Module 2 SharonCroxford_2017_7SugarBrowningAndCara_UnderstandingTheScien.pdf

Copyright 2017. Routledge. All rights reserved. May not be reproduced in any form without permission from the publisher, except fair uses permitted under U.S. or applicable copyright law. Chapter 7 Sugar, browning and caramelisation LEARNING OUTCOMES After completing this chapter, you should be ab...

Copyright 2017. Routledge. All rights reserved. May not be reproduced in any form without permission from the publisher, except fair uses permitted under U.S. or applicable copyright law. Chapter 7 Sugar, browning and caramelisation LEARNING OUTCOMES After completing this chapter, you should be able to: • describe the main sugars in food • list the sugar content of some common foods • explain enzymatic browning, its uses and ways to minimise it • outline the Maillard reaction, caramelisation and dextrinisation, and conditions required for each reaction to occur • give examples of sweets and the sugar syrup required for their production. Sugar The main sugars of interest in food are monosaccharides and disaccharides. Sucrose (table sugar) is a disaccharide made from the monosaccharides glucose and fructose (see Food Focus 7.1). Lactose, the sugar in milk, is made from glucose and galactose. Maltose, used in the production of beer and breakfast cereal, is made from two glucose units. Oligosaccharides, several saccharides joined together, are found in dried beans. Table 7.1 lists the sucrose, lactose, glucose and fructose content of some common foods. Animal products have little or no sugar, except milk and some milk products, which contain lactose. 192 EBSCO Publishing : eBook Collection (EBSCOhost) - printed on 8/6/2023 11:54 PM via EDITH COWAN UNIVERSITY (ECU) AN: 2489874 ; Sharon Croxford.; Understanding the Science of Food : From Molecules to Mouthfeel Account: s3681727.main.ehost Native Files.indd 192 2/08/2017 11:13 AM Sugar, browning and caramelisation Plant products have varying levels of sugar and proportions of the different sugars. Sugar, like starch, is a moderate energy source, providing 17 kJ/g. (Spotlight on Special Diets 7.1 describes elimination and management of the intake of specific dietary sugars.) Table 7.1 Sugar content of common foods (g/100 g edible portion) Food Description Beef Chuck, trimmed, raw 0.0 0.0 0.0 0.0 Kangaroo Loin, raw n.d. n.d. n.d. n.d. Fish Bream, raw 0.0 0.0 0.0 0.0 Chicken Breast, lean, raw 0.0 0.0 0.0 0.0 Egg White 0.0 0.0 0.4 0.0 Yolk 0.0 0.0 0.2 0.0 Lentil Dried, boiled, drained 0.5 0.0 0.0 0.0 Baked beans Canned in tomato sauce 0.2 0.0 1.2 0.7 Chickpea Canned, drained 0.5 0.0 0.0 0.0 Tofu Firm 0.0 0.0 0.0 0.0 Almond With skin, raw 4.8 0.0 0.0 0.0 Peanut With skin, raw 5.1 0.0 0.0 0.0 Sunflower seed Sucrose Lactose Glucose Fructose 2.0 0.0 0.0 0.0 Milk Regular fat (3.5%) 0.0 6.3 0.0 0.0 Yoghurt Regular fat (4.0%), natural 0.0 5.0 0.0 0.0 Cheese Cheddar style, regular fat (32.8%) 0.0 0.1 0.1 0.2 Ricotta style, reduced fat (8.7%) 0.0 2.0 0.0 0.0 Parmesan style, shaved 0.0 0.0 0.0 0.0 Bread White 0.0 0.0 0.2 0.3 Pasta White, boiled 0.1 0.0 0.0 0.0 Rice White, boiled 0.0 0.0 0.0 0.1 Apple Granny Smith, unpeeled, raw 0.2 0.0 2.9 5.6 Banana Peeled, raw 0.0 0.0 6.7 6.2 Tomato Raw 0.0 0.0 1.1 1.2 Cucumber Unpeeled, raw 0.0 0.0 0.6 0.6 Butter No added salt 0.0 0.0 0.0 0.0 Chocolate Dark, high cocoa solids 51.4 0.4 0.1 0.1 Potato New, peeled, raw 0.2 0.0 1.3 0.0 Corn Fresh on cob, raw 0.2 0.0 2.0 1.5 Oats Rolled, raw 0.0 0.0 0.0 0.0 n.d. no data Source: FSANZ (2010). 