Few people can resist the temptation of ice cream. Most would succumb more often to the soft creamy texture and sweet taste but for the saturated fats and calories – about 60kcal in a single scoop.
A typical ice cream contains 30% ice, 50% air, 5% fat and 15% sugar solution by volume, with added milk proteins, emulsifiers and stabilisers. Dairy or milk fat – comprising around 65% saturated fat – is the most commonly used fat. In the UK, unless the product specifically says ‘dairy ice cream’, other vegetable fats, such as coconut oil – about 90% saturated fat content – and palm oil, are also used.
‘There is a technical reason why saturated fats are used to make ice cream,’ says Chris Clarke, ice cream expert and author of The science of ice cream. ‘Saturated fats tend to be solid over the temperature range in which the microstructure of ice cream is formed (+5 to -5ºC),’ he explains. Unsaturated oils, in contrast, are liquid at these temperatures.
Making ice cream involves stabilising billions of air bubbles needed to give ice cream its smooth texture, and which would otherwise simply fuse together and come out of the mixture. The fat droplets stabilise the air bubbles by partial coalescence, Clarke explains. They need to be somewhere between liquid and solid state so that they can stick together but remain as separate fat droplets: if they are completely liquid, they coalesce; if they are completely solid, they bounce off each other. But when the fat droplets are partly liquid and partly solid they stick together and partly coalesce, forming a coating on the surface of the air bubbles that stabilises them – also giving ice cream its smooth texture.
Fat has several functions in the final product. Not only does it give the ice cream the right air structure, and the smooth creamy taste as it melts in the mouth, it also increases its viscosity and some flavours are soluble only in fats.
Fat reduction
However, there has been some success in reducing the saturated fat content of ice cream. In the early 2000s, Carte d’Or came on the UK market offering a creamy product with just 35kcal/ scoop. This was achieved by improving the homogenisation process, whereby the fat is broken up into small droplets in the ice cream mixture, and by adding water soluble biopolymers to increase the viscosity of the unfrozen matrix. Unilever scientists used higher pressures during homogenisation to create smaller fat droplets, which gives a bigger surface area for the same amount of fat, or the same surface area for less fat, explains Clarke.
Other scientists have been trying to reduce the content of saturated fats by using blends of coconut oil with unsaturated vegetable oils. This is challenging not least because in some countries like the US you can’t call something ice cream unless it has milk fat in it. And simply changing the fat content is unlikely to work, because a delicate balance of emulsifier and milk protein is needed to achieve partial coalescence. ‘You can’t swap one ingredient for another without having unintended effects,’ says Clarke.
Douglas Goff, professor of food and nutritional science at the University of Guelph, Canada, comments: ‘We have been evaluating ways of trying to enhance the unsaturated oil content and still get good structuring. Blending is one approach. We have also been looking into the effect of gelling the oils with ‘organogelators’, such as rice bran wax, to see the effect of gelled oils on ice cream structure. This is ongoing work, but we have shown definite improvement with gelled oils, compared with the oils on their own.’
Another promising approach is to change the composition of the milk fat so that it is less saturated. This can be done by including foodstuffs that contain unsaturated oils, such as rapeseed, into the feed given to cows. Dairy company Friesland Campina already produces milk that is lower in saturated fat by this method. The resulting milk fat contains a complicated distribution of fatty acids, and enough long chain C14 to C18 fatty acids are present to give the right kind of structure to the ice cream, Clarke says. It would be possible to achieve a similar effect by fractionating milk fat but this is labour intensive and therefore less likely to be commercialised, he points out.
Making huge quantities of this milk fat so that all ice cream could be made this way would be a challenge, but it could certainly be done for moderate quantities by local farmers, Clarke says.
Sugar replacements
The other ‘naughty but nice’ ingredient in ice cream is sugar. Sucrose is commonly used and, like fat, it has several important functions in ice cream. It provides the sweetness and is a freezing point depressant. The more sugar, the lower the freezing point and the less ice in the final product, Clarke explains.
And since freezing point depression depends on the molecular weight of the added solute, a lower molecular weight sugar will give a larger freezing point depression.
The amount of sugar can be reduced to some extent by replacing sucrose, a disaccharide, with the monosaccharides fructose or glucose/dextrose while still obtaining the same freezing point depression but the final product wouldn’t be as sweet. There is the added complication, says Clarke, that too much glucose could crystallise out of solution, giving a gritty texture to the ice cream.
‘To reduce the sugar content of ice cream,’ says Clarke, ‘you need to add something that is not only as sweet as sucrose without the calories but also provides the same freezing point depression.’ Simply replacing sucrose with artificial low-calorie sweeteners would give an ice cream with too much ice.
Some researchers are looking to sugar alcohols to do the job – maltitol, from maltose, is a promising candidate for replacing at least some of the sucrose because it has similar sweetness and freezing point depression profiles, but like all the sugar alcohols too much has a laxative effect. Other scientists are investigating different saccharides, such as trehalose, which has the added advantage of being an antioxidant, and stevia, a zero-calorie sweetener from the South American plant Stevia rebaudiana. Whether any of these will be used in commercial products will depend on whether sufficient quantities can be obtained to make ice cream economically.
The focus of much research in this area over the past couple of years has been to produce ice creams with special dietary requirements, such as low GI (glycaemic index) ice cream. Goff explains: ‘In the no sugar (reduced glycemic index) area, combinations of maltitol or lactitol plus a sweetness boost with something like sucralose or aspartame or stevia work well. Addition of soluble fibres, such as inulin, beta-glucan or soya polysaccharide, can also lower GI.’
There are two issues: one is getting the sweetness right; the other is matching the freezing curves, since the freezing point depression effect from sucrose is very important for ice cream’s firmness and texture – its ‘scoopability’.
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