Something to think about to use less energy…
https://www.builditsolar.com/Projects/Cooking/cooking.htm
https://solarcooking.fandom.com/wiki/Solar_Cooking_Wiki_(Home)
https://permies.com/t/65217/DIY-Solar-Oven

1 liter of whole milk
1 tablespoons of stevia / erythritol mixture
1 tablespoon of sugar
100 ml of cream (optional)
1 vanilla stick or vanilla extract (optional)

1 teaspoon baking soda (optional)
10 grams of unflavored gelatin (optional)

Really, you only need milk and sugar. That's it. I like my dulce de leche more creamy, so I add cream.
Sweeteners can be added to taste. I find any store bought dulce de leche overly sweet. They add as much as 1/2 ratio of sugar/milk. Practically ruins the experience.
Erythritol crystals melt at 121 degrees celcius, but do not reduce (chemistry).
Gelatin is to make the dulce de leche denser with a shorter cooking time. It's cheating and not necessary.
Baking soda helps reduce lumps in the dulce de leche. Some recipes have it, but I am suspicious of this ingredient on the end outcome.

OVEN ATTEMPTS:

I have attempted using an oven to make dulce de leche, because I thought heat is more evenly distributed, and will keep the mixture from burning on the bottom of the pot. Milk has a boiling point of 95 degrees celcius, however, you can set the oven slightly above this temperature. Maybe an infrared thermometer would help to check the temperature of the milk surface.

As opposed to an oven, cooking with a pot on top of a flame can superheat the milk adjacent to pot surface, causing it to boil over and spill from the pot. The bottom of the pot in an oven is unlikely to reach such a high temperature as that of a flame.

In an oven, the top surface of the milk is at more of a danger of burning from the top coil of the oven, from radiative heat transfer.

Also, if you set the oven temperature too high, the fat will separate, or perhaps break down, and you won't get a creamy mixture: just runny oil with chunky parts.

For some reason, it takes FOREVER in the oven. More than 5 hours. Is the moisture not allowed to escape, thus keeping the water in the milk from evaporating?

NORMAL INSTRUCTIONS:

Place milk, cream, and sugar, and the optional vanilla, in a stovetop pan, and bring to 90 C.
At 90 C, you can optionally add baking soda, and optionally add gelatin.
Stir every so often. You will require progressively more stirring as the milk thickens. You can also turn down the temperature as it thickens to reduce a temperature gradient between the surface and the inside of the dulce de leche. Some people add marbles to encourage self mixing, and perhaps they also create a surface at a higher temperature than the surrounding dulce de leche. I suspect that commercial production of dulce de leche involves ovens with automatic mixing.

Raw milk and milk cream, stirred with powdered milk to thicken, sweetened with stevia and erythritol.

Tevias brand drops ingredients

Also excellent stirred with Instant Toasted Barley (Ecco brand).

Full instructions for making raw milk yogurt at:
https://nourishedkitchen.com/raw-milk-yogurt

I'm going to try adding culture to yoghurt at room temperature, rather than 110F (43C) as suggested. Or maybe I'll use my oven.

Which bacterial culture should I buy?

Fresh milk, even if refrigerated, starts to turn sour within 2-3 days. Could you preserve milk by having it turn to yogurt instead?

Update: found these articles:

MESOPHILIC YOGURT: MAKING YOGURT AT ROOM TEMPERATURE, Ashley Adamant 2021
How To Activate Your New Mesophilic Yogurt Culture, positivelyprobiotic.com

If one were to genetically engineer yogurt bacteria that proliferated at 4 degrees Celsius, you could put some in fresh cow milk and place the milk in the fridge. Instead of having refrigerated milk go bad if it isn't consumed within the first few days, you would have sour flavored yogurt.

This isn't a how-to, rather some research notes. The conclusion at the moment is that making yogurt from milk at 4C would require a special yogurt bacterial strain that could outcompete other bacteria at 4C. The same way the normal yogurt strain can outcompete other bacteria at 43C.

Yogurt is stored in the fridge after it reaches maturity. Otherwise it goes sour. See: https://www.quora.com/How-did-one-store-yogurt-or-curd-before-the-invention-of-the-modern-refrigerator

So it would still be possible to drink yogurt that passed its optimal taste. But how do you know when not to drink it? “Whey typically runs clear, so if that water on top of your yogurt is looking a little cloudy, better play it safe and toss it out.” tastingtable.com

4 degrees celsius is the maximum temperature recommended for refrigerators. Would it be good to genetically engineer a yogurt culture that proliferates at 4C? This could extend the life of the milk as yogurt, with no heat and minimal effort. The idea is to add the culture right after opening the milk carton, or milking from the cow, and it would slowly become yoghurt. Raw milk would then still be healthy past 2-3 days.

The purpose of boiling is to kill unwanted microbes, but milk from a carton or straight from a healthy cow should be relatively free of microbes? According to the nourishedkitchen.com article above, raw milk has some naturally occurring non-harmful bacteria, but they will keep you using fresh starter culture rather than reusing your yogurt.

Regular yoghurt bacteria will not be active at 4 degrees Celsius.

Lactobacillus delbrueckii subsp. bulgaricus:

“ The optimum temperature and pH conditions for lactobacilli growth are 30–40 °C and 5.5–6.2, respectively; however, the Lactobacillus genus is diversified and belonging bacteria can grow in temperature ranging from 2 to 53 °C. ”. Growth Kinetics of Probiotic Lactobacillus Strains in the Alternative, Cost-Efficient Semi-Solid Fermentation Medium, Śliżewska and Chlebicz-Wójcik 2020

“ The optimum growth temperature for lactobacilli lies between 30 and 40°C but they can grow at temperatures ranging from as low as 5°C to an upper limit of 53°C, depending on the species. ”

Streptococcus thermophilus: (searching…)

“ Because those organisms grow over different temperatures ranges, the term ‘cold’ can be used with respect to their behavior only in a relative sense, with reference to the minimum temperature for growth of each organism. Although a defining minimum temperature is often given for each group-40, 5, and <0°C for thermophiles, mesophiles, and psychrotrophs, respectively (3)-the minimum temperatures for growth of individual organisms within each group vary widely. ” Microbial Control with Cold Temperatures, Colin Gill 2001

From Wikipedia: “Yogurt made with raw milk can be contaminated with bacteria that can cause significant illness and death, including Listeria, Cryptosporidium, Campylobacter, Brucella, Escherichia coli and Salmonella.[41] Yogurts can also be contaminated with aflatoxin-producing Aspergillus flavus, Aspergillus parasiticus and Aspergillus nomius.”

Proliferation capacity of unwanted microbes at 0-4 Celsius:

Growth of Listeria monocytogenes at refrigeration temperatures, Walker et al 1990

I think even the healthier option oils are not healthy to consume. So an ideal diet would not have oil. Unfortunately, I live among families that are going to continue to use oil.

I have in memory that certain oils are healthier, such a first cold pressed olive oil with low acidity content. Also healthy are coconut oil, avocado oil, almond oil, etc. These are all relatively expensive.

I reserve this space to compilate research on healthier oil options.