What is Reducing Sugar?
Reducing sugars aid in browning by reacting with proteins during baking. They are carbohydrates containing a terminal aldehyde or ketone group which can undergo oxidation reactions. By reacting with free amino groups in amino acids, peptides and proteins, reducing sugars can form unique compounds essential for the desirable flavor and aroma of baked goods.
Examples of reducing sugars include:
Sucrose needs to be hydrolyzed and decomposed to its glucose and fructose molecules to act as a reducing sugar.1,2
Structure of glucose and lactose
All mono- and disaccharides have reducing capabilities with smaller sugars reacting faster than larger ones. Reducing sugars impart several effects in baked goods, such as:3
- Sweetening: Small reducing sugars are sweeter than larger ones
- Tenderizing: interfere with gluten formation, protein coagulation and starch gelatinization
- Shelf life improvement: limit the amount of water available for microbial deterioration
- Provide food for yeast: common sugars can be fermented by yeast
- Color and flavor: reducing sugars such as glucose upon heating in the presence of a free amino group undergo non-enzymatic browning reactions. Products of this reaction include desirable color compounds mainly melanoidins.2 Caramelization, on the other hand, takes place by heating sugar in acidic media to produce unique colors, flavors and aromas.2
Typical carbohydrates like glucose, fructose and sucrose have a caloric value of 4 kcal/g. One drawback of including reducing sugars in baking is the potential development of acrylamides derived from the second order reaction of reducing sugars and L-aspargine. Acrylamide is a known neurotoxicant and carcinogenic agent found in some baked, fried or roasted food (<1.5 ppm).2
Reducing sugars have important contributions to baked goods such as breads, muffins, cookies, bagels, tortillas, cakes and pastries. While Maillard reactions can occur at room temperature, in the case of milk solids and lactose, caramelization requires high temperatures 160 -170 °C (320-340°F).3
Efficiency of reducing sugars in undergoing browning reactions follows this order:3
fructose > glucose > lactose > sucrose > maltose > isomalt
Some consideration when working with reducing sugars include:3