usually skimmed of the yeasty foam (barm) that collects on the top. The reason, because like a bread starter, lots of other microbes are present in it, notably lactic acid bacteria, which give the beer an unpleasant acidic tang if they aren't removed. The main by-products of the beer-making process are therefore dense cultures of fermenting microbes, which when added to bread dough result in a terrific rise. Thus throughout recorded history, wherever you find beer, you find bread.

So then people went to breweries to get their yeast in the old days? Depending on the society and the time period in which they lived, yes. However most people who drank beer in the pre-industrial ages simply made their own (or perhaps lived and worked on an estate on which the landlord's kitchen brewed its own...plus made its own wine, baked its own bread, butchered its own meat...you get the idea). Back-country peasants generally did without luxuries like beer, and hence ate very dense breads leavened by the sourdough method alone.

So you see that in many respects beer and bread went hand-in-hand. But then how do we get from goopy beer barms to cake and/or dried yeast? For that we need to look to the historical epicenter of all that was good and noble in the baking and pastry arts: the Austro-Hungarian Empire and its capital, Vienna. For it was there (or thereabouts) that chemists first isolated yeast (you mean it's a microbe???) and turned it into the compressed cakes that were so popular from the mid-1800's all the way up until roughly World War II. That's when Fleischmann's Yeast, a company founded by two Austrian brothers in Cinncinnati in 1868, invented active dry yeast so soldiers could enjoy a little taste of home in the field.

The 1980's witnessed the next great advancement in the world of commercial yeast with the invention of "instant" yeast. Invented by the Dutch (no slouches themselves at fermenting yeast), it can be added directly to bread doughs without having to "prove" it in warm water first. It therefore provides all the ease-of-use of fresh cake yeast (still favored by most professional bakeries) with the store-ability of active dry.

One notable exception to the starter microbiology I sketched out below is San Francisco sourdough. Set out a bowl of flour-water slurry there, and a different thing will likely happen. Namely, that your starter will very likely culture a very different and interesting yeast by the name of Candida humilis (formerly called Candida milleri, formerly called Saccharomyces exiguus) which is notable in that it doesn't consume maltose, one of the primary products of enzymatic activity. And that leaves the door wide open to a unique bacterium that lives in the Bay Area and goes by the name of Lactobacillus sanfrancisco. L. sanfranciso, as you might expect, is a prodigious producer of acid (lactic acid as well as acetic acid, which is what gives vinegar its sour tang). L. sanfranciso also produces an antibiotic-type compound by the name of cycloheximide which not only suppresses competing bacteria, but also competing yeast (though obviously not the Candida humilis, which the L. sanfranciso further indulges by excreting a little glucose as it works).

Some makers of commercial sourdough starters claim that the cycloheximide produced by L. sanfranciso makes it impervious to invasion by microbes in climates outside of San Francisco. It's the kind of balderdash you only get when someone is trying to sell you something. All kinds of microbes (including most other types of lactic acid bacteria) employ similar sorts of anti-competitive countermeasures. To imply that L. sanfranciso has the biggest gun in the microbial world regardless of environment is just plain silly.

God only (and I mean God only) knows what other kinds of yeast and bacteria (and yeast-bacteria relationships) are out there, and that might produce fabulous bread. We know so much about San Francisco sourdough because it's been studied exhaustively by food scientists. Outside of that micro-climate, very little similar work has been done. So who knows what might be going on in the micro-climate of your own back yard? There may be something exceptional there just waiting for discovery.

So what's going on in a starter bowl? The answer: lots of stuff. Beneath that happy, bubbly veneer there's war is going on, between all manner of lower forms of life, all of them vying for supremacy. Sort of like the presidential primaries. Bu-dum-bum.

Starters are not what most people think they are: cultures made up solely of "wild" yeast. Nor are they a simple tag-team of one type of wild yeast and one type of flavor-creating bacteria. Rather they are a brewpot of dozens (even hundreds) of kinds of both (plus the odd mold or two thrown in just for fun). There are however certain strains that dominate according to conditions of temperature, moisture, pH and the availability of food. On the yeast side it's usually — but by no means always — Saccharomyces cerevisiae, "sugar fungus of the beer-making kind" in translation from the Latin, the fermentation world's most common (and popular) microbe. On top of the bacteria heap are usually two or three types of lactic acid bacteria, which outnumber the yeast by about 100-1, and vary so dramatically from place to place even the most committed microbiologist could never catalog them all.

But what are all those bugs actually doing in there? Well, if you're a yeast, you're consuming simple sugars. Flour as I mentioned yesterday contains enzymes — enzymes which, in the presence of moisture, begin to dismantle starch molecules that the germ (sprout) would normally use to feed itself. Of course in this case there is no germ, it's either been removed (in the case of white flour) or ground to powder (in the case of whole grain flour). But then the enzymes don't know that. They're simple organic molecules with a job to do. They set down to work slicing those big indigestible starches down into easy-to-handle simple sugar molecules. The yeast dig right in, gobbling down the sugars and giving off carbon dioxide and alcohol in return.

In the meantime, the lactic acid bacteria are doing much the same thing. Except when they consume sugar, they give off mostly lactic acid, the tangy tasting compound that gives yogurt its zing and bread quite a bit of its flavor. Pretty neat really, though there is so much more to the story.

Microbial (also called "biological") leavening is the grandaddy of all leavening techniques. It dates back to when the first human mistakenly left his bowl of gruel sitting out while he rushed off to join the village mastodon hunt. The result was what we now call a "starter". And while the biology of microbial leavening can be a bit complex, mechanics are pretty simple: yeasts, as a by-product of their consumption of sugar, create bubbles of carbon dioxide in a dough. Those bubbles fill with steam in the oven, expanding the loaf and lightening its texture. Elementary, my dear Watson.