Lecture 5: Demystifying your Ingredients - Sugar Substitutes (Part II)

In our last lecture, we began our journey into the world of sugar substitutes.  Our study of how sugar alternatives were discovered and created left us with a storied history that is so complex that we've been forced to spread the richness across a trio of lectures.  In Part I, we discussed a couple sweeteners that are no longer in use because of their mind-numbing toxicity or alleged carcinogenicity.  Here, we will take a much more conservative approach and look into how other natural sugars, besides glucose, have been used to sweeten foods over the years.

Corn Syrup

Figure 1. Most carbonated drinks contain corn syrup...and so
do most sweet food items you find in the grocery store

Everyone is familiar with corn syrup.  It is the choice sweetener for most food manufacturers, as the commercial food industry has decided that cane sugar is too expensive to use in their products.  Corn syrup is used in mass quantities to sweeten everything from soft drinks to canned foods to candy (Fig. 1).  It definitely has a different taste than real sugar - have you tried Coca Cola Life, Pepsi True, or Berghoff or Goose Island Root Beer, which are all made with real sugar?  If you compare a drink sweetened with corn syrup and one sweetened with cane sugar, you will notice right away that the one with the real sugar is much sweeter.  In fact, sucrose is 2-4 times as sweet as corn syrup.

Why is corn syrup less sweet than pure cane sugar?  Corn syrup is made up of many different
saccharides (sugar molecules) that your taste buds are simply not that sensitive to.  Corn syrup starts as starch, the long, interconnected glucose chains that pasta is made of - if you've had pasta before, you'll know that starch is not sweet.  Through a process called acid hydrolysis, corn starch is broken down from polysaccharides (made of many glucose molecules) into oligosaccharides (chains of 3-5 glucose molecules). While closer to glucose in structure - these smaller glucose chains also aren't perceptibly sweet.  Following more hydrolysis, these oligosaccharides are broken down into single glucose molecules.  Finally, something your taste buds can recognize.

Manufacturers can control how thick and sweet the syrup will end up by controlling how much the starch molecules break down: the more oligosaccharides that are left behind, the thicker and less sweet the syrup will be.

Because of how thick it is, corn syrup is used not only for its sweetness, but it is also used as a thickening agent.

Figure 2. Glucose is converted to fructose to make HFCS

Now you can see why corn syrup isn't as sweet as pure sugar.  So what do food manufacturers do when they want a sweetener as cheap as corn syrup but as sweet as sucrose?  They convert corn syrup into the ever-notorious high-fructose corn syrup.  To synthesize high-fructose corn syrup, the regular corn syrup is subjected to complicated chemical process that converts glucose to fructose (see Figure 2).  High-fructose corn syrup is sweeter than regular corn syrup because fructose is sweeter than glucose.  Pretty straightforward, right?

 Until recently, high-fructose corn syrup was the sweetener of choice in most prepared foods because of its low price and high sweetness (which means that less has to be used).  Despite this advantage, high-fructose corn syrup has been targeted as a primary cause of obesity and heart disease, and therefore has been removed from many foods in favor of "healthier" sweeteners.  However, similar to the attacks on other sweeteners, the veracity of these health claims have been called into question, and it is still not known for sure whether or not high-fructose corn syrup is worse for you than normal corn syrup.  As scientists, we recommend that if you need something sweet but you don't want to use real sugar, use non-caloric artificial sweeteners.

Figure 3. Clockwise from above: Glucose, mannitol, and sorbitol.

Sorbitol

Now we will step farther away from glucose and look at a couple sugar alcohols (Figure 3). 

Sorbitol is a common sugar substitute that is primarily used in mass-produced ice cream, sugar-free gum, and candy for diabetics.  Sorbitol is called a "nutritive sweetener" because it provides dietary energy (i.e. calories), albiet not as much as sucrose (table sugar).  Sorbitol can be found in nature, for example, in fruits, but most of the sorbitol that is used in manufacturing is derived from corn syrup.  As we mentioned above, corn syrup is composed of a glucose chains of different sizes.  To make sorbitol, the sugar chains in corn syrup are further broken down into single glucose molecules, and these are converted into sorbitol through a different chemical process called hydrogenation.  Glucose and sorbitol are very similar in structure, but this little difference accounts for a big change in how the body reacts to it. Sorbitol is used as a sugar substitute in sugar-free candy because, unlike sucrose, it is not easily absorbed through the intestines, and therefore doesn't reach the blood stream. This aspect of sorbitol is very helpful for diabetics.

