3

First of all, I'm in Korea, and sorry for my bad English.

I'm trying to make yogurt at home, so I've searched out the best yogurt machine on the internet.

Something weird is that most yogurt machines do not have any temperature controllers. I thought that a yogurt machine would keep steady temperature of milk. But how? If temperature of milk is at 80 centigrade, does a yogurt machine keep temperature of milk at 80 centigrade? or let milk be cool, and keeps milk at the optimal temperature for making yogurt, which is in this case around 40 centigrade.

Have a good day everyone!

3

There are a variety of ways to make yogurt. One that might be "easy" would to use a thermal cooker (giant thermos flask) if you happen to already have one, which would be under the "maintain temperature" (actually temperature will fall, but slowly) classification - but if you already have one, it's no extra cost or equipment needed.

Others have a thermostat and heater, but it's rarely an adjustable thermostat. I am currently using a dehydrator which is adjustable as my temperature control device; I have in the past used an oven with a standing pilot light (not adjustable, but a good temperature.)

Regardless of which type you use, you need to cool the milk (after scalding) to 35-40c before adding the yogurt culture (a small amount of any yogurt with live culture) or you will kill the culture - so you can't just dump 80C milk into "whatever" and expect yogurt, regardless of temperature controls or lack of temperature controls.

My research and personal testing W.R.T. the tradeoffs of time and temperature has lead me closer to 35C (for up to 24 hours) than 40C as a default setting. On the other hand, some go as high as 45C.

  • ... and of course for very precise temperature control (≤1°C), mason jars in an immersion circulator bath. Complete overkill unless you already have the circulator. – derobert Jan 20 '16 at 17:20
0

I built a yogurt incubator, so I know a bit about how they work.

Yogurt incubators without temperature controls will have a preset temperature, and they'll heat the contents to this point with a thermostat or a temperature controller such as a modified PID controller. (Engineering: normal PID controllers are not suitable for cooking, since the heat-up phase will create a large integral error term, which will result in compensatory overheating.*) For a consumer product, it wouldn't make sense to hold an object at its current temperature, because that would assume the user set the initial temperature correctly. It would be a food safety issue to let the user implicitly set the temperature, without giving clear controls.

Next, it's not possible to hold an object at its current temperature if you don't know what that temperature is. Yogurt incubators often (usually?) allow you to use your own container, and there's no good (inexpensive) way for the device to read the liquid's temperature without touching it. If it used a typical thermometer (whether it's a thermistor or thermocouple), the milk would cool down by the time the device managed to get an accurate reading. On the other hand, if you are setting the temperature (arbitrarily heating to 43°C, for instance), all you need to do is measure the temperature of the plastic housing or the air--eventually, the milk will match this temperature.

*Note: my yogurt incubator does use a PID controller, but it's my product rather than a consumer product, so it only needs a certain amount of idiot-proofing. If I don't put overly hot or cold milk into it, it won't overcompensate and overshoot the desired temperature.

  • The integral error term is from heat retained within the food but not at the thermostat sensor? – rackandboneman Apr 21 '18 at 18:11
  • @rackandboneman As I understand it, the integral error term measures error due to the proportional term being wrong (or insufficient). The proportional term responds immediately, but if it's not perfectly tuned or if the system requires a nonzero output even when the sensed value == the setpoint, error will build up in the integral term, and eventually that error will be enough to correct the deficiency with the proportional term. If the food starts cold, the system will act like the proportional term is insufficient, building up a huge integral term. When the food gets hot, it'll overshoot. – piojo Apr 22 '18 at 9:48
  • @rackandboneman But can you explain what you mean by heat in the food versus the sensor? – piojo Apr 22 '18 at 9:48

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.