Sugars

Sugars are the basic building blocks of carbohydrates. They are classified chemically according to how many sugar units they contain (2, 3). The term ‘sugars’ is conventionally used to describe the mono- and disaccharides in foods (also known as total sugars).

The three most common monosaccharides (single units) found in foods are glucose, fructose and galactose. Where two of these monosaccharides combine they form a disaccharide (two units) – the main disaccharides in foods are sucrose (glucose & fructose); maltose (glucose & glucose) and lactose (glucose and galactose). If three or more sugars are combined they produce oligo-saccharides (3 – 10 monosaccharide units) and polysaccharides (more than 10 monosaccharide units) See IFST Information Statement on Carbohydrates [in preparation].

Sugars can also be hydrogenated to produce polyols which can be used as food additives. The polyol’s name typically follows the root of the sugar name e.g. xylose is hydrogenated to produce xylitol; maltose to produce maltitol. Polyols are classified as carbohydrates, but not sugars (see labelling and IFST information statement on carbohydrates [in preparation]).

Most sugars in nature are D- isomers however some do exist as L-isomers, notably L-arabinose, L-rhamnose (6-deoxy L -mannose) and L-fucose (6-deoxy L-galactose). These L-sugars are found in plant cell wall polysaccharides (Non starch polysaccharides NSP; hemicelluloses).

Sugars can exist in different forms – they are built up of a carbon backbone with several hydroxyl groups attached, they can be present as aldehydes or ketones and change between different acyclic (straight chain) form and ring forms 5 atom (furan) and 6 atom (pyran) rings. The proportion of the different anomers is dependent on the stereo chemistry and the most thermodynamically stable form will be present in the highest abundance e.g. in D-glucose which is an aldo-hexose (i.e. aldehyde form; 6 carbons). It’s most stable form is β-D-glucopyranose (present at 64 %) in an equilibrium with the other anomers. This interchange of different forms is known as mutarotation (4). The 6 carbon sugars notably glucose, fructose and galactose are present in mono-, di- and polysaccharides. The 5 carbon sugars e.g. xylose and arabinose are found mostly in polysaccharides e.g. hemicelluloses and pectins, whereas ribose and in particular deoxy ribose (also 5 carbon sugars) are the backbone sugars in RNA and DNA.

Sugars can also be classified according to their chemical reactions e.g. reducing and non-reducing sugars. The carbon 1 on the ring form of carbohydrates can be reduced to form other compounds e.g. hydrogenated to produce polyols (see below). If blocked or linked to another sugar in for example the di-saccharide sucrose the C1 is not available for reaction and the sugar is known as a non-reducing sugar.  This reactivity is important in food manufacture – reducing sugars can react with other ingredients e.g. amino groups in proteins to form more complex compounds e.g. Maillard reaction products which deliver characteristic colour and flavour to products. Non-reducing sugars cannot react and typically will hydrolyse to their component mono-saccharides (reducing sugars) before they take part in the Maillard reaction. Sucrose in particular reacts to produce glucose and fructose (the reaction is known as inversion from the change in optical rotation in a polarimeter) which then react to give colour and flavour. This reactivity of sugars is fundamental to the range of properties that sugars can deliver and can change the properties e.g. sweetness, freezing point depression and other colligative properties where the number of molecules present is important.

The multi-functionality of sugars provide versatile, traditional ingredients that deliver different properties to many food products, illustrated in the table below: 

Table 1 Sugars Functionality in Food Products (after 6)

Properties of sugars

Sweetness

The sweetness of sucrose is seen as the principal standard of sweetness. All sugars and sweeteners are compared to sucrose with respect to sweetness intensity usually at a defined concentration (either 5 % or 10 % w/v) - see Table 2. The sugars each have a different sweetness intensity and sweetness quality – for example fructose has a very clean, fast onset of sweetness which is quickly cleared from the palate and complements citrus flavours. Glucose has a slower onset of sweetness with a greater linger and complements caramel flavours. The sweetness intensity can also be affected by the temperature and pH of the test solution. In acid solution sucrose is hydrolysed to its component mono-saccharides, glucose and fructose (inversion). This can also change the sweetness intensity as there is sweetness synergy between sucrose and fructose giving an increased sweetness at different concentrations.

