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I have read the answers to this question - How to thicken yogurt and I am not sure if my question is on topic or not - this is the procedure I followed to make yogurt in a mountainous region and I am not able to make yogurt twenty four hours after I started the procedure

1) Boiled unpasteurized milk
2) Cool the boiled milk to where when I touch the milk it feels warm. 
3) I added three teaspoons of branded yogurt(3 % fat)
4) Stir for couple of minutes with teaspoon

I am not sure if you are required to put a lid over the milk as it ferments (I did not) but today morning when I looked it is half milk and half yogurt. It still tastes milk like. The bottom portion is yogurt and the top portion is milk. I am in a mountainous region and it is fairly cold right now(the coconut oil has frozen). I reckon it is around 13-14 degrees centigrade plus rain and I am wondering if my milk culture is "gone" or is there a way to rescue it ? In the morning I did wrap some woolens around it and did put a lid on top of the vessel. I am looking to know what I did wrong here.

  • Is your bought yoghurt absolutely certainly live? Insulating won't help once it's already cold, but there are vacuum flask methods if the temperature is the issue. – Chris H May 29 '17 at 14:06
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    The answer is in the question you linked. Are you keeping the recommended temperatures during the process? Get a cheap food thermometer to check and find a way to keep your yogurt warm during the night (put it in a cooler box or oven). – Luciano May 29 '17 at 14:13
  • @ChrisH - how do I find out if it is live or not ? Is there a simple test ? The marker says made on 26th May. Made with pasteurized milk and live cultures. – gansub May 29 '17 at 15:26
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    @ChrisH - I think if there's a layer of yogurt at the bottom, the starter had to be live or else it would only be a container of milk. It seems to me more likely the temperature got low so not all the milk got converted. – Megha May 30 '17 at 4:46
  • @Megha I think you're probably right though the I didn't want to assume anything about proportions or mixing. – Chris H May 30 '17 at 5:48
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Not covering it was unwise - it accelerates cooling and also allows other (unwanted) bacteria to populate it.

Not insulating it to keep it warm was your main problem. I seem to recall that some cultures in cold climates that traditionally fermented milk products would do so in a bag worn next to the skin, under clothing and blankets, to keep the ferment warm at body temperature. You can probably improvise something a bit less primal, but it's there as an option. A well-insulated box is generally simple enough, but waiting until the milk was already cool to "wrap some woolens around it" is too little, too late - you need the insulation to keep the heat in, as it does not warm the milk up when it's already cold.

I have decent success with a 35-37C temperature for a long time (24 hours) - but without a means to hold the milk at temperature (perhaps set it above the warm computer? Carefully!) I'd start in the 45C range and wrap it up in a lot of insulation to hold the heat as long as possible. Aside from woolens, styrofoam or cardboard are fairly inexpensive or even free insulating materials that are widely available. You can nest several cardboard boxes inside each other to make multiple layers for better insulation. If you are making a small quantity of yogurt you can add bottles or jars of warm water for additional thermal mass (wrap them all up together inside the insulation/box.)

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You need to keep your developing culture at a steady temperature. Fluctuations in the temperature can allow less desirable microorganisms to grow. The ideal temperature for incubating yogurt is 113° F (45° C). I found replacing the oven light with an incandescent 60 watt bulb gave me the heat I needed when I left the light on.

Experiment to see which wattage provides you with the right temperature. I used a thermometer to check and left the light on long enough (30 minutes) to make sure it was accurate. Still, I immersed my container of milk and culture in a large soup pot of warm water to provide even warming throughout.

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Most store-bought yoghurt cultures are thermophillic; you need to provide heat to grow them. Generally, the ideal temperature range is between 50C and 58C for me, but I noted that Chefsteps has a 43C recommendation as does the comment below.

You do need to maintain that temperature range for a few hours at least.

It is best to thin down your bought yoghurt with enough milk so that it mixes easily and readily with the bulk of your milk, otherwise you will either get lumpy or gritty results or you will have two layers as you described. The idea is that you should disperse the culture as widely as possible throughout the milk, otherwise, in what you described, you will have fermentation happening slowly and only at the interface between the two layers.

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  • Too hot by a good 10C – Ecnerwal May 30 '17 at 0:09
  • @Ecnerwal. For me, I found that 54C is optimal in terms of time but I have not done a cell count exercise to confirm that. I use two brands of Greek natural (not Greek-style). In three different climates (which should not matter too much), I have not been able to get it to work within a reasonable amount of time (say 6 hours) much below the mid 40s. I sometimes sous vide, and sometimes just use a water bath in an oven. Just checked online and saw Chefstep has 43C. – user110084 May 30 '17 at 9:50

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