Ingredients matter Ice cream is a complex system of foam, containing a gas (air) dispersed as small cells in a partially frozen continuous phase. In the continuous phase, fat is dispersed as an inner phase in an emulsion, where the milk solids and stabilizers are in a colloidal solution and sugar and salts form a true solution. Three ingredient types exert the most influence on a typical ice cream’s ability to withstand heat shock: milk solids, sugars, and additives in the form of emulsifiers and stabilizers. Milk solids and sugars both have a lot to say when it comes to the freezing point of the ice cream, so it is important to get both the types and amounts used exactly right. Fat, while important for the product’s structure, has little effect on its ice crystals, since it works in the fat phase and not in the water phase. Solid impact Milk solids act to stabilize ice cream and provide body for the product’s networked structure. But they are an expensive part of the product, making their reduction a desirable move for many manufacturers. If less milk solid is used, however, there will be a greater proportion of water in the product. And all that moisture needs to be crystallized – resulting in a watery mouth feel and greater instability. % water frozen as ice crystals using 8% fat and 11% msfn 100 90 80 70 60 50 40 30 20 10 0 0 100 90 80 70 60 50 40 30 20 10 0 -10 -20 -30 °C 0 -10 -20 Figure 1: Effect of different sugar compositions on the melting curves of ice cream. -30 °C product’s freezing point. Adding a lot of sugar lowers the freezing point, helping to achieve the softness needed, for example, for scoopable ice cream applications. But doing so also makes the product more vulnerable to damage along its journey, with more free water moving to attach to the existing ice crystals and causing them to grow. Too little sugar, on the other hand, makes the ice cream harder. Even with the same fat and milk solid content, the percentage of water frozen into ice crystals will vary if sugar levels or types are adjusted. Figure 1 shows the effect of a different sugar composition alone on the freezing curve of ice cream containing 8% fat and 11% milk-solids-nonfat (MSNF). Sugar, of course, comes in different types. Ice creams typically use a combination of refined sugar and, for example, glucose syrup. This combination affects the texture and the freezing point of the product, too. Some producers even add a little salt to the mix, further depressing the freezing point and adding to heat shock susceptibility. Emulsifier effects Luckily (or rather, the result of over 100 years of development), modern, vegetable-based emulsifiers are available that can go a lot of the way toward protecting ice creams from heat shock. As Figure 2 visualises, emulsifiers are surface-active ingredients due to their hydrophilic-lipophilic prop- Figure 2: An emulsifier is a molecule with ambiphilic properties (part of the structure is hydrophilic and other moieties are lipophilic). In a multiphase system the emulsifier will adopt a favourable position with respect to energy. The emulsifier reduces surface tension between the phases Choosing a milk solid that can combat heat shock requires manufacturers to have a good eye for quality. First, the milk solid needs to be in a form that is best able to support the formation of the desired texture. Most often, a powder is used, but milk or cream is also relatively common. Sugars: Small changes, big effect Sugars play twin roles in ice cream: helping to control taste and the E E Addition of emulsifier (E) to the liquid/liquid interface: Reduces interfacial tension Stabilizes emulsion E E E 2
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