Table 2 Sweetness of Sugars (adapted from 7)

Texture

Sugars also provide texture to foods e.g. in confectionery where the crystalline texture is provided by small sucrose crystals. Sugars also interact with other ingredients e.g. proteins and carbohydrates to provide structure and texture. Sucrose increases the starch gelatinisation temperature in baking and therefore starch gels set at a higher temperature and provide excellent texture to cakes (8). In addition the dissolved sucrose competes with the gluten protein to provide a more tender crumb texture. Sucrose also provides the snap to biscuits and in 1985 Doescher and Hoseney (9) reported that only sucrose would deliver the snap and surface crazing in dough biscuits and that glucose and fructose were not as effective. The particle size of sugar can also have an impact on the dough characteristics in biscuit manufacture. Coarser sugar crystals dissolve more slowly in the dough and give a more compact biscuit whereas finer crystals dissolve rapidly and give greater dough spread (8).

Colour & Flavour formation

Sugars play a major role in the development of characteristic colours and flavours in food products. There are two principal reactions where sugars deliver flavour and colour. These are caramelisation and the Maillard reaction. These are known as non-enzymatic browning as they do not require enzymes to deliver their products and are wholly chemical. Enzymatic browning involves enzymes, principally polyphenol oxidase, and is the route to producing brown discolouration when an apple is cut.

Caramelisation typically occurs in high sugar systems with low nitrogen and is accelerated by the addition of alkali (pH >9) or acid (pH<3). It is essentially decomposition of the sugar molecules and is a removal of water followed by isomerisation, condensation and polymerisation giving a mixture of low and high molecular weight compounds. (10)

The Maillard reaction is a cascade of reactions starting with the reaction of the carbonyl group of a reducing sugar with the amino group of an amino acid followed by Amadori rearrangements (isomerisations) and formation of dehydration, fission and Strecker degradation products producing a mixture of low and high molecular weight heterocyclic nitrogen compounds (melanoidins) similar to those produced by caramelisation. (11)

Many different factors can influence the route and rate of the two reactions that result in characteristic flavours and colours e.g. pH, type of amino acid and sugar(s), temperature, time and other ingredients. For example, fructose (keto hexose) reacts very quickly to give characteristic caramel, treacle flavours and rapid colour formation. Ribose and Xylose (aldo-pentoses) react more slowly to give meaty flavours which are used widely in the snack food industry. Sucrose as a non-reducing sugar must hydrolyse to glucose and fructose before taking part in either of the two reactions.

The characteristic colour and flavour of brown sugars is produced in the sugar manufacturing process and will also contain many colour and flavour precursors (products part way down the colour/flavour cascade) and so can provide additional colour and flavour to food products with less processing or cooking.

More recently the Maillard reaction has been recognised as a route to production of acrylamide in food products (12). This route is usually associated with the presence of the amino acid asparagine found in potatoes and some cereal products.  See IFST Information Statement on Acrylamide.

Amorphous and Glass formation

Sugars are highly soluble and when they reach saturation point they will crystallise. This is important in some food products e.g. fudge or set honey, where the sucrose (fudge) and glucose (honey) provide the characteristic texture of the product. In relatively pure solutions the concentration at which crystallisation occurs is governed by the solubility product and saturation point of the specific sugar. However if other sugars are present they act as a ‘sugar doctor’ which prevents/delays the crystallisation and typically results in a super-cooled liquid or glass. This mechanism is the basis for many confectionery products e.g. high boil sweets where sucrose is combined with 42 DE glucose syrup and boiled to a constant temperature before cooling. The sugars inhibit each other and produce a characteristic glass which combined with colours and flavours give the range of sugar confectionery products (13).

