I recently made a quince dessert in which I wanted yeasty, fermented flavors to play a strong supporting role. I decided to simply take a few of the sweet liquid components I would have used anyway and add yeast to them, allowing them to begin to ferment. They turned out delicious, but the coolest and most delicious thing to come out of the experiment was the discovery that sweetened cream can be whipped (at least partially) just by the addition of yeast! That discovery opens the door to all sorts of other yeast foams.

Yeasts survive by metabolizing carbohydrates and, in many cases, releasing alcohol and carbon dioxide. When you introduce typical brewer's or baker's yeasts to an anaerobic or low-oxygen environment (such as a liquid), the yeasts begin to propagate, while feeding on the available sugars and reproducing, thereby "colonizing" the liquid. As the yeasts metabolize, they begin to release tiny bubbles which are molecules of carbon dioxide (as well as releasing molecules of ethanol). When these bubbles are trapped by, for example, capping a bottle that contains active yeasts and available sugars, you get a carbonated beverage (see, e.g., the champagne method of producing sparkling wines). But when a liquid contains a sufficient quantity of emulsifiers or solid particles for the liquid itself to trap the bubbles produced by the yeast, all those tiny bubbles actually start to foam the liquid.

This action can be observed somewhat when proofing yeast before making bread dough. One mixes a quantity of yeast in warm water. If the yeast is active, bubbles appear on the surface of the water. However, these bubbles are not very stable and disappear quickly, being replaced at a slow rate by more bubbles. If, however, one adds a sufficient quantity of emulsifer to the water--lecithin, for example--the yeast will produce a loose foam similar to what chefs these days call an "air."

This is an interesting discovery for me, though of course it has probably occurred to lots of people before. We are already in the habit of using stabilized bases in order to create various types of foams, from the more traditional sabayons and whipped cream, to the recently popular airs, espumas, and similar foams. This means we can use those same bases, but introduce air by the natural process of fermentation, instead of by manually whipping, blending, or charging with compressed gases.

In this case, I scraped a vanilla bean into some just-above-fridge-temperature heavy whipping cream, then stirred in enough sugar to make it nicely sweet. Finally, I added a small amount of active dry baker's yeast, covered the mixture, and left it out at room temperature for about an hour before returning it to the freezer. When I came back to the cream several hours later, it was partially whipped! This yeast was, apparently, a top-fermenting yeast, and indeed, the container of cream now had a thick layer on top of foamy cream with a unique whipped texture, with apparently unchanged cream below. The part that had been somewhat whipped was definitely less rigid even than what is referred to in cream whipping as "soft peaks," but instead it had much smaller and more uniform bubbles that gave it a delightful, "thick" texture. In fact, it was a lot like cappucino foam. It had the characteristic yeasty, complex flavors of fermented products, but the fullness of whipped cream. It was, in short, delicious.

I ended up taking the partially fermented cream and whipping it further with a whisk, all the way to stiff peaks. I then spooned it onto the quince dessert. It functioned much like normal whipped cream, but with a unique and tasty flavor. The proof of the deliciousness came after the dish had been served to the guests and I began the long process of cleaning up. Over the next hour or two, I ended up finishing off the leftover yeast-whipped vanilla cream straight from the mixing bowl, one finger-full at a time.

Posted by Barzelay on 2010/02/23 @ 1:30 | Comments (5) | Food Additives, Science, Technology

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