193 EBSCOhost - printed on 8/6/2023 11:54 PM via EDITH COWAN UNIVERSITY (ECU). All use subject to https://www.ebsco.com/terms-of-use Native Files.indd 193 2/08/2017 11:13 AM Spotlight on Special Diets 7.1 Low FODMAP diet Irritable bowel syndrome is a common disorder of the digestive system affecting approximately one in seven adults, with symptoms including bloating, wind, abdom­ inal pain, diarrhoea and constipation. The syndrome is characterised by chronic and relapsing symptoms and is diagnosed and managed by gastroenterologists. Diag­ nosis involves detailed analysis of symptom patterns and exclusion of other condi­ tions, such as ulcerative colitis and Crohn’s disease, by colonoscopy and gastroscopy testing. Dietary triggers that may induce symptoms of irritable bowel syndrome can include food chemicals (salicylates, amines, glutamates), gluten, caffeine, excess fat and excess alcohol. The Low FODMAP diet is also a treatment option, with scientific research showing that poorly absorbed small carbohydrates in food can be a cause of symptoms in some people. These are given the technical term of FODMAPs (fermentable oligo­ saccharides, disaccharides, monosaccharides and polyols). Oligosaccharides include the fructooligosaccharides found in wheat, rye, onions and garlic, plus the galactooligosaccharides found in legumes. Disaccharides include lactose in milk, soft cheese and yoghurt, and the common monosaccharide avoided is fructose, found in honey, fruit such as apple and highfructose corn syrup. Sugar polyols (sugar alcohols such as sorbitol and mannitol) are found in some fruits and vegetables and artificial sweeteners. Many people with irritable bowel syndrome are treated with the Low FODMAP diet, which involves a restrictive initial phase (approximately 6 weeks) and a liberation phase to gradually reintroduce and test tolerance to certain FODMAPs. As there are individual differences in tolerance levels, no two Low FODMAP diet plans are identical, and people on the diet require management by an accredited practising dietitian. It is advisable in a food service setting to ask the customer about personal food preferences, rather than relying on the full list of restricted foods. There are also suitable substitutions (such as the use of garlic-infused olive oil instead of crushed garlic). Monash University, in Melbourne, has developed a comprehensive database of the FODMAP content of food and a smartphone application called the Monash University Low FODMAP Diet, which is regularly updated with new laboratory results and details of ongoing research. (Monash University, 2016) 194 EBSCOhost - printed on 8/6/2023 11:54 PM via EDITH COWAN UNIVERSITY (ECU). All use subject to https://www.ebsco.com/terms-of-use Native Files.indd 194 2/08/2017 11:13 AM MIKEY BOYLE Food Focus 7.1 Sugar Sugar is the second-largest Australian export product after wheat, and the Australian raw sugar industry is one of the largest in the world. Refined sugar features in most Australian homes and restaurants as white table sugar, and various types of sugar ingredients, derivatives and products are used extensively in the food industry. History Unlike some other sugar-producing countries, which grow beets as their sugar source, Australia bases its sugar production solely on sugarcane farming. The first viable sugarcane plantations were established in Queensland in the 1860s, and indentured labourers were brought in from South Pacific islands until the early 1900s. Migrants from Italy and other European countries then became the predominant cane cutters, eventually establishing small, family-operated farms. Cutting cane by hand is arduous work, particularly in humid, tropical conditions, and innovation led to the manufacture of mechanical cane harvesters and loaders by the 1960s. Once characterised by iconic fires set at each harvest, the industry has since changed practices to green harvesting. One of the best known Australian sugar brands, CSR, was established around 1855. The initials stand for Colonial Sugar Refining. Growing regions and farming There are approximately 4400 cane farm­ ing entities primarily operating along Australia’s eastern seaboard, u ­ nderpin­ning Finely ground icing or powdered