This rule has largely lost its relevance in today’s kitchen. Stainless steel is a nonreactive material and does not harm yogurt culture. The rule dates back to copper, untinned iron, and aluminum utensils in older kitchens; these metals reacted with acid and spoiled both flavor and color. The key factors in fermentation are temperature, cleanliness, and leaving the mixture undisturbed.
Not using a metal spoon when making yogurt is one of those kitchen rules passed down from one generation to the next. The usual explanation is that metal harms the culture. In today’s kitchen, that reason does not hold up; stainless steel is a nonreactive material and does not interact with yogurt bacteria. Yogurt stirred with the same steel spoon also sets without a problem. This makes the rule a good example of kitchen wisdom passed down without being tested. In fact, the same yogurt in the same bowl sets the same way whether stirred with a steel spoon or a wooden spoon. You can test the rule by setting two bowls of yogurt side by side.
The rule has a historical origin, and there was a real observation behind it. The pots and spoons used in older kitchens were made of copper, iron with its tin lining worn off, or aluminum. These metals react with acidic foods; in a highly acidic product such as yogurt, they can leave a metallic taste and cause a color change. So the rule arose not from a false observation, but from materials that are no longer generally used. Earthenware and wooden vessels were preferred at the time because they did not cause this problem.
This part still applies today. Do not use aluminum or unlined copper vessels with acidic dairy products; this applies not only to yogurt but to any mixture containing lemon or vinegar. Stainless steel, glass, and food-grade plastic are safe. So the right rule is not “do not use metal”, but “do not use reactive metals.” Scratched pots or pots with peeling coatings are also not used with acidic mixtures for this reason. That is why glass or stainless steel vessels are preferred for acidic mixtures.
The variables that actually determine fermentation are different, and there are three of them. First is temperature: milk is cultured at around 45°C / 113°F, and the culture dies above 50°C / 122°F. Second is cleanliness, because the mixture will sit warm for hours. Third is leaving it undisturbed: do not move the fermenting container at all, because jostling breaks the curd and makes the yogurt watery. If you also warm the container with hot water beforehand, the temperature will not drop suddenly when you add the milk. When these three variables are managed correctly, the yogurt will set no matter what kind of spoon is used to stir it.
What should you do in practice? Mash the starter with warm milk, stir it into the pot, wrap the mixture, and leave it to rest. The spoon material does not change any of these steps. Still, you can choose a wooden or silicone spoon as a matter of habit; it does no harm. The important thing is not to look for blame in the wrong place when yogurt does not set. First check the milk temperature, then the freshness of the starter. Third, check the fermentation time and how well the container maintained its temperature.