Blog Directory - Blogged foodliterate: Change is Good

Saturday, January 5, 2008

Change is Good

Happy New Year! I'm back from the holidays and ready to continue our discussion on starches. Last post I talked about unmodified starch and its properties. Today, I'm going to talk about modified starches as you are far more likely to see these in the foods you buy and now you can understand more about why they were chosen.

Unmodified starches, like the corn starch that is in your pantry, thickens really well but has some major limitations. When heat is applied to starch and water the starch swells - also known as gelatinization. The starch granules separate as they absorb water and the slurry becomes viscous (thick). When this gel cools the starch molecules start to realign causing 'set-back' or an additional thickening - think cold gravy. Unfortunately as the realignment occurs, the starch molecules form hydrogen bonds and those squeeze out water - this is called syneresis. And as if this particular issue wasn't enough, unmodified starches can't be frozen. Well you can freeze them, but what they turn into once thawed isn't pretty. Plus as you have probably noticed, unmodified starch won't thicken in cold water and they clump terribly in hot water. Essentially there are a lot of problems that need to be overcome for use in processed foods.

So, what are we to do? Modify the starch to suit our needs of course. There are a number of chemical and physical modifications that are done to native (unmodified) starches to imbue them with the desired characteristics. Modified starches were developed in the 1940's and over the years the range of modifications has increased. Some of the modifications have led to the development of freeze-thaw stable starches, retort stable starches (like in canned soups), fat replacement (low-fat salad dressings), emulsification, pulpy starches, and instant starches.

One of the most common chemical modifications is cross-linking. It is achieved by reacting compounds, usually esters, with the hydroxyl groups on the starch molecules, i.e. cross-linking the starch molecules to each other. This modification provides resistance to temperature, acid and shear. Another common modification is substitution as it is used to provide freeze-thaw stability. This process involves scattering anionic groups, like acetates and phosphates, at the starch molecules to prevent the realignment of the starch molecules so retrogradation/syneresis can't occur.

Physical modifications can be used to produce a cold-water swelling or instant starch. The starch is slurried (mixed with water), then heated with steam to hydrate/gelatinize the molecules and then dried. These starch molecules are now porous and quickly resolubilize when reintroduced to liquid. Our instant puddings just wouldn't be the same without them!

Fibers are also used in processed foods, to provide not just nutrition, but also texture, moisture retention, and mouthfeel. One of the newest forms of functional fiber is resistant starch; while not technically a fiber, it is a starch, the body processes it physiologically like a fiber. It reaches the intestines where is is fermented by the intestinal flora just like a fiber. Resistant starch are primarily used in baked goods and cereal products, but new applications are being developed.
Well, we have spent a good bit of time on carbohydrates, but with the new year, its time for a new topic. Let's move on to fats - and not the kind we are all trying to work off from eating too many holiday treats!

No comments: