Is it possible to skip this step, and use milk straight from the fridge? Would the result be terribly different?
The biggest reason to heat milk to almost boiling before fermenting is that it improves the texture of the yogurt.
During fermentation the bacteria consume lactose and produce lactic acid which causes the milk proteins to denature and coagulate trapping most of the fat. The proteins involved are primarily the casein proteins.
When this happens, there is still quite a bit of protein left that isn't bound up in the new casein mesh. All of the albumin proteins are water soluble and will not add to the structure of the yogurt.
These albumin proteins denature when they are heated. For this reason recipes universally call for the milk to be heated to 190 and then cooled. The albumin is denatured and is able to tangle up with the casein during fermentation and add to the yogurt structure.
Skipping this step will make a very profound difference to the structure of your yogurt. Without it your yogurt will be thinner and much more fragile. When you scoop it there will be more whey and all that albumin will wash out in it.
According to Harold McGee (On Food and Cooking pp 48)
"These treatments improve the consistency of the yogurt by denaturing the whey protein lactoglobulin, whose otherwise unreactive molecules then participate by clustering on the surfaces of the casein particles. With the helpful interference of the lactoglobulins, the casein particles can only bond to each other at a few spots, and so gather not in clusters but in a fine matrix of chains that is much better at retaining liquid in its small interstices."
Casein is the milk protein that gels to form yogurt, encapsulating whey in a "spongy" matrix. Casein floats around in milk in the form of globules, or micelles. In fresh milk, the suspended micelles bump into each other and bounce away, going off in different directions.
When an acid is added to the milk, the interactions between the protein micelles are modified, and now instead of bouncing off each other, they stick together when they meet.
Now here is the reason why you need to heat the milk and failing to do so results in sub-par yogurt:
As the milk is heated, the microstructure of the protein micelles changes, they become bumpy. Micelles in unheated milk do not have these bumps.
The bumps on the micelles make it so only a limited number of sites on its surface are available to interact with other micelles. This means that the micelles can only adhere to each other in branched chains.
In unheated milk, there are no bumps to prevent adhesion, so ALL sites are available, and instead of forming a spongy matrix, the proteins form a curd. This means that less liquid can be trapped in the "pockets" of the matrix. Therefore, you will have runnier and grainier yogurt.
Check out this link for technical details and diagrams: http://www.medicinalfoodnews.com/vol01/issue5/kalab.htm
The specific details in the expert answers are great, and really helpful in understanding the chemical and biological processes taking place in making yogurt. Understanding more about the underlying science makes anything more fun in my experience. Even after making yogurt for years, there is more that can be learned!
The original question asked whether this step could be skipped in making yogurt. Here is an empirical response. I haven't experimented fully to confirm all the assumptions that I make (having "learned" them from anecdotal sources), but have developed a procedure that works well for me.
I did try making mesophilic yogurts like the "Caspian Sea" strain mentioned in another post. It was very easy, no heating cycle at all, and "incubation" consisted of leaving the jars at room temperature for 12-24 hours. The process is super easy, and I really liked the flavours of most of these strains, which tend to be more tart. But alas, I was the only person in the household to like the tartness, so I went back to the more common thermophilic strains.
I have been making thermophilic yogurt successfully for about a year or more without the pre-heating step, just incubation. This greatly simplifies the process. However, there are several items that I think are crucial to getting this to work well.
I only use milk from a previously unopened container to reduce the risk of unwanted bacteria being introduced to the milk. (In the rare events that I have raw milk available, I use the pre-heating stage to pasteurize the milk.)
I use glass jars with metal or plastic lids freshly "sterilized" by the hot wash and hot dry cycles of a dishwasher, for the same reason.
There seem to be differences in the resulting yogurt between different dairy brands etc of milk. My best success has been with Organic Valley Whole homogenized milk. I have avoided anything that has been ulta-pasteurized (UHT) because of general online chatter, though I haven't verified it myself. Perhaps as an earlier contributor suggested, the UHT process may achieve the denaturing effect - this could be an interesting experiment. I am guessing that even the regular flash pasteurization may affect the proteins and thus the consistency of the finished yogurt.
I use about 1 teaspoon of starter (from previous batch or favourite store brand of yogurt with live cultures)per quart or litre of milk.
Simply put the teaspoon of yogurt in each jar (I prefer the quart or litre size as I make 4 of these at a time. Then pour in the cold milk directly. Put the jars in the incubating chamber.
A good ice chest (cooler, Esky, whatever you call it) works well as an incubating chamber with hot tap water to maintain the temperature. Use enough water to immerse the part of the jars below the lids, and close the lid of the cooler to keep the temperature stable.
The water temperature should be about 105-120F or 40-50C. The cold milk will affect this temperature, so check up and adjust the water an hour or so after introducing the jars. I usually try to have the water temperature in the upper half of the temperature range as it will cool slightly overnight in the cooler.
Usually overnight or so, roughly 10-12 hours, will complete the process. You can tilt the jar to see that it has gelled. Then just transfer the jars to the refrigerator. Longer incubation seems to yield a more tart flavour which may be a personal preference.
The result is a thinner consistency than if the higher temperature denaturing process is used, but it is very smooth and creamy. Since our household uses about 2 litres of yogurt a week, the simplification of the procedure is welcome. An entire batch can be prepared in about 10 minutes in the ice chest, the water temperature checked and adjusted an hour or so later, and in the morning have yogurt ready to put in refrigerator. Never do I get the graininess that can occur sometimes from the denaturing process etc. The differences in consistency can be a personal preference.
Occasionally, if the incubation temperature is lower than it should be, and I end up with yogurt with a viscosity only slightly thicker than milk. It still tastes great and works well in beverages or smoothies. Some people may actually prefer this.
With a wide variety of milk, bacterial cultures, and procedures, there are myriad varieties of yogurt that can be made at home. This is part of the fun!
There's a specific culture of yogurt that can be fermented at room temperature without keeping the milk warm that produces the desired texture. The "Caspian Sea" yogurt strain works well with room temperature fermentation and, in my experience at least, required no heating.
I was able to obtain this strain a few years back from friends who were growing it; it works as long as you're regularly using and replenishing it, but it can spoil eventually. I believe you can find vendors for this kind of product at health food stores or online, but I don't know how you'd be able to distinguish quality or efficacy, so I can't speak to the best way to obtain the culture.
Keep in mind, though, that this is a much more viscous and lower-acid yogurt than you may be used to. Because of these characteristics, it became quite popular in Japan and among Japanese immigrants to the US, which is how I first encountered it.
I have been making yoghurt using 100 per cent powdered milk and not only do I not heat it above fermentation temperature but it makes the thickest creamiest Greek yoghurt I have ever eaten. Just mix the milk up with loads of extra milk powder because this really affects the thickness of the yoghurt. Some of these comments puzzle me because normal pasteurised milk has already been super heated during its process. I could understand if it was straight from the cow..