Under certain conditions sugars can also exist in an amorphous state. However, it is very unstable and the preferred state is crystalline (14). The sugar amorphous phase is very important in certain food products e.g. chocolate. During the refining stages of chocolate manufacture sucrose is subjected to very high physical pressure and amorphous sugar is produced in situ. The amorphous sugar is highly absorptive and changes the flavour and flow properties of the liquid chocolate. The change of physical state of the sugar is fundamental to the taste, texture and shelf life of chocolate (15) and other food products (14).

Preservation

Water is ubiquitous in foods and the amount and availability determine the shelf life of many products. Sugars are highly soluble and thus influence the water activity aw in many systems.  The ability to retain water and in some cases even attract water (hygroscopicity) can also influence the texture of many foods. Traditionally sugars have been used to preserve foods providing nutritious and safe foods outside the usual harvest season e.g. jams, preserves, chutneys. The ability of sugars to deliver humectancy is a colligative property i.e. the number of molecules influence the effect rather than the amount or molecular weight. Therefore glucose and fructose (invert) have a greater humectant effect than sucrose and are widely used as humectants in bakery products (16).  Problems can occur if sugars are reduced or replaced in food products and a well-documented outbreak of botulism in 1989 was associated with the replacement of sugar with aspartame in hazelnut puree products. Sugar was providing the microbiological hurdle in the processing and replacement with aspartame changed the aw and allowed the microbial growth of Clostridium botulinum in the product (17).

Fermentation

Sugars are readily broken down by yeasts, bacteria and moulds. In many food products this is undesirable hence the need for preservation (see above). However in some products the fermentation of sugars is essential to deliver the desired characteristics e.g. in bread production the formation of carbon dioxide is fundamental to the texture of a dough. On an industrial scale sugars are widely used to produced alcoholic beverages, yeast, citric acid, xanthan gum, enzymes, antibiotics and more recently to produce bioethanol for fuel use (18).

The functionality of sugars and their impact on health have been reviewed by Clemens et al (19)

Sugars are one of the few food ingredients that are specified under regulations which define their characteristics (purity, moisture and non-sugar composition and colour) and also provide reserved descriptions which must be used in labelling. In the EU, Directive 2001/111/EC relating to certain sugars intended for human consumption (29) outlines the products, characteristics and analysis. The Directive is implemented separately in each of the home countries of the UK, for example Statutory Instrument 2003 No 1563; The Specified Sugar Products (England) Regulations 2003, (30).  The regulations cover sucrose derived products e.g. sugar, sugar solution, invert sugar syrup etc., glucose derived products e.g.  glucose syrup, dried glucose syrup, dextrose monohydrate etc. and also fructose. For iso-glucose where the percentage of glucose is higher than fructose it should be labelled as glucose-fructose syrup and where fructose is present at a higher percentage e.g. in HFCS it must be labelled as fructose-glucose syrup. Sugar is a reserved description and legally should only be used when referring to a purified and crystallised sucrose with specific characteristics (including colour). Internationally the same definitions and terms are used in the Codex standard for sugars (31).

In a nutrition declaration or nutrition labelling the information required is specified in EU Regulation 1169/2011 on the provision of food information to consumers (21). The regulation outlines the expression and presentation of nutrition declaration in Annex XV and states that labelling should be ‘carbohydrate - of which sugars’ and defines sugars in Annex I  ‘sugars’ means all monosaccharides and disaccharides in food but excludes polyols. In Annex XIV the regulation provides energy values for components in foods – for carbohydrate (including sugars) the figures to be used in labelling are 17 kJ/g and 4 kcal/g. Reference Intakes for sugars are also specified in Annex XIII – based on a total energy intake of 8400 kJ/2000 kcal the reference intake for sugars is 90 g/day. 

The body of scientific evidence on sugars and their impact on health has been the subject of many extensive reviews by expert committees in recent years (1, 26, 27, 34, 35 and 36). The main concerns raised by the reviews have been on obesity and dental health (see below); the more recent reviews (1, 34) have raised additional concerns about the over-consumption of sugar-sweetened beverages. All have presented recommendations on the intake of sugars in relation to energy intake. These recommendations are summarised in Table 3.

