A couple months ago, I bought two rubber stoppers, some brewer's yeast, and two plastic airlocks at a local homebrewing store, and set about transmuting grape juice to wine, a noble alchemy celebrated by Spike Your Juice. At the time, the girls weren't very interested. The oldest daughter in particular was put off that I was, as near as she could tell, ruining a perfectly good bottle of Northland Cran-Grape juice by turning it into alcohol.
This time around, I have two bottles on the table: Welch's grape juice, and a store brand apple cider. This time, the girls are watching what's going on, and they're asking questions.
- Why are the drinks bubbling?
- Why did the apple juice start to foam first, but the grape juice foam more?
- What's pushing some of the foam in the grape juice container climbing up into the airlock?
I'd already explained the last time through this, that the brewer's yeast is multiplying and feeding on the sugars in the grape juice. The yeast produces alcohol as a waste product, but it also produces carbon dioxide gas -- or, as Middle Daughter put it, the yeast is pooping and peeing into the juice. The drinks are bubbling, she recalled, because of the carbon dioxide.
And from that, she deduced the physics of the operation: The carbon dioxide had nowhere to go except up, and so up it went, through the stopper and into the airlock. When it reached the airlock, it filled the top bend, and then slowly, inexorably, began to be pushed down by the ongoing rush of other carbon dioxide gas produced as the fermentation process continues apace. And because the juice is full of bubbles, sometimes those bubbles are rising into the airlock, taking miniscule amounts of juice with them.
Not bad for a 9-year-old.
But why does the grape juice have more foam? The girls already understood that the yeast is feeding on the sugars in the juices, so Oldest Daughter speculated that there might be more sugar in the grape juice, which would mean the yeast is producing gas more quickly. She checked the nutrition labels and, sure enough, Welch's grape juice has 36 grams of sugar per serving, as opposed to the 28 grams in the apple cider.
I'll have to ask her later if she can think of a way to test her hypothesis.
And of course, there's the nature of the chemical change under way in the juice. Like most other living organisms, yeast requires food, oxygen and warmth to live; unlike many other organisms, it does not require the oxygen to be chemically unbonded. I explained it last time, though I really should see if the girls remember and understand how exactly this is working.
When I added the brewer's yeast on Wednesday, there already was a fair amount of oxygen dissolved in the juice. The yeast very quickly used this oxygen as it began to consume the sugar in the juice; and during this period, it produced no alcohol. Once that oxygen disappeared, the yeast switched from aerobic metabolic processes to anaerobic. It still consumed oxygen, but that oxygen was present in the sugars, chemically bonded to the hydrogen and carbon items that also comprise sugar molecules.
It's getting less energy from the sugar than before, since the yeast has to consume the oxygen after breaking the bonds between the oxygen and other atoms, but as a direct result, it's producing the alcohol that we want. Like the carbon dioxide, the alcohol is a waste product that eventually will make the juice too toxic for the yeast, just as our own environments eventually would become too toxic for us if we allowed them to fill up with our waste products. The difference is, we like the waste products of the yeast, and the girls' mother and I are looking forward to enjoying them when the yeast culture has run its course.
Physics, chemistry and biology. I wish all my hobbies were this educational, and that all homeschooling lessons could have so rewarding a conclusion.
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