sugar the economy of many towns, like Bunda­ berg, in Queensland. Sugarcane (Saccharum officinarium) is a tropical grass that has long stalks growing densely together to a height of 2–4 m. Sugarcane cuttings, known as setts, are planted on cultivated, fertilised land, and up to 12 stalks grow from each, forming a sugarcane stool. At least 1.5 m of rainfall is required during the typical 10- to 18-month growth cycle before harvest. During the cane harvest, in the drier months of June–December, machinery cuts each stalk into shorter billets for transportation. Milling Sugarcane is milled shortly after harvesting, to prevent evaporation of the sugarcane juice. Many mill owners have investments in transportation infra­ structure, such as railway networks. Sugarcane billets are weighed, processed and put through a 195 EBSCOhost - printed on 8/6/2023 11:54 PM via EDITH COWAN UNIVERSITY (ECU). All use subject to https://www.ebsco.com/terms-of-use Native Files.indd 195 2/08/2017 11:13 AM shredder that breaks the fibrous stalks and ruptures the cells. Rollers are used to separate the sugarcane juice from the fibrous material, called bagasse. Sugarcane juice goes through a process of purification and concentration via crystallisation. Sugar crystals can be developed at the desired size by controlling the amount of sugarcane syrup added to the boiling mixture. Finally, the raw sugar crystals are separated from the syrup using centrifugation and dried in bulk bins. The bagasse is recycled and used to fuel boiler furnaces at the mill sites. Molasses is the dark syrup left over from the centrifuge process. Other byproduct residues are used to make fertiliser. Refining Refining begins with affination, a process that mixes the raw sugar with hot, concentrated syrup to soften the outer coating of the crystals. After centrifugation the crystals are separated from the syrup again and dissolved in hot water to form the sugar liquor. This is purified by carbonation (adding carbon dioxide and lime to form a calcium carbonate precipitate) or by phos­ phatation (adding phosphorous and then removing the precipitate). The resulting liquid is decolourised and concentrated by boiling in a vacuum, then seeded with fine sugar crystals and grown to the desired size. Finally, the mixture and the crystals, called a massecuite, is centrifuged again, to remove the crystals from the syrup, and dried. Sugar products There are many variations in the refining process, which produce different sugar products, including a range of crystals like white, caster, brown and dark brown; raw and demerara sugar; and pure icing sugar and icing mixture (with 5% starch to prevent caking). Treacle is made from syrup in a similar way to molasses, as is golden syrup, which is further decolourised. Brown sugar is made from a mixture of sugar crystals and colouring from the dark syrup. Innovation has seen the launch of sugars with low glycaemic indexes, made from raw sugar sprayed with molasses, which lowers the glycaemic index to 50, compared with typical values of over 60 in refined table sugar. Most refineries also produce liquid and invert sugars for food industry use. Invert syrup is a pale-coloured sweetener made by the acid hydrolysis or inversion of a solution of white refined sugar. It contains equal proportions of the reducing sugars glucose and fructose. It has crystalinhibiting characteristics and humectant properties which can extend the shelf life of certain products. Consumer usage In its purest form, sugarcane stalk is juiced into a refreshing drink in Asian countries like Vietnam. Household sugar usage is widespread in Australia, from a sweetener in tea and coffee to a common baking ingredient. Added sugar is also widely used in food manufacturing, from soft drinks to breakfast cereal. In Australia and New Zealand, the nutrition information panel on food labels does not