Heating and Cooling of Food

As we have seen, heating of food is one of the oldest and most powerful ways of making food safe and stable, whether cooking a burger on a barbecue or pasteurizing juice, but is also a potentially highly damaging thing to do to many food products. So, it makes sense that a key principle of processing food is to understand how to control the flow of heat as precisely as possible. In Chapter 8, I introduced how we can maximize the efficient transfer of heat into and out of food in a kitchen in simple systems, like pots on stoves. In practice, in large-scale processes, to transfer heat efficiently from hot to cold, and in this way keep the lords of thermodynamics happy while minimizing damage to the food being heated, we need to use clever pieces of equipment, called heat exchangers (reflecting the fact that, just as the cold part of the system gets hotter, so the hot part gets colder in the deal; fair exchange is no robbery). To visualize a heat exchanger, imagine a simple metal tube, through which a cold liquid is flowing from one end to the other. Now surround that tube with a larger one, through which a hot liquid flows (as shown in Figure 11.1). The wall of the inner tube is exposed to cold on the inside and hot on the outside, and this temperature gradient is the pump that transfers heat across that wall, in nature’s obsessive quest for equality in all things temperature-y. So, now we have two tubes laid horizontally in concentric neatness, say with a hot and a cold liquid flowing in from the left-hand side; as they exit at the right-hand side, the outer hot liquid will be colder, and the inner liquid will have gained the lost heat and thus become hotter. If the tubes were sufficiently long, then both would come out at exactly the same temperature.