Dietary sugars are known to have medical implications for humans from causing dental caries to obesity. This book aims to put dietary sugars in context and includes the chemistry of several typical subclasses eg glucose, galactose and maltose. Modern techniques of analysis of the dietary sugars are covered in detail including self monitoring and uses of biosensors. The final section of the book details the function and effects of dietary sugars and includes chapters on obesity, intestinal transport, aging, liver function, diet of young children and intolerance and more. A must have for the bookshelf of anyone with an interest in diet and nutritional health.
About the Author
Victor Preedy is currently Professor of Nutritional Biochemistry and Director of Genomics Centre, King's College London and Professor of Clinical Biochemistry at King's College Hospital London. After graduating with a BSc degree in Physiology with Pharmacology and Biology, Professor Preedy carried out a period of research on protein metabolism in the Department of Nutrition at the London School of Hygiene and Tropical Medicine. After the successful award of his PhD he studied aspects of cardiac protein metabolism at the National Heart Hospital. After 4 years, he then moved to the MRC Clinical Research Centre in Harrow, which was followed by his appointment as a lecturer to Kings College in 1988. He was promoted to Reader in 1995 and Professor in 2003. Professor Preedy has published over 550 articles, which includes over 160 peer-reviewed manuscripts based on original research and 90 reviews as well as 35 books.
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Chemistry, Analysis, Function and Effects
By Victor R. Preedy
The Royal Society of ChemistryCopyright © 2012 The Royal Society of Chemistry
All rights reserved.
Sugars in Honey
SEVGI KOLAYLI, LAÏD BOUKRAA, HÜSEYIN SAHIN AND FATIHA ABDELLAH
Honey is produced by honey bees from the nectar of plants, as well as from dew. For a long period of human history, honey was an important source of carbohydrates and the only widely available sweetener, until the production of industrial sugar began to replace it after 1800. The natural product mainly consists of carbohydrates and a small amount of other compounds such as phenolics, proteins, amino acids, minerals, vitamins, pigments and organic acids (Figure 1.1). Honey has been used in folk medicine since the early ages of human history and in more recent times their role in the treatment of burns, gastrointestinal disorders, asthma, infected wounds and skin ulcers has been reinvestigated. The composition, nutritional value, appearance and sensory properties of honey differ in relation to its botanical origin and the geographical area where bee hives are located.
Sugars are the main constituents of honey containing about 95% of honey dry weight. In general, honey sugars contain 70% monosaccharide and 10–15% disaccharides and oligosaccharides composed of glucose and fructose (Ouchemoukh et al. 2010). Honey sugars are responsible for many of the physicochemical properties of honey, such as sweetness, viscosity, hygroscopy, granulation and energy value (Cavia et al. 2002). Sugar composition has also been used to distinguish honey samples by botanical origin or geographical origin. Sugar's composition is affected by contributions of the plant and environmental conditions. Sugars contained in nectars are mainly fructose, glucose and sucrose, but their relative proportions are usually variable, however, they are quite consistent for certain botanical families. On the other hand, the relative amount of the two major monosaccharaides (fructose and glucose) is useful for the classification of monofloral honeys, as well as the fructose : glucose and the glucose : water ratios. The minor sugars have a relatively low value for the determination of botanical origin. For example, honeydew honey contains a higher amount of di- and trisaccharides, especially melezitose, which are present in blossom honeys. The numerous di- and trisaccharides in honey are produced by microbial activity and enzymatic reactions in the intestinal tract of the aphids and during honey ripening. The small differences in the sugars' spectra of blossom honeys are explained by the fact, that di- and trisaccharides are mainly produced through trans-glycosylation or enzymatic reversion by the alpha-glycosidase in honey (Ruoff 2006). Honey sugar profile was used to detect adulteration with cheap invert sugars.
1.2 Main Sugars
The main sugars are the hexose monosaccharides fructose and glucose, which are produced by the hydrolysis of the disaccharide of sucrose (Table 1.1). Fructose is the main monosaccharide that is found in three main forms in the diet: as free fructose (in fruits and honey), as a constituent of the disaccharides sucrose or as fructans, a polymer of fructose usually in oligosaccharide form (in some vegetables and wheat) (Shepherd and Gibson 2006). Fructose, a natural sugar found in honey and many fruits, is consumed in significant amounts in Western diets. Fructose is approximately two times sweeter than sucrose, glucose is less sweet and maltose even less sweet and therefore fructose is responsible for honey sweetness and is commonly used as a bulk sweetener (Rizkalla 2010). In nearly all honey types, fructose prodominates and only a few honeys, such as rape (Brassica napus), dandelion (Taraxacum officinale) and blues curls (Trihostema lanceolatum). Rape appears to contain more glucose than fructose (Kakoniene et al. 2010). The relative amount of the two monosaccharides fructose and glucose is frequently useful for the classification of unifloral honeys (Bogdanov 2009).
