Before we move forward from our previous chapters’ exploration of the importance of the microbiology of food from its many different angles to start to focus on how we process food to, among other effects, control that microbiology, we need to consider one more basic constituent of food. This is because, even after several earlier chapters in which the key functions of proteins, sugars, lipids, and other rather high-profile food constituents were discussed, we have yet to discuss explicitly the one that is perhaps the most significant of all. It was mentioned many times of course, lurking in the background like a supporting character actor in a movie who doesn’t dominate the foreground activity but is a key part of the scene. This magically powerful ingredient is water, yes water, that represents the majority of most food products, and without which most of their properties and characteristics would not exist. We have seen already how water can appear in food in many guises, depending on whether it deigns to interact with the other constituents present, leading to apparent logical surprises like the fact that a melon (a solid?) has actually more water per gram of its weight than milk (a liquid?), just because in one case the water is absorbed and robbed of its innate fluidity, while in the other no such restrictions apply. Besides influencing texture in a completely fundamental way, though, water influences behavior of just about every other molecule in food, from the structure of a protein (and hence the texture we perceive) to the suspension of oil droplets in the many food products that are emulsions. As well as this, almost all the dynamic changes we encounter in food, for better or for worse, depend on water. Microbes require water to live, as we can see when we preserve food by removing it (in drying), or else denying it more subtly by adding substances such as sugar or salt, which can suck the very water out of bacterial cells like molecular vampires.