By Don Mercer
Associate Professor, Department of Food Science University of Guelph
Have you ever opened a container of yoghurt only to be greeted by a layer of water floating across its surface? Or, have you watched helplessly as the filling of your lemon meringue pie begins to weep moisture and fail to hold its shape when you slice it to serve to your guests? If so, you’re quite familiar with the frustrating process of “syneresis”.
Syneresis is basically the oozing of a liquid from a gel structure while the gel is standing, or as it is being cut. It’s a serious problem that food processors have had to face and overcome in delivering quality products to meet the demands and expectations of the consumer.
“Gels” are created from a three-dimensional network of large molecules which are cross-linked with each other to such an extent that they trap water and hold it in place. The term itself is a shortened version of the word “gelatine” which was used to make some of the first jellies. You might think of the gel structure as being like a tangled fish net. The tighter the mesh and amount of entanglement, the greater will be its ability to hold water.
Looking back, tremendous strides have been made in controlling syneresis in yoghurt and other gel-based products, including pie fillings. Much of this has been due to understanding interactions at the molecular level. Such knowledge has gone a long way to generating tailor-made gelling agents designed to fit specific applications in thickening and stabilizing various products.
When the gel structure of a product like yoghurt is relatively weak, water can escape quite easily. As the yoghurt gel structure shrinks in on itself, the free water is pushed out and travels to the surface within the confines of the container. One way to prevent this moisture seepage would be to strengthen and increase the number of bonds between the large molecules that have been used to form the gel. However, this could result in a firm rubbery mass with an unpleasant texture, and an equally unappealing mouth-feel.
Being able to enhance the water-holding properties of a gel without affecting its texture has been an objective of Food Scientists. Their main focus has been on a group of compounds classed as “hydrocolloids” or “gums” which have a rather amazing ability to bind large amounts of water.
When you read ingredient lines on products you will see some of these gums listed by their names: guar gum, xanthan gum, locust bean gum, and carrageenan, in addition to others. Not only are they used in gels, but they are used to modify the viscosity, or thickness, of salad dressings and sauces.
One of the nice things about the hydrocolloids we see in our foods today is that they come from natural sources. Carrageenan is extracted from edible seaweed, and has the ability to form strong bonds with proteins such as those found in dairy products, in addition to other food sources. Xanthan gum is produced in a fermentation process. Guar gum is extracted from guar beans, and locust bean gum is obtained from the beans of the carob tree (bet you didn’t see that coming!).
When used at concentrations as low as one-half percent in water, these gums are capable of promoting appreciable thickening which makes them idea for food uses. Three grams of xanthan gum in a hundred grams of water can create a gel that retains the water quite well.
The next time you use a gelled or thickened product, take the time to check the ingredient line and see what type of gum is being used to do the job.
A small amount of carrageenan in this yoghurt has provided the appropriate mouth-feel, texture, and gel strength
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77th Institute of Food Technologists (IFT) Annual Meeting and Food Expo, Chicago, USA
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