Glucose is the second main sugar in honey and varies from 25 g to 42 g per 100 g. The actual proportion of glucose and fructose in any particular honey depends largely on the source of nectar (Anklam 1998). It is reported that a F : G ratio of 1.14 or less would indicate fast granulation, while values over 1.58 are associated with no tendency to granulation (Tosi et al. 2004; White et al. 1975). The F : G ratio found in chestnut honeys is higher and, thus, these honeys are not prone to crystallization (Tezcan et al. 2010). It is also suggested that the ratio of fructose to glucose could be used to typify honey samples from different origins, moreover it may indicate the tendency of honey to crystallize (Mendes et al. 1998).
Sucrose, which is composed of fructose and glucose linked together, is a disaccharide. It comprises a little over 1% of the composition of honey. Honey contains other disaccharides which make up over 7% of its composition. Some of the disaccharides in honey are sucrose, maltose, kojibiose, turanose, isomaltose and maltulose. Sucrose content is important to detect heavy sugars feeding of the bees or adulteration by direct addition of saccharose. According to some studies, the amount of sucrose has been used to distinguish the adulteration of honey samples by sugar syrups (Özcan et al. 2006). For example, supplementary feeding of honey bees with saccharose syrup caused a higher saccharose level in honey (Özcan et al. 2006). Other studies reported that sucrose content of honey was not an effective property for distinguishing pure blossom honey from adulterated (with sucrose syrup) honey (Guler et al. 2007).
Generally, honeydew honey compared with blossom honey contains higher amounts of di-, tri-, and higher oligosaccarides, non-reducing sugars, such as melezitose, maltotriose and raffinose, which are usually not found in blossom honeys (Diez et al. 2004). However, Weston and Brocklebank (1999) had studied oligosaccharide fraction in samples of manuka, heather, clover and honeydew honey from New Zealand by high-performance anion-exchange chromatography with pulsed amperometric detection. The monosaccaharide amount was the lowest in honeydew honey among these honeys, while oligosaccharide amount was the highest in the study. In addition, isomaltose (maltulose), kojibiose, turanose (gentiobiose), nigerose and maltose were the major oligosaccharide component, and isomaltose was the major component in heather honey.
Honey sugars are mainly formed by the action of several enzymes on nectar sucrose. Although adult bees can use glucose, fructose, sucrose, trehalose, maltose and melezitose, they cannot use rhamnose, xylose, arabinose, galactose, mannose, lactose, raffinose, dextrin or inulin. The differences in carbohydrate utilization between larvae and adults may be due to the absence of appropriate enzymes. Food enters the alimentary canal by way of the mouth of the bee and passes through the esophagus to the honey stomach. Invertase, in the honey stomach, breaks down the sucrose of nectar to the samples mono saccharides, glucose and fructose present in honey (Özcan et al. 2006).
Oligosaccharide profiling could also be used to check the authenticity of honey types and adulteration. For example, theanderose, a glucosyl-sucrose, was detected in all cane sugar samples examined and has not been detected in any of the beet sugar samples. It was proposed that the presence of theanderose is a better indicator for distinguishing cane and beet white sugars (Morel du Boil 1996).
Maltose, a reducing sugar, is a disaccharide, produced by breakdown of starch by enzymatic digestion, and a partial hydrolysis. It is present in germination grain, in a small proportion, in corn syrup.
Raffinose is a trisaccharide composed of galactose, fructose and glucose. It was reported to be present in heterofloral honey (0.3–0.4 mg/g). The origin of raffinose in floral honey is not clear; it is suggested that rafinose could be a nectar constituent or could be present through honeydew contamination (Kakonienè et al. 2010). Clarck et al. (1992) reported that raffinose amount in the range of 300–1200 mg/kg is indicative of beet sugars, since no cane sugar tested contained observable raffinose. It is reported that raffinose and melezitose were observed only in chestnut and fir honey samples among the 469 monofloral honeys, such as the rape, sunflower, acacia, lavender and cinder heather (Table 1.2). Melezitose was found in fir honey and chestnut honey, 2.22 ± 0.48 and 0.42 ± 0.31 g per 100 g of honey, respectively, and was not detected in cinder heather, lavender, acacia, rape and sunflower.