distinguish between added (extrinsic) and naturally occurring (intrinsic) sugars and lists only total sugar content, which can be confusing for consumers seeking to lower their sugar intake. (Australian Sugar Heritage Centre, 2010; Canegrowers, 2010; Sugar Australia, 2014; University of Sydney, 2016) 196 EBSCOhost - printed on 8/6/2023 11:54 PM via EDITH COWAN UNIVERSITY (ECU). All use subject to https://www.ebsco.com/terms-of-use Native Files.indd 196 2/08/2017 11:13 AM AUSTRALIAN SUGAR MILLING COUNCIL AUSTRALIAN SUGAR MILLING COUNCIL Mechanical green harvesting Sugarcane billets Brown sugar crystals coloured with syrup Humans have an innate sweet preference THOMAS KELLEY AUSTRALIAN SUGAR MILLING COUNCIL AUSTRALIAN SUGAR MILLING COUNCIL A mature sugarcane field AUSTRALIAN SUGAR MILLING COUNCIL Young sugarcane growing near a refinery 197 EBSCOhost - printed on 8/6/2023 11:54 PM via EDITH COWAN UNIVERSITY (ECU). All use subject to https://www.ebsco.com/terms-of-use Native Files.indd 197 2/08/2017 11:13 AM UNLOCKING KEY FOOD CHEMISTRY REACTIONS Enzymatic and nonenzymatic browning Browning in food occurs through enzymatic and nonenzymatic reactions, with some reactions desirable and others not. Enzymatic browning results in brown discolouration of fruits, vegetables and seafood. Nonenzymatic browning reactions include the Maillard reaction and caramelisation, and are responsible for a range of flavours and colours that are associated with many foods. There are two stages in enzymatic browning. Firstly, in the presence of oxygen, and at a pH between 5 and 8, the endogenous enzyme polyphenoloxidase (also called PPO) hydroxylates (that is, adds hydroxyl to) phenolic compounds (polyphenols) in food from monophenols to diphenols and then oxidises the diphenols to quinones (aromatic compounds with even numbers of carbons double bonded to oxygens –C(=O)–). The second stage in the browning is nonenzymatic and converts quinones to melanins (a complex, insoluble dark compound derived from tyrosine, an aromatic amino acid) (see Figure 7.1). Melanins are dark in colour and are seen in food as brown pigmentation. Foods in which enzymatic browning is desirable include tea, coffee beans, cacao Figure 7.1 Production of melanins from phenols 198 EBSCOhost - printed on 8/6/2023 11:54 PM via EDITH COWAN UNIVERSITY (ECU). All use subject to https://www.ebsco.com/terms-of-use Native Files.indd 198 2/08/2017 11:13 AM Sugar, browning and caramelisation and dried fruit in which a dark final colour is acceptable (for example, plums as prunes, grapes as raisins and sultanas, and dried figs). There are many foods in which enzymatic browning is undesirable and is avoided wherever possible through manipulating the food during preparation and cooking. Sweet potato, potato, pear, peach, apricot, mango, banana, apple, avocado, lettuce, eggplant, mushroom, prawn and lobster can all brown when peeled and cut and left in the open air. Disrupting the cell walls of the foods liberates the membrane-bound enzyme polyphenoloxidase, and in the presence of oxygen it reacts with the phenols. Therefore, both cell disruption and oxygen are required for enzymatic browning to commence. There are several ways to minimise enzymatic browning. Addition of acid (such as lemon juice) in the kitchen or citric or ascorbic acid in larger scale food production leads to a lower pH, which deactivates the enzyme. Heat treatment (such as blanching in water at 100°C) and high-pressure processing are also used to minimise the browning. High-pressure processing up to 600–700 KPa selectively deactivates enzymes in vegetables, fruit juice, seafood and dried fruit, acting in the same way as denaturation, by disrupting the tertiary structure of the protein and inhibiting the active site on the enzyme (Chakraborty et al., 2014). Avocado dip benefits from the application of high-pressure processing, which