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The Functional Role of Sugars in Food
By David D. Kitts, Department of Food Science,
Faculty of Agricultural Sciences, University of British Columbia
From the consumer’s point of view, sugars are primarily associated with sweetness. But sugars have numerous other functions, which make them important ingredients in many foods.
Sweetness
Sweetness is the most recognized functional property of sweeteners. Our
preference for sweetness, regarded as innate, is apparent soon after birth
and prior to postnatal learning, and decreases with older age.
Sweetness is also associated with feelings of pleasure and appreciation or reward, which contribute to the appeal of sweet foods. The combination of sugars and fats in confections provide a sweet taste and texture that compliment each other. In beverages, sucrose provides sweetness without altering the subtle flavours of the beverage.
Texture
Sugars make an important contribution to the way we perceive the texture
of foods, commonly referred to as ‘mouthfeel’. For example, glucose syrups
in ice-cream provide body and texture, perceived as smoothness. Adding
sugar syrup helps prevent lactose crystallization, which would cause a
sandy, grainy texture that is sometimes associated with frozen dairy products.
In candy-making, controlling the rate and extent of sugar crystallization provides a vast array of different textures. These range from the soft textures of fondants and fudges, where crystallization is minimized, to hard candies where crystallization results in a desired grainy or crystalline structure. Honey has a non-crystallization property, and can therefore be used in confectioneries to maintain a soft, smooth consistency.
In bakery products, sugar is recrystallized as water is removed during baking, resulting in a crisp texture. This crispness is increased by the effects of browning (Maillard reaction) which takes place when reducing sugars (e.g. glucose and fructose) and nitrogen-containing ingredients (e.g. protein) are heated together.
Sugars also act to tenderize bakery products by slowing the rate at which starch molecules become interlinked and proteins break down. Glucose, fructose, sucrose and maltose are used in bread making to increase dough yield and prevent excessive stickiness.
Preservation
In many products, sugars play an important role in preservation. The addition
of monosaccharides, such as glucose or fructose, to jams and jellies inhibits
microbial growth and subsequent spoilage. Sugars have a great affinity
for water, slowing moisture loss in foods like baked goods and extending
the shelf life of these products. Both honey and invert sugar (a fructose-glucose
mixture formed when sucrose is broken down by acids or enzymes) help retain
moisture due to their high fructose content, as do sorbitol (the sugar
alcohol of sucrose) and corn syrup.
Sugars are added to canned vegetables both to maintain firmness and minimize oxidation when the can is opened. Inhibiting oxidation reactions not only protects against deterioration of texture and flavour, but also the loss of colour resulting from the breakdown of pigments. Finally, the interaction between sugars, such as sucrose, and water, controls the moisture in products like cakes and biscuits, to prevent drying out and staleness.
Fermentation
Sugars, which are used to activate yeast for fermentation, are important
in the brewing and baking industries. The type and the amount of sugar
added to the dough in baked products can increase dough yield by influencing
the rate of fermentation. Sugars, such as sucrose, glucose, and fermentable
corn syrups, significantly contribute to sweetness and softness in white
breads.
In contrast, sugars are either omitted or used in much lower amounts for hard crust breads, such as Italian or French breads. In these breads, yeast is activated by sugars that are formed when enzymes present in the flour act on starch.
Sugars that remain after fermentation affect flavour, contribute to the colour and texture of crusts (through non-enzymatic browning and caramelization reactions), and influence the overall texture of the product.
Appearance
The reactivity of glucose on heating contributes to the subtle orange-red
colour in bread crust that is a result of browning (Maillard reaction).
Caramelization of fructose produces a dark brown crust. Breads that contain
sucrose often yield a darker, richer-coloured crust than breads prepared
with glucose.
Freezing Point
Sugars are effective in lowering the freezing point of a solution, which
is important in manufacturing frozen desserts and ice-cream products.
Monosaccharides and corn syrups, containing a high proportion of low molecular
weight sugars, are the most effective at lowering the freezing point.
This property ensures smaller ice-crystals and greater smoothness of the
product. The use of corn syrup sweeteners in sherbets also helps prevent
crystallization of sugars and promotes a smoother product.