Erlose is an intermediate trisaccharide in the metabolism of nectar sugars by honey bees. It is made from sucrose by transglucosylation of the α-D-glucosyl group of a molecule of sucrose to the fourth position of the glucose moiety of another sucrose.
Difructose anhydrides (DFAs) are pseudodisaccharides produced by condensation of two fructose molecules by means of a caramelization reaction which takes place during heating of sugars (Montilla et al. 2006). The concentration of DFAs in honey samples is recommended as an indicator of honey adulteration, with high fructose corn syrup or invert syrup, allowing the detection of values as low as 5% (w/w).
1.3 Adulteration of Honey
For centuries, the purity and naturalness of commercialized honey has been questioned. Because honey composition is highly variable, adulteration is very easy to do by overfeeding with inexpensive sweeteners such as saccharose syrups, corn syrups, high fructose syrups, invert syrups and saccharide variants. Overfeeding bees with saccharide or invert saccharide derivatives is practiced by beekeepers to increase honey production.
Sucrose analysis has been frequently used to determine the adulteration, but the test is not adequate, because worker bees convert saccharose to glucose and fructose by digestive enzymes (Ruiz-Matute et al. 2010). The ratio of fructose and glucose indicates honey adulteration. However, some researchers have reported that saccharose, fructose and glucose amount can be used to distinguish pure honey from adulterated honey. Maltose is usually present in honey in low quantities (30 mg/g) and is suggested as a marker of natural honey. Higher amounts of maltose concentration may indicate adulteration of honey by sugar syrup or starch hydrolysate. Maltose and isomaltose are anomers and honeydew honeys showed significanlty higher isomaltose concentrations than flora honeys.
It was suggested that high maltose : isomaltose ratio may indicate adulteration of honey by starch hydrolysate; on the contrary, low maltose : isomaltose values may indicate the use of high fructose-containing syrup (Kakonienè et al. 2010). In recent studies, some of the carbohydrates such as inulin, inulobiose, inulotriose, were used as adulteration markers. For example, inulobiose is not present in honeys, but is detected in adulterated honeys.
1.4 Crystallization of Honey
Crystallization of honey is an undesirable process because it affects its textural properties, making it less appealing to the consumer, and in many cases, it results in increased moisture of the liquid phase, which can allow naturally occurring yeast cells to multiply, causing fermentation of the honey. Glucose may crystallize as a α-D-glucose monohydrate with the stable crystalline form below 50 1C, as an α-D-glucose anhydrous, stable form between 50 °C and 80 °C and b-anhydrous form, stable above 80 °C (Young 1957). In solutions saturated with fructose, the transition temperature from glucose monohydrate to anhydrous glucose has found to be below 30 °C (Cavia et al. 2002). Glucose solubility is lower than fructose; a high glucose ratio may facilitate the crystallization of honeys. This natural phenomenon happens when glucose, one of the three main sugars in honey, spontaneously precipitates out of the supersaturated honey solution. The glucose loses water (becoming glucose monohydrate) and takes the form of a crystal (a solid body with a precise and orderly structure) (Assil et al. 1991). The crystals form a lattice that immobilizes other components of honey in a suspension, thus creating a semisolid state (McGee 1984). The tendency of honey to crystallize depends primarily on its glucose content and moisture level. The overall composition of honey, which includes sugars other than glucose and more than 180 identified substances such as minerals, acids and proteins, also influences crystallization. Additionally, crystallization can be stimulated by any small particles – dust, pollen, bits of wax or propolis, air bubbles – that are present in the honey. These factors are related to the type of honey and are influenced by how the honey is handled and processed. Storage conditions (temperature, relative humidity and type of container) may also influence the tendency of honey to crystallize.
Excerpted from Dietary Sugars by Victor R. Preedy. Copyright © 2012 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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Table of Contents
Dietary Sugars in Context; Chemistry and Biochemistry; Analysis; Function and Effects; Subject Index