extends its chilled shelf life by up to 6 weeks without the addition of preservatives. Browning can also be minimised by inactivating the enzymes through addition of compounds that remove oxygen from the food system, that remove metals (polyphenoloxidase is a copper-containing enzyme) or that alter the pH (Dauthy, 1995). Two key types of nonenzymatic browning are the Maillard reaction and caramelisation. The Maillard reaction requires a sugar and an amino acid, while caramelisation occurs in the absence of protein. Both reactions yield similar brown pigments critical to the perception of the quality of food from a sensory perspective. The brown crust on a loaf of bread and the browning of pastry, cake and biscuits, and of meat are all due to the Maillard reaction. (See Chef’s Insight 7.1 for advice on getting the most out of the Maillard reaction.) Very small amounts of sugar are required to produce significant changes in the colour, aroma and flavour of many cooked and heated foods through nonenzymatic browning. The sugars in Table 7.1 are all six-carbon sugars involved in nonenzymatic browning; however, there are some five-carbon 199 EBSCOhost - printed on 8/6/2023 11:54 PM via EDITH COWAN UNIVERSITY (ECU). All use subject to https://www.ebsco.com/terms-of-use Native Files.indd 199 2/08/2017 11:13 AM UNLOCKING KEY FOOD CHEMISTRY REACTIONS sugars, such as xylose and ribose, that are important for reactions in food like meat, where the brown colours from roasting, grilling and frying are significant. Ribose is available in the form of deoxyribonucleic acid (also called DNA) and ribonucleic acid (also called RNA) in either deoxyribose or ribose form. Proteins in food provide the amino acids for the Maillard reaction to take place, although different amino acids produce some variations in the final aromatic compounds. Maillard reaction The Maillard reaction is named after the French chemist Louis-Camille Maillard, who first reported the phenomenon in 1912. A detailed description of the process was first documented by American chemist John Edward Hodge, in 1953, and the main steps that he observed are still used to explain the reaction today. A complex series of reactions come together to produce the colours, flavours and aromas of many foods usually exposed to a high temperature during cooking. They require a reducing sugar (a sugar with an aldehyde carbonyl group (C=O) or with the potential to form an aldehyde), an amino group (amine, amino acid or protein) and heat. (See Chapter 2 for a description of reducing sugars.) Stage 1 The first stage in the Maillard reaction is the condensation of the carbonyl group on the reducing sugar and the α-amino group on the amino group. In Figure 7.2 glucose is used as an example of a reducing sugar. Glucose reacts with an amino group to produce a carbinolamine intermediary (a functional group that has an amine and a hydroxyl group attached to the same carbon) that rapidly degrades to a Schiff base (a compound with a carbon double bonded to nitrogen with R-groups that are not hydrogen R1-(H)C=N-R 2, named after Hugo Schiff, an Italian chemist). The Schiff base cyclises to N-substituted glycosylamine, (a cyclic structure with an amine joined via a glycosidic link to a carbohydrate). Rearrangement of the unstable N-substituted D-glycosylamine yields a range of products that are more stable (known as Amadori compounds, (named after Mario Amadori, an Italian chemist)) if derived from aldoses. Heyn compounds result from ketoses. In the case of aldoses, Amadori rearrangement results in 1-amino-2-deoxy-2-ketose. This reaction is acid catalysed and occurs 200 EBSCOhost - printed on 8/6/2023 11:54 PM via EDITH COWAN UNIVERSITY (ECU). All use subject to https://www.ebsco.com/terms-of-use Native Files.indd 200 2/08/2017 11:13 AM Sugar, browning and caramelisation at room temperature and in both acid and alkaline environments, although a higher pH is preferred where the amino component is in the basic form (R-NH3+). These products do not contribute to the flavour or colour of food but do reduce the available amount of essential amino acid lysine in the product, due to its engagement in condensation. Reactions to this point are all reversible (see Figure 7.2) (Vistoli et al., 2013). Figure 7.2 M  aillard reaction, stage 1: condensation of glucose and an amino group to produce Amadori compounds Source: Adapted from Nursten (2005) citing Hodge (1953) and Vistoli (2013). 