Table 3 Sugars Intake – recommendations from Expert Committees

*a maximum level of intake of 25 % of energy or less due to decreased intakes of certain micronutrients observed at these consumption levels was suggested.

Calories & Obesity

The SACN review (1) reported that randomised control trials conducted in adults indicate that increasing or decreasing the percentage of total dietary energy as sugars when consuming an ad libitum diet leads to a corresponding increase or decrease in energy intake.  Similarly the WHO Guidelines (36) report, a meta-analysis of randomised controlled trials in adults, suggests an association between reduction of free sugars intake and reduced body weight. Increased intake of free sugars was associated with a comparable increase in body weight.  However the EFSA expert panel (34) found that the evidence relating high intakes of sugars (mainly added sugars), compared to high intakes of starch (i.e. the same calories), to weight gain is inconsistent for solid foods. However they concluded there is some evidence that high intakes of sugars in the form of sugar-sweetened beverages might contribute to weight gain. EFSA concluded that the available evidence was insufficient to set an upper limit for the intake of added sugars based on the effect on body weight.

A recent EFSA expert panel provided a new scientific opinion on free and added sugars in relation to certain conditions (35).  A level of sugars intake at which the risk of chronic metabolic diseases is not increased could not be identified over the range of observed intakes, and thus, a tolerable upper intake level UL or a safe level of intake could not be set. Based on available data and related uncertainties, the intake of added and free sugars should be as low as possible in the context of a nutritionally adequate diet. 

Dental Health

The SACN Committee (1) reported that prospective cohort studies indicated that higher consumption of sugars and sugars-containing foods and beverages, but not total sugars, was associated with a greater risk of dental caries. EFSA in 2010 (34) reported that frequent consumption of sugar-containing foods can increase the risk of dental caries, especially when oral hygiene and fluoride prophylaxis are insufficient. However the available data did not allow the setting of an upper limit for intake of added sugars on the basis of a risk reduction for dental caries, as caries development related to consumption of sucrose and other cariogenic carbohydrates does not depend only on the amount of sugar consumed, but it is also influenced by frequency of consumption, oral hygiene, exposure to fluoride and various other factors.

Type 2 Diabetes

The SACN report (1) concluded that there is no association between the incidence of type 2 diabetes mellitus and total or individual sugars intake. However, prospective cohort studies indicate that greater consumption of sugar-sweetened beverages is associated with an increased risk of type 2 diabetes mellitus.

Consumption

Consumption of sugars, along with other components in the diet, is notoriously difficult to quantify accurately with widely suspected underreporting. The main source of dietary information in the UK is derived from the National Diet and Nutrition Survey (36). The data from the latest survey indicates that mean intakes of total sugars were around 95 – 103 g/day in adults, about half the total sugars comprise sucrose 40 – 50 g/day, with glucose and fructose each accounting for 15 – 18 g/day, lactose 10 – 13 g/day and maltose 5 – 8 g/day. Mean intakes of non-milk extrinsic sugars (NMES) exceeded the daily reference value (DRV) set by COMA (25) i.e. no more than 10 % of total dietary energy with the highest intakes in children 4 – 10 years (14.7 %) and 11 – 18 years (15.4 %). Adults aged 65 and over had mean intakes (11.2 %) closest to the dietary reference value, DRV.

DEFRA Family Food statistics provide a complementary measure of sugars intake based on foods and drinks purchased. The 2014 data (37) indicates an average total sugars intake for 2014 of 115 g/day/person and an average NMES intake of 73 g/day/person. The family food survey is carried out every year and from 2001 included eating out, confectionery and soft drinks. Comparing the data from 2001/2 to the 2014 figures it indicates a reduction in consumption of 15.4 % for total sugars (136 g to 115 g) and 20.65 % for NMES (92 g to 73 g).

The SACN report 2015(1) recommends that free sugars intake should not exceed 5 % of energy intake. Public Health England (PHE) (38) calculated this figure to be equivalent to no more than 30 g/day for children aged above 11 and adults. To put this into perspective, the sugar ration at the end of WWII was 8 oz (227 g) a week for adults (39). This equals 32 g/day.

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