Antioxidant Activity
Many carbohydrates are excellent scavengers for metal ions. Glucose, fructose
and the sugar alcohols (sorbitol and mannitol), have the ability to block
the reactive sites of ions such as copper, iron and, to a lesser extent,
cobalt. This characteristic of monosaccharides aids in food preservation
by retarding catalytic oxidation reactions. Furthermore, Maillard reaction
products are known to have antioxidant properties in food systems. For
this reason, some mixtures of Maillard reaction products have been employed
in the food industry as food additives for biscuits, cookies and sausages.
Conclusion
Sugars in foods have multifunctional roles that go beyond the basic perception
of sweetness. They contribute in many ways to a safe and varied food supply.
Replacement of the many functions of sugars in foods cannot be readily
achieved by other ingredients.
REFERENCES
Clarke MA. Sugars in food processing. Int Sugar J 1997; 99:114-26.
Davis EA. Functionality of sugars: physiochemical interactions in foods.
Am J Clin Nutr 1995;62:170S-7S.
Desor JA and Beauchamp GK. Longitudinal changes in sweet preferences in
humans. Physiol Behav 1987;39:639-41.
Hartel RW. Controlling sugar crystallization in food products. Food Technology
1993;47(November):99-107
Jeffery MS. Key functional properties of sucrose in chocolate and sugar
confectionery. Food Technology 1993;47(January):141-4.
The Many Roles of Sugar as an Ingredient in Prepared Foods
- Contributes sweetness
- Enhances flavours
- Improves appearance (browning, colour preservation)
- Acts as a preservative
- Keeps foods moist
- Tenderizes
- Provides a base for yeast fermentation
- Keeps sauces smooth
- Keeps cooked fruits and vegetables firm
- Increases volume of cakes and other baked goods
- Contributes to the texture of baked goods and confectioneries
- Lowers the freezing point (prevents coarse ice crystals)
DEFINITIONS
Caramelization
A browning reaction, caramelization results from the action of heat on
sugars. At high temperatures, the chemical changes associated with melting
sugars result in a deep brown colour and new flavours. Examples are the
browning of bread when toasted, or the darkening of maple sap when heated
to make maple syrup.
Maillard Reaction
The Maillard reaction, sometimes called nonenzymatic, nonoxidative browning,
results from chemical interactions between sugars and proteins, at high
heat. An amino group from a protein combines with an aldehyde or ketone
group of a reducing sugar to produce a brown colour in a variety of foods,
including fried foods and baked goods such as breads.
Reducing Sugars
Reducing sugars, such as glucose, fructose, maltose and lactose, contain
a free aldehyde or ketone group, which allows them to be easily oxidized
(to lose electrons and take up oxygen). This characteristic allows them
to combine with nitrogen at high temperatures to cause browning (Maillard
reaction), to inhibit food spoiling that results from oxidation, and to
retain bright colours in food.
REFERENCES
Freeland-Graves JH and Peckham GC. Foundations of Food Preparation. 6th
ed. Toronto, ON: Prentice-Hall of Canada, 1996.
Ockerman HW. Food Science Sourcebook. 2nd ed. New York, NY: Van Nostrand
Reinhold, 1991.
UPDATE: A NEW ROLE FOR SUGAR IN FOOD PRESERVATION
Drs. John Shi and Marc Le Maguer, of the Department of Food Science at the University of Guelph, have recently developed a new technique called “superior osmotic dehydration”. They hope this technique will eventually result in better tasting, more nutritious, environmentally-friendly dried foods for consumers.
The process relies on the principle of osmosis. When two solutions of differing concentration are separated by a semi-permeable membrane, there is a natural tendency to equalize the two concentrations across the membrane. Osmosis is the movement of water and dissolved substances through the membrane to equalize this concentration difference.
The principle of osmosis has been used for some time in the food industry. Food material of plant or animal origin is immersed in concentrated solutions of water, containing solutes such as sugar or salt. There is a transfer of water out of the food (dehydration) and a simultaneous transfer of solute into the food (impregnation). By controlling the extent of dehydration and impregnation, it is possible to modify the functional properties of foods.
There is a growing interest by the food industry in the process of osmotic dehydration, with the goal of extending the shelf life while enhancing the overall quality of the final products.