201 EBSCOhost - printed on 8/6/2023 11:54 PM via EDITH COWAN UNIVERSITY (ECU). All use subject to https://www.ebsco.com/terms-of-use Native Files.indd 201 2/08/2017 11:13 AM UNLOCKING KEY FOOD CHEMISTRY REACTIONS An alkaline environment promotes the open-chain form of the reducing sugar, which allows the reaction to occur. In more acidic food the reaction is less likely to occur unless the food product contains sucrose, which in acidic conditions hydrolyses from its nonreducing state to its component, reducing monosaccharides, glucose and fructose. The reaction rate increases with higher temperature. The temperature required for real browning in food is above 140–150°C, although some references suggest a lower temperature: this is related to the reaction progressing slowly at a lower temperature. At a lower temperature and over a longer period (during storage of food), the Maillard reaction may not proceed to produce a discernible change in colour of a food product but may proceed far enough to create compounds that are not digested by humans. The amino acids lysine, arginine, tryptophan and histidine are particularly susceptible, due to their basic nature. Excess or lack of moisture inhibits the reaction. The type of sugar also influences the rate of reaction, with pentoses reacting more quickly than hexoses, and hexoses more quickly than disaccharides. Stage 2 The second stage involves the breakdown of 1-amino-2-deoxy-2-ketose to hydroxymethylfurfural and other breakdown products. This stage is referred to as sugar dehydration. Two breakdown pathways start with enolisation (production of an alkenol, an alkene with alcohol, shortened to enol) of the 1-amino-2-deoxy-2-ketose, with the pathways pH dependent. At an acidic pH, 1,2-dicarbonyls are formed, while at a basic pH, 2,3-dicarbonyls are formed. In Figure 7.3 the 1,2-dicarbonyl is referred to as 3-deoxyglucosone and the 2,3-dicarbonyl as 1-deoxyosone. 3-Deoxyhexosone rapidly dehydrates to form furfurals, including hydroxymethylfurfural (also called HMF). A series of reactions with amino acids then yields brown pigments referred to as melanoidins (see Stage 3 below). Hydroxymethylfurfural does not lead to significant quantities of melanoidins; however, concentrations in food are used to detect deterioration. 1-Deoxyosone forms reductones (which contain the group –C(OH)=C(OH)–), furanones, pyranones and α-dicarbonyls, which react with amino acids to produce melanoidins. Furanones and pyranones are sources of important flavours related to browning (Nursten, 2005; Vistoli et al., 2013), providing a burned, sweet caramel aroma (see Figures 7.4 and 7.5). 202 EBSCOhost - printed on 8/6/2023 11:54 PM via EDITH COWAN UNIVERSITY (ECU). All use subject to https://www.ebsco.com/terms-of-use Native Files.indd 202 2/08/2017 11:13 AM Sugar, browning and caramelisation Figure 7.3 Maillard reaction, stage 2: sugar dehydration Source: Adapted from Nursten (2005) citing Hodge (1953) and Vistoli (2013). Figure 7.4 2-Furanone Figure 7.5 4-Pyranone In addition to the pathways described above, an additional breakdown pathway is sugar fragmentation. α-Dicarbonyls contain two carbons on the carbon backbone that are double bonded to oxygen. In Figure 7.3 one of the α-dicarbonyls is depicted as the generically named 