Sunday, May 31, 2009

Microbes in the Kitchen! (with minor Edits)

Eeek! No! Not germs in the kitchen! Grab the bleach, where is the steamer? LYSOL ME!

OK, calm down. It's all going to be fine. Trust me, I'm a microbiologist. We all know (or we all should know) that bacteria and fungi are everywhere. Almost literally everywhere on Earth's surface and even fairly deep into the crust. Don't worry about it too much, they were here first. It's their planet. We merely provide a few new niches for them to grow in. Sometimes they find a nice niche in us and can cause harm, but that's not the kind of thing I'm going to be talking about in this, the inaugural post of the Microbes in the Kitchen mini-series.

I'm going to talk about the niches that we make for bacteria and yeasts intentionally. Food. Good food. Spoiled rotten food that's supposed to be spoiled rotten and will make you feel spoiled rotten eating it. And making it, too.

Let's start easy. Yogurt. After reading this, you will be able to make your own delicious spoiled milk product. The thing that makes it work, this time, is one of many species of Lactobacillus. As the name implies, it eats lactose, and it's a long rod shape (the bacillus part). It's Gram positive (maybe I'll get into that later, if anyone is interested, but basically, it tells us about the cell wall), and it's a facultative anaerobe, meaning that it generally prefers low-to-no oxygen in the environment.

As it eats the lactose, its main by-product is lactic acid. It's the lactic acid that does the work to make milk into yogurt. It also puts the sour in sourdough and the distinct sourness into a good lambic, but those are subjects for later posts. A nice side-effect of this lactose munching is that people who don't express the lactase gene into adulthood (the true lactose intolerant folks) can generally tolerate yogurt just fine, thanks.

OK, so with that intro out of the way, let's get cookin'. I promise that this one is super easy. If you can boil water successfully, you're well on your way to making good yogurt. First we'll need to get a high quality Lactobacillus culture. Fortunately for us, we don't have to call the ATCC for this one. Believe it or not, you can procure a very high quality culture that has been selectively bred for centuries for this specific purpose at the local grocery store. Just buy a little tub of plain yogurt. (I told you this would be easy, don't you trust me?) Heck, it doesn't even have to be plain, since all the sugar and fruit and stuff will be diluted out anyway, but I started with plain. This time, though, all we had was strawberry fruit on the bottom, since I ate all of the last batch.

Lactobacillus culture? Check.

Now we need Lactobacillus food.

While you're there in the dairy isle, grab a 1/2 gallon of whole milk. (BTW, how do you get the internet in the dairy aisle? It never works for me.) The lactose sugar in the milk is our friendly bacteria's food. The milk fats and protein, once denatured by the lactic acid will stick together and get all intertwined to give us that great yogurt texture. If you're worried about using whole fat milk, well, I suppose you could try low-fat or maybe even non-fat, but I make no guarantees that it'll work and, frankly, if a little bowl of 4% fat yogurt in the morning doesn't fit into your diet plan, well, I pity you.

Culture? Check.
Culture medium (bacteria food)? Check.

You do have a thermometer at home, right? No? OK, fine, grab that little instant-read one over there. I hesitate to ask this, but without a thermometer in your kitchen... you do have a pot that can hold a half-gallon of milk, right? Whew. And some plastic containers to hold the yogurt? Yeah, that Chinese takeout soup tub'll work fine, it'll hold a 1/2 gallon, wont it? OK, we can use 2.

(EDIT, PLEASE NOTE: I didn't mention this earlier, but it is very important that your containers are as clean as possible. Wash them in VERY HOT water, or, if available, use the sanitize cycle on your dishwasher. If your hot water doesn't darn near scald you, use a 10% bleach solution to rinse the containers. Give them a good rinse with the bleach and let them sit for 10 minutes, then rinse with clean tap water (nothing too nasty in there if you're on a city supply.) If you're not on a city supply (well, etc.) boil your containers and lids for 10 minutes.)

