The Big Freeze

As the couch potato's mantra has it, ice-cream is one of the three basic food groups (along with pizza and beer).

And as befits a basic foodstuff, it's been around for a long time - some scholars believe sorbets were invented by the Romans, and were first frozen using the snows from Mount Vesuvius.

Remarkably, the basic processes to make ice-creams have changed little since then - the basic technique still involves beating air into a mixture of cream, sugar and eggs while reducing its temperature until it freezes. But the actual equipment has, of course, changed a great deal. And so have the ice-creams.

One notable development in recent years has been the move away from traditional single-flavour ice-creams to the more complex blends and mixtures developed by companies such as Ben & Jerry's. The company's products, with their mixtures of ice-cream flavours, swirls of sorbets and other textures, and large 'inclusions' - lumps of chocolate, fruit or other non-ice cream components - give an impression that most products are made simply by throwing lots of ingredients into a pot and seeing if what comes out tastes good. Of course, the reality is very different.

The Ben & Jerry's production facilities are among the most complex in the industry, and exemplified many of the processes needed for modern ice-cream production. The process starts at the blending tank, a 1000-gallon mixing vessel which produces the three different base mixtures for the company's products: sweet cream base, chocolate base and yogurt base.

The vessel is equipped with a high-speed agitator and movable deflection plates which prevent unmixed material from collecting at the sides. The mixing process is a recipe-based batch system analogous to many others in the food industry, with ingredients pumped from storage tanks into the main vessel. Because the ingredients' properties may change from batch to batch, the company employs 'Mix Masters' to control the ingredient quantities in each mix, rather than using a fully automated system.

Once mixed, the base passes through a series of strainers into storage tanks, before it is pasteurised at 83 degrees C for 20sec using a system of plate heat exchangers. First, a regenerator section, where the cold mixture entering the system is warmed by the hot mixture on its way out; then a heater section, where the mixture's temperature is raised to 183 degrees C by steam.

The mixture is then homogenised by pressurising it to 2000psi and allowing it to expand through a nozzle. This breaks down large fat particles in the mixture, giving the ice cream a smooth texture. The homogenised mixture passes back through the cooling section of the pasteuriser, where cold water reduces the temperature from 63 degrees C to 3 degrees C. This is known as the High Temperature for a Short Time (HTST) process, and is used by most ice-cream manufacturers who work on a medium to large scale.

The mixture is then pumped into a tank room where it cures for at least four hours, before entering the production stage. First, flavourings are added in 300-gallon mixing vessels. It then passes into a 750-gallon continuous freezer supplied by Tetra Pak Hoyer, the market leader in ice-cream freezers, where it is simultaneously whipped, to introduce 15-20 per cent air into the mixture, and cooled rapidly from 2 degrees C to -5 degrees C. The speed of cooling is controlled carefully - fast freezing gives small ice crystals and a smooth consistency.

The frozen ice-cream - still fairly soft - is then mixed with the other ingredients characteristic to Ben & Jerry's ice cream. 'Swirls' of caramel, fudge or other mixtures are added in a variegator, with the swirled mixture first cooled to -9 degrees to -3 degrees C so that it doesn't melt the ice-cream. Inclusions such as fruit, chocolate chunks and chocolate-coated peanut-butter filled pretzels - it's a matter of taste - are added in a feeder which incorporates a screw-thread augur to carry the inclusions and star-wheel which rotates at a variable rate to control the amount of inclusions in the ice-cream.

The finished product is filled into the retail containers, which are sealed, and the containers are loaded into a spiral hardener, a deep-freezer which reduces the temperature from -5 degrees C to -23 degrees C in two and a half hours, at which point it is ready to be transported to shops.

This process is fairly typical among large producers - smaller companies tend to operate less complex machinery. Homogenisation, in particular, is still mainly confined to the larger producers, making more than 250litres of ice-cream per hour. However, as homogenisation not only improves the texture and taste of the product, but also lengthens its shelf life, many smaller producers are beginning to install equipment.

Smaller operators also exemplify one of the main divides in ice-cream manufacture - they generally buy second-hand machinery. Ice-cream is a seasonal business, and a good summer will boost demand across the whole industry. Many smaller manufacturers prefer to purchase reconditioned machinery, and ice-cream industry supply specialists like Charles Wait Process Plant offer a range of reconditioned equipment.

Smaller equipment is often favoured by companies who are expanding. Rather than producing larger batches, with the scale-up of machinery and the changes in production techniques this entails, they often prefer to expand their range of flavours by producing more batches of similar size to their current run.

Sidebar: Topping out ceremony

Air Products - one of the largest companies researching new technologies for the ice-cream industry - produces a piece of equipment called the Cryo-ZAT (the ZAT stands for Zero Adhesion Technology) which uses liquid nitrogen to chill the walls of the ice-cream moulds. This ensures that the product does not stick to the mould and enables the producers to make complex three dimensional shapes.

The fast-freezing technique also allows companies to make products with decorated surfaces. Adding decoration such as sauces, nut flakes or chocolate to an ice-cream has the same difficulties as adding inclusions to the mix - they can melt the ice-cream.

But rather than chilling the decoration, the Air Products has developed a method of 'crusting' the top of the ice-cream - fast-freezing the surface to create a layer of extremely hard ice-cream that can withstand the addition of a decoration, and which also ensures that the decoration sticks to the surface.