CARBOHYDRATE NEWS asked Dr. Shi and Dr.
Le Maguer about their new technique.
How is your process different from treatments currently being used?
Our process is unique in that it combines two processes. Firstly, we use
a physical skin treatment, which allows us to modify the properties of
the skin of the food (eg. tomato). The result of these modifications allows
more water to leave the tomato and less of the osmotic solution to enter
it. We combine this process with a 2-step osmotic treatment (the first
step using sugar and the second step using salt) for pre-concentration
of tomatoes which can be used to produce intermediate moisture tomatoes.
What are the advantages of your process?
The final products are superior in sensory quality (better colour, texture,
flavour) to those directly dehydrated. This treatment enhances the “fresh”
quality. Secondly, unlike hot air drying for conventional dehydration,
this process does not use a lot of energy.
As well, it extends shelf-life without the effects on taste, texture and
colour that are common with hot-air drying and canning.
How would your process affect the nutrient composition of these foods?
Osmotic treatment is operated at room temperature or lower, and water
is removed without a physical phase change. So, almost all natural nutrients,
including those that are sensitive to heat, are retained. In fact, the
concentration of nutrients is increased in dehydrated products.
When will we see products using your method on the market?
A number of foods which have been processed by osmotic pre-treatment are
already on the market. They include fruit snacks such as apple slices,
apricot pieces and banana chips; frozen vegetables, such as green peas,
mushroom pieces, and diced carrots; and dried and salted fish such as
salmon and cod. We hope our technique will enhance the quality of these
types of products.
Burley VJ. Sugar consumption and cancers of the digestive tract. Eur J Cancer Prev 1997;6:422-34.
Forty human studies on the relationship between dietary intake of sugar and cancers of the digestive tract are reviewed. Specifically, risk for stomach, small intestine, colon and rectum cancers are examined. The author concludes that there is insufficient evidence to support the view that dietary sugar increases risk of stomach cancer. Further, there are too few well-designed studies to assess the effect of sugar intake on the risk of cancers of the colon and rectum.
Hegenbart S. Understanding starch functionality. Food Product Design 1996;January:23-34.
The general structure and function of starches is reviewed. The properties of a variety of starches used in the food industry are explored and their functionality compared. The article confirms that, like sugars, the many functional roles of starch in foods are not easy to duplicate.
Kalergis M, Pacaud D and Yale JF. Attempt to control the glycemic response to carbohydrate in diabetes mellitus: overview and practical implications. Can J Diabetes Care 1998;22(1):20-9.
Factors that influence the glycemic response to carbohydrate, the glycemic index, and methods of controlling the glycemic response to carbohydrate are reviewed. The authors conclude that sugar is no more detrimental to glycemic control than most starchy foods.
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Carbohydrate News is an annual publication of the Canadian Sugar Institute (CSI). CSI maintains a scientific library and comprehensive computer database of current literature pertaining to carbohydrate, sugar and health. CSI also publishes resource material for health professionals, educators and the public.
CSI gratefully acknowledges the contributions made by the Editorial Board, in addition to Susan Fyshe, M.H.Sc., RD, for her role in editing this newsletter; Gérald Fortier, for his French adaptation; and Marie Breton Dt.P., and Paul-Guy Duhamel M.Sc., Dt.P. for their additional review of the French adaptation of Carbohydrate News.
Harvey Anderson, Ph.D.
Professor, Department of Nutritional Sciences
Faculty of Medicine
University of Toronto
N. Theresa Glanville, Ph.D.
Professor, Department of Human Ecology
Mount St. Vincent University
David D. Kitts, Ph.D.
Associate Professor, Department of Food Science
Faculty of Agriculture
University of British Columbia
Rena Mendelson, D.Sc., RD
Associate Vice President, Academic
School of Nutrition
Ryerson Polytechnic University
Alison M. Stephen, Ph.D.
Professor, Division of Nutrition and Dietetics
College of Pharmacy and Nutrition
University of Saskatchewan
Huguette Turgeon O’Brien, Ph.D., Dt.P.
Professor, Department of Food Sciences and Nutrition
Faculty of Agriculture and Food Science
Laval University
This publication may be reproduced provided the source is acknowledged.
Publié en français sous le titre: «Glucides-Info»