I have been making yogurt since 1965. After initial trial with a yogurt making kit the method became simplified into equal quantities of evaporated milk and boiling water poured into jars that then have a spoonful of the last batch of yogurt added. These jars then go into a polystyrene milk-bottle keep-cool container that I got in 1962 as an offer with Kellogg’s cornflakes! Left overnight there is perfect yogurt in the morning. There is some pleasure in still using this battered polystyrene box, up-cycling it in current jargon, for almost 50 years – technology you can trust.
The past few years I have had some mild allergy to “standard” milk that I don’t have with organic milk. Now I am making yogurt with organic milk that needs heating to 82C to kill bacteria and prepare the milk protein to help setting, and then cooling to 43C before adding some of the last batch of yogurt.
The cooling takes over an hour and to start with I kept going to check because the final temperature is critical. Then I thought that I would make use of Newton’s rule-of-thumb about bodies cooling in still air. It was a delight to see the near linear plot of log(temperature difference) against time. This I now use by taking the starting temperature and one other reading at a later time that an excel spreadsheet then turns the time I need to go and add the yogurt – ping.
The temperatures are measured with an IR thermometer that I got to help with research on house insulation. The measurements of temperature and time are rough – same eyes reading both and jotting down – but fit for purpose.
Final points: The top and bottom temperatures of the jar are different and half-way up the jar gives a reasonable average. The milk in the jar is hotter than the temperature on the outside, in my case by about 2C. I add one dessert-spoon of yogurt and that cools the milk by about 1.5C . The final milk temperature I aim for is 43C, so 43C on the outside is just about right.
At last into that courtesy-of-Kellogg’s 1962 insulating box.
Probably all this is over done, but it is irresistable to let a bit of our skills affect everything we do.