In healthy people, when one's blood stream contains too much sugar, the pancreas releases insulin. Insulin causes one's cells to take sugar up and store it.  Diabetics, however, do not have insulin (or their bodies are insensitive to it), so if a diabetic's blood cells are full of sugar, they do not have a mechanism to release it into the blood stream and dispose of it.  Since sorbitol never makes it into the cells of the body in the first place, it is a safe and healthy sugar alternative for diabetics.

Unfortunately, there is a downside to consuming a sugar that cannot cross the intestinal wall - it can cause diarrhea.  To explain this phenomenon requires a little background in biochemistry:

All of the fluids in your body (including the food mush that your stomach makes out of what you eat) contain mostly water (called the solvent), and also some proteins, sugars, and fats (called solutes).  All of these fluids are separated by membranes (for example, the membranes that compose the cells that make up the wall of the intestine).  These membranes are sometimes permeable to water, and are sometimes permeable to different proteins, sugars, and fats. 

Your body wants to keep the concentration of water and these solutes equal on both sides of these membranes.  For instance, if the water to sugar ratio inside your intestines is 1000:1, then the water to sugar ratio on the outside of your intestines is going to be 1000:1.  If you take some sugar away from the outside of the intestine, some sugar will move (or diffuse) from the outside to the inside of the intestine to balance the sugar-to-water ratio across the intestinal wall (See Fig. 4).

Figure 4. A an example of the diffusion process.  If you dump a
bunch of sucrose into the intestine (top) , over time, it will
 diffuse into the wall of the intestine (bottom) until the sucrose 
concentration is the same on both sides of the membrane.

Another example: when you eat sucrose, the sugar concentration in your intestines goes way up, so a lot of it will diffuse to the outside of the intestines to balance out the sugar concentration across the intestinal wall (Fig. 4).  Sorbitol, however, cannot cross the intestines that easily.  Therefore, when you eat sorbitol, the sugar concentration in your intestines goes up, yet most of it will not cross the intestinal wall, and therefore the sugar-to-water ratio across the intestinal wall will not balance out.

So how does your intestine rectify this problem?  Since the intestines cannot kick out sorbitol, they pull in water in order to balance out the sugar to water ratio across their wall.  What does excess water in the intestine cause?  Diarrhea!  Yes, diarrhea is a common side of effect of eating too much sorbitol, which is why it is recommended that individuals with disorders of the gastrointestinal tract avoid sorbitol.

Okay, now that I've ruined your appetite (ironic for a food blog, right?), I think its about time to move on to another sugar.

Mannitol

Mannitol is another sugar alcohol that is found everywhere in the grocery store.  Chemically, mannitol is very similar to sorbitol, but it is derived from a different sugar, mannose.  Mannitol is found widely in nature as well, most notably in tree sap and mushrooms.  Like sorbitol, mannitol provides energy to the body (although, not nearly as much as sucrose).  Mannitol also does not cross the intestinal wall very easily, and it has a high melting point, which is why it is used in chocolate (chocolate is heated and cooled several times to smooth it out).  For these two reasons, mannitol is an ideal sugar choice when manufacturing chocolate for diabetics.  However, just like sorbitol, the fact that it does not cross the intestinal wall means that it causes diarrhea, a side-effect that causes many diabetics to avoid "sugar free" chocolate.

Figure 5. The powdered stuff you see
on your gum is mannitol

Unlike sorbitol, mannitol is a desiccant, which means that it does not hold onto water.  Even when the humidity outside is 98%, mannitol still stays dry!  Food manufacturers take advantage of this amazing property when packaging hard candies and gum: mannitol provides a natural barrier against water, therefore candy and gum that are coated in mannitol do not stick to their wrappers (Fig. 5).

***
Another advantage of sorbitol and mannitol: unlike sucrose, mannitol and sorbitol cannot be broken down by the bacteria in your mouth.  When sucrose is broken down, the remaining byproducts increase the acidity of your saliva.  In the mouth, acidic saliva can wear away at the enamel on your teeth.  Since sorbitol and mannitol are not broken down by oral bacteria, they do not increase the acidity of your mouth, and therefore they do not rot your teeth.

References and Further Reading:

Figure 4 was borrowed from biologyguide.net
 http://sweetsurprise.com/hfcs-myths-and-facts
http://www.wisegeek.org/what-is-sorbitol.htm
http://www.caloriecontrol.org/sweeteners-and-lite/polyols/mannitol