Fermented Vie

November 5, 2008

I spent the last 24 hours in the microbiology and bioprocess laboratory, trying to grow yeast, for reasons I’ve forgotten about.

It started at the previous Friday, when we formulated a nutrient medium for the fermentation. It consisted of several nutrients, 0.1-4 g/L. And the sustrate used is glucose. In this experiment, we used 40 g/L glucose, since glucose is supposed to be the limiting substrate.

The nutrient and the substrate were autoclaved seperately, since glucose and protein will react in high temperature, resulting in a compound that is supposedly poisonous. At least for some microorganism. I read it somewhere in Bailey and Ollis. The nutrient and substrate will make a 2 L solution. So we used 1800 mL to dilute the nutrient, and 200 mL to dilute the glucose

Then, on Monday, the starter is inoculated with Saccharomyces cerevisiae aka yeast. The 150 mL starter is incubated in a shaker, at 30 degrees, overnight. The minimum incubation time is 16 hours, though a lot of practicants preferred more than 24 hours.

On Tuesday, the pain began. The starter is inserted into the fermentor. Then a sample is taken every one or two hours. It was tested in a spectrophotometer, to determine the cell concentration and the glucose concentration. And then a sample of the air from the outlet is also taken with a syringe half-filled with NaOH. Then the syringe is shaked, to mixed the air and the NaOH completely. The mixture is then titrated with HCl, using phenolphthalein as the indicator. This is done to know the concentration of carbon dioxide exerted by the yeasts. The analysis is simple enough.

But imagined having to stay up all night doing it.

At least it’s all behind me now. My next experiment is supposedly an easy one. (yay!!!)

We worked in the same lab with another group of three, who were making yoghurt. They used two substrate: fresh milk (“yuck…”) and full cream (“still yuck…”); and two types of starter culture: a mixture of Lactobacillus bulgaricus and Streptococcus thermophilus, and Bividobacteria (if I’m not mistaken).

They were titrating about 200 samples all night long (yes, all night long), to know the concentration of acid formed, and also testing the acidity with a pH-meter. Then there were several cups of fresh ‘home-made’ yoghurt that were to be tested for organoleptic (I don’t really know what organoleptic means, it’s more or less how much the tester really likes the taste of the test subject).

This group consisted of a woman (let’s call her Miss N), and two men (Mr Y and Mr B). I asked Miss N, who is going to test the cup of yoghurt that she just taken out from the incubator. She answered, “Whoever. But me.” *lol*

Then Mr Y had his first mouthful of their ‘home-made’ yoghurt.

“Sour”, said he, wincing his eyes a bit, “the texture is good, though”.

Mr B (who was always a bit…well…) took his first mouthful, and without even wincing a bit, said, “Yes, it’s sour…, and yes, the texture is good.”

He took another mouthful, “It’s sour”.

And yet, despite the sourness, another mouthful.


Notice that it was about 8 to 9 am in the morning, and none of us have had a good night sleep, or (more importantly) anything to eat. By the time Mr B was taking his third mouthful, I involuntarily shuddered. It felt like my stomach was already producing extra acid from seeing the yoghurt-devouring. And it was not just one cup. There were lots of cups.

And Mr B finished almost all of it.



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