Incr-Edible Science: Bacteria and Yogurt Making

So far we have looked at the different types of bacteria, building our own model:

We’ve experimented with growing bacteria cultures on agar in petri dishes testing which area of the kitchen was the dirtiest:

And finding out which was dirtiest the loo or the sink?

We are currently testing the efficacy of various cleaners against the bad bacteria present in our bathroom and kitchen, as well as testing the effect antibiotic has on bacteria grown from our own nose.  I thought it would be good to have a look into one of the good bacteria, found naturally in our digestive tract, and found in copious amounts in live yogurt.

Growing bacteria, growing yogurt

Yogurt is simply a culture rich medium.  It seems weird to think of it as anything other than a food stuff, yet it is to all intents and purposes a place to breed bacteria.  When we make yogurt from milk, we are using the milk as a breeding ground for certain bacteria.  The change in consistency shows that the milk is full of live bacteria.  Or does it?  We’ve always assumed it does because it says so on the packet:

But we all know we shouldn’t believe everything we read……so I asked the children how we could test for the presence of bacteria in the yogurt.  Between them they decided to grow some in a petri dish.  We made up some fresh Agar using this method, and zigzagged some of the yogurt onto the surface of the set agar, taking care not to break through the surface.  We left it for a week and it rewarded us with some wonderful bacterial colonies:

The yogurt from which we grew the bacteria claimed it contained three different types of bacteria – Bifidobacterium, Lactobacillus acidophilus and Streptococcus thermophylus.  In our growth sample we were able to identify two different colonies, marked as shown below:

Once we were certain that there were bacteria present in our small pot of bought yogurt, we decided to grow our own bacteria farm in the form of some home made yogurt.  We have been making home made yogurt for years and I think the children would have a fair idea how to go about it.  I did wonder, though, whether they had given any thought to why they do what they do.

Making our own home made yogurt

So I set up our slow cooker full of milk and popped it on a high setting, letting it reach almost boiling point (I don’t use a thermometer, but when there are little bubbles forming at the edge I know it is about ready).  Then I gathered my young scientists around me and asked why?  They immediately replied correctly with ‘to kill of any bacteria’.  Oooh, I thought, this is going to be a doddle!  However, this was the end of their understanding.  When asked why we needed to kill off the bacteria, I was met with four blank stares!  After a hint or six, T12 finally said that it was heated to kill off all the bacteria which may have been present that were not wanted or required to make yogurt.

I then turned the slow cooker off.  Turning round, I asked why?  I got many answers, none of them what I was looking for, until having exhausted all other avenues, L11 hesitatingly guessed it was to cool it down so when we added the yogurt from the pot the heat wouldn’t kill the bacteria present.  Yay!  We were getting somewhere.  We left it for a few hours to cool whilst we went to the gym:

I once again gathered the troops.  T12 collected some of the cooled down milk in a bowl:

B3 added two spoons of the bought live yogurt, whilst the older ones and I discussed why we only added such a small amount.  The reason is to allow for maximum space for bacterial growth.  More starter doesn’t result in a thicker yogurt but a thinner one.  If you wanted to continually make your own yogurt then just save a small amount from the batch to use in the next batch.  I always forget so always have to buy:

Once the two are combined well, this starter mixture is poured back into the crock pot:

And again mix thoroughly to ensure even distribution throughout of the starter:

Once mixed, cover with a couple of towels and leave overnight:

And the next morning you should have a large pot full of beautiful, thick yogurt.  Don’t mix, else it will lose its thickness.  I fill up some plastic containers which I keep just for the job, and store them in the fridge.  Remember, the longer you leave them the tarter the yogurt becomes due to the bi-product of the bacteria building up.  Taste it straight from the crock pot.  Seriously, it is not at all like the yogurt bought from the shops:

There are many ways of doing the same thing, some of them haphazard and some of them organised and ‘proper’.  Mine is the former, which whilst almost always successful may make some nervous.  If you are someone who enjoys recipes and the like I have included a couple of links which will instruct you very thoroughly on the how to of growing yogurt bacteria and making your own yogurt.  My attention span, being very close to that of gnat, disallows me from even entertaining the thought of using them (they are long).

1. Grow yogurt on petri dish to prove it contains live cultures

2. Making yogurt using proper procedure