Back in the kitchen we have everything we need, so let's get going. Put the milk in the pot (that's the hard part), then put the pot over medium to medium high heat. Check the temperature every now and then, but if you notice the milk almost starting to boil, with some nice steam you should be about there. Your target is 180 to 190 degrees F (80 to 88 ish C). (EDIT: This step is to re-pasteurize the milk. It makes sure there is nothing too nasty in your milk. Gotta be careful when we're playing with bacteria.) (EDIT: If the milk ever forms a skin, just scoop it off. It can leave little harder curds in the yogurt if you don't) Like so:

When it gets there, remove the pot from the heat and let it cool to 120 F (~49 C). This will feel very warm, but you can leave your hand on it for a while. Like so:

Now put 3 or 4 big spoonfuls of the starter culture (this time, the yogurt you bought, next time some you made) into the plastic container (the Chinese takeout tub, in this case) and add about a half cup of the warm milk, thusly (the color is the strawberry, don't panic):


Stir them together, so that the starter is nice and thin so it'll mix evenly into the rest of the milk. Then pour the starter and milk mix into the warm milk and stir it well. So:

Pour this into as many soup tubs as necessary to hold it all and slap on the lids. Good.

Now we have to keep them warm for a while, so put them in the oven with the oven light on but don't turn the oven on. Yikes! When was the last time you cleaned that thing? I'm not putting my new yogurt in there. Just wrap them in a clean towel. That'll keep 'em warm. All that's left now is to wait. Give the Lactobacillus 4 or 5 hours to grow, multiply, eat lactose and pee lactic acid. All comfy in their warm place:

Again, wrap 'em up tight and leave them alone. After that time, unwrap and check that the yogurt is relatively set. Gently tilt the container and see if the yogurt stays pretty much in place (good and thick). If so, put your new yogurt in the fridge to set up over night and you'll have some great plain yogurt in the morning. No sugar, no preservatives, no binders and stabilizers, just yogurt. Like mama never used to make. Without all the additives, it will leak a bit of liquid. Just pour it off, it's water and lactic acid. This stuff should be good for a week or so.

I like it with a little honey and granola. It's the best yogurt you've ever had (if you're my typical American reader who gets everything pre-packaged and over processed at the store).

Next up: Greek cheese using the yogurt you just made.


spurge said...

Great post. I may have to give that a try. Even if it seems allot like what I do at work.

Do the Lactobacillus out compete any potential contaminating organisms?

Do you sterilize your containers?

Lab Boy said...

I haven't sanitized (sterilize is virtually impossible in the standard kitchen) the containers in the past. It would be a good idea as a precautionary measure, but I do wash them in very hot water before I use them. The Lactobacillus will outcompete most things that settle out of the air.

The point of going to 190 F is basically to re-pasteurize the milk (assuming it was in the first place, dunno where you get your milk).

I'll add a few notes to the post to clarify that. Thanks for asking. I end up doing so much of the micro stuff automatically that I forget to think about it when writing it up.

spurge said...

We recently had some contamination issues in the lab so it was on my mind.

A friend of mine has some Indian starter Lactobacillus I may try.

Lab Boy said...

AARGH!!! That I understand. I hate contamination issues! Bacteria are frustrating enough on their own, but try to de-contaminate a lab infested with a 189 bp amplicon that has been sprayed around by a clueless intern! For a real-time PCR assay! AARGHHH!

No, using the laminar flow hood with an amplified product to set up your gel is not cool!

spurge said...

So far we have been spared any DNA/RNA contamination.

It happened again today. I am glad I am not running the fermenter.

That is what we get when we put two bioreactors on the same bench using different organisms.

Lab Boy said...

Glad I don't have to use bioreactors.
Just good old-fashioned tubes and plates for me, thanks.
Did you remember to tell whoever did the work to stand on one leg while working? Maybe a chicken sacrifice would help.

spurge said...

I just swear at the bugs and threaten them.

Lab Boy said...

Ah, the classics.