Okay -- here we go.............

Pasteurization is the process of heating the mix in order to kill harmful bacteria. There are many ways to pasteurize milk products, but each of these methods focuses on two points: temperature and time. Every method is a combination of the temperature and the time the product is held at that temperature. In general, the higher the temperature, the shorter amount of time required to pasteurize. At Ben & Jerry's we pasteurize at a High Temperature for a Short Time (HTST).

The pasteurization process begins with the transfer of mix from the Surge Tank and ends with the mix being pumped into the pasteurized storage tanks (the storage tanks are in our Tank Room, which we'll talk about in a separate chapter). The Surge Tank is a 1000-gallon stainless steel tank that serves as a holding tank for the mix before it goes to the Balance Tank to begin the process. The Balance Tank acts as a "reserve" tank -- it allows the Mix Master to prepare a new batch of mix (in that big ol' Blend Tank we discussed in the last chapter, remember?) without having to wait. So, it's actually the Balance Tank that supplies the mix to the Pasteurizer for the pasteurization process. As the mix flows into the Pasteurizer, the level of mix in the Balance Tank drops; the dropping level alerts the surge pump, which automatically turns on and transfers more mix from the Surge Tank to the Balance Tank, and so on, as the process repeats itself (it's called a "continuous" process).

Here's what goes on inside the Pasteurizer:
As the mix flows through the Pasteurizer, it's being heated and cooled at the same time. Sounds impossible, right? Well, the Pasteurizer makes it possible through a nifty process called "heat transfer". The mix actually helps to heat itself as it travels through a machine called the Press, which is made up of three sections: Regenerator, Heater, and Cooler. Each section consists of a number of narrow plates that are stamped with grooves and pressed together. As the mix enters and exits the press, it passes through all three of the sections. The heat passes from the heater to the mix and back again later.

The booster pump transfers the mix from the Balance Tank to the first stage in the pasteurization process which occurs in the Regenerator. The Regenerator is made up of plates that have liquids of different temperature on each side. On one side the cold (40 degrees), raw mix enters, while on the other side the hot (180 degrees), pasteurized mix exits. Because the two liquids pass so closely, the heat from the hot mix is transferred to the cold mix. The temperature of the cold mix is preheated (143 degrees) as it continues along its path to be pasteurized at the final temperature of 183 degrees.

The Heater portion of the press functions on the same principle as the Regenerator, except that steaming hot water circulates on one side and the unpasteurized mix flows on the other. In this stage, heat from the water is transferred to the mix and raises its temperature to 183 degrees, completing the heating portion of the pasteurization process. The mix then travels through the holding tube where it is held at this temperature for a specific amount of time (20 seconds). The mix then travels to the Cooler and eventually to the pasteurized storage tanks.

There is a safety device called the "flow-diversion valve" located at the outlet of the holding tube. It serves to send the mix back to the balance tank if the mix is not at the required temperature.

Once the mix has reached the pasteurization temperature of 183 degrees and has been held at that temperature for 20 seconds, it is finally cooled. The Cooler, once again, operates in the same way as the two previous stages. Heat moves from the hot mix to the cool water. This drops the temperature of the mix to approximately 38-40 degrees. The mix leaves the cooling stage on its way to the pasteurized storage tanks in the Tank Room, completing the pasteurization process.

Whew! There's a lot going on in there, eh? Now you know why we insisted on breaking up the pasteurization/homogenization process into two processes, each with its own machinery.

If you're ready to enter The Homogenizer, now is the perfect time. (If you want to skip it, you'll miss the cool part where we talk about fat globules and other stuff...)