1-methyl-2,3-dicarbonyl, and, as the name suggests, the two double bonded oxygens are at carbons 2 203 EBSCOhost - printed on 8/6/2023 11:54 PM via EDITH COWAN UNIVERSITY (ECU). All use subject to https://www.ebsco.com/terms-of-use Native Files.indd 203 2/08/2017 11:13 AM UNLOCKING KEY FOOD CHEMISTRY REACTIONS and 3. In a review of the literature Smuda and Glomb (2013) proposed five main pathways for the degradation of α-dicarbonyls: retroaldol cleavage, hydrolytic α-cleavage, oxidative α-cleavage, hydrolytic β-cleavage and amine-induced β-cleavage, which result in a breaking down of larger compounds to smaller compounds. In this review the authors questioned the validity of some of the pathways cited in the literature. In summary, however, numerous pathways can be taken to create a multitude of products, and food scientists are continuously working to understand more about these complex reactions. Retroaldol cleavage (retroaldolisation), oxidative α-cleavage (fission), Strecker degradation and some additional dehydration reactions are described below. An example of an α-dicarbonyl, shown in Figure 7.6, is 1-deoxyosone, a six-carbon chain molecule; however, α-dicarbonyls include both longer and shorter chain molecules. Retroaldolisation yields a shorter chain α-dicarbonyl and glycolaldehyde, and further degradation of these products leads to acetic acid, glycolic acid and glyoxal, an α-dicarbonyl. The same base α-dicarbonyl, Figure 7.6 Retroaldolisation of 1-deoxyosone Source: Adapted from Nursten (2005) and Vistoli (2013). 204 EBSCOhost - printed on 8/6/2023 11:54 PM via EDITH COWAN UNIVERSITY (ECU). All use subject to https://www.ebsco.com/terms-of-use Native Files.indd 204 2/08/2017 11:13 AM Sugar, browning and caramelisation Figure 7.7 Oxidative α-cleavage of 1-deoxyosone Source: Adapted from Nursten (2005) and Vistoli (2013). 1-deoxyosone, undergoes oxidative α-cleavage to yield acetic acid and erythronic acid (see Figure 7.7). It is important to note that the figures in this section are examples of reactions that occur in sugar degradation and that there are many sugars involved in these pathways, which result in a variety of compounds. α-Dicarbonyls react with amino acids via a process known as the Strecker amino acid degradation (named after Adolph Strecker, a German chemist) to produce α-aminocarbonyls and aldehydes, the latter of which provide flavour in food (see Figure 7.8). Further condensation of α-aminocarbonyls produces heterocyclic pyrazines, pyrroles, oxazoles, oxazolines and thiazole compounds, along with many other compounds associated with the aroma and flavour of cooked food (Vistoli et al., 2013). Pyrazines are volatile compounds with strong earthy, nutty, roasted, toasted caramel-like aromas, such as those found in tea, coffee, bread crust, fried beef and cocoa (see Figure 7.9). The amount of pyrazines produced depends on the type of sugar available for reaction. Pyrroles have roasted aromas and are associated with savoury biscuits or crackers, bread crust and nutty aromas (see Figure 7.10). They produce strong flavours that might be overwhelming in some foods. Oxazoles are nitrogen containing and have fresh green, nutty and 205 EBSCOhost - printed on 8/6/2023 11:54 PM via EDITH COWAN UNIVERSITY (ECU). All use subject to https://www.ebsco.com/terms-of-use Native Files.indd 205 2/08/2017 11:13 AM UNLOCKING KEY FOOD CHEMISTRY REACTIONS Figure 7.8 Strecker amino acid degradation Source: Adapted from Nursten (2005) and Vistoli (2013). sweet aromas (see Figure 7.11). Thiazoles are derived from sulfur-containing amino acids and have toasted aromas of popcorn, cereal, coffee, roasted peanut, cooked beef, broth and potato chips, although their aromas are also associated with tomato and wine (see Figure 7.12) (Belitz, Grosch & Schieberle, 2009). Other key cyclisation products include 5-hydroxymethylfurfural; the reductone furaneol (2,5-dimethyl-4-hydroxy-3(2H)-furanone), known for its caramel-like aromas; maltol (3-hydroxy-2-methyl-4-pyranone); and isomaltol (1-(3-hydroxy-2-furanyl)-ethanone). Isomaltol results from the degradation of 2,3-dicarbonyls, including 1-deoxyosone (used in the example above), which Figure 7.9 Pyrazine Figure 7.10 Pyrrole 206 EBSCOhost - printed on 8/6/2023 11:54 PM via EDITH COWAN UNIVERSITY (ECU). All use subject to https://www.ebsco.com/terms-of-use Native Files.indd 206 2/08/2017 11:14 AM Sugar, browning and caramelisation Figure 7.11 Oxazole Figure 7.12 2 -Acetyl-2-thiazoline (cooked rice aroma) is easily converted to maltol. Both isomaltol and maltol are furans and are chemically related to furanones (see Figures 7.13 and 7.14). Isomaltol and maltol are commonly associated with Maillard degradation of lactose, although starch and sucrose also produce maltol and isomaltol in some conditions (Kroh, Fiedler & Wagner, 2008; Vistoli et al., 2013). Pyrans are also produced, and these are chemically related to pyranones. Figure 7.13 Isomaltol Figure 7.14 Maltol The Maillard and Strecker degradation reactions produce other highly volatile compounds, including aldehydes, such as glycolaldehyde (described above), glyceraldehyde and aldol; and ketones, such as acetol and levulinic acid (see Figures 7.15–7.18). Figure 7.15 Glyceraldehyde Figure 7.16 Aldol 207 EBSCOhost - printed on 8/6/2023 11:54 PM via EDITH COWAN UNIVERSITY (ECU). All use subject to https://www.ebsco.com/terms-of-use Native Files.indd 207 2/08/2017 11:14 AM UNLOCKING KEY FOOD CHEMISTRY REACTIONS Figure 7.17 Acetol Figure 7.18 Levulinic acid The reactivity of typical sugars involved in the Maillard reaction differs with only 0.0002% of glucose and 0.7% of fructose in the open-chain reactive form, resulting in a seven-fold increase in reactivity for fructose. Fructose must tautomerise to glucose before it can react. This reaction involves the double bonded carbon and oxygen at carbon 2 rearranging to carbon 1 via an enediol (hydroxyl groups attached to both carbon atoms of a carbon–carbon double bond) in an alkaline environment (Vistoli et al., 2013). Stage 3 The final stage of the Maillard reaction is the production of melanoidins, the brown and caramel colours associated with many foods, through dehydration and addition of amine groups to the hydroxymethylfurfural and other breakdown products. The compounds are larger than the aroma and flavour molecules from which they derive and are the least well understood of all the processes involved. Melanoidins in food are of high molecular weight (greater than 12–14 × 103 Da), and the sizes of the compounds grow with increased heat and time in food. It is likely that these high molecular weight melanoidins are derived at the later stages of the Maillard reaction from polymerisation with lower molecular weight melanoidins and other Maillard reaction products (Wang, Qian & Yao, 2011). One pathway sees the dicarbonyl compounds reacting with high molecular weight molecules to create carbohydrate-based melanoidins, and this pathway is identical to the degradation of sugars seen in caramelisation (Kroh, Fiedler & Wagner, 2008). The type of melanoidin produced depends on the type of food, the temperature and the time taken in preparation and cooking (Wang, Qian & Yao, 2011). Melanoidins have both positive and negative biological effects, with some antioxidant and antimicrobial benefits and prebiotic and antihypertensive activities that are current focuses of research. 208 EBSCOhost - printed on 8/6/2023 11:54 PM via EDITH COWAN UNIVERSITY (ECU). All use subject to https://www.ebsco.com/terms-of-use Native Files.indd 208 2/08/2017 11:14 AM LIZZIE O'HALLORAN Chef’s Insight 7.1 Perfecting the steak: Matt Wilkinson The steps towards cooking the perfect steak start long before you light the barbecue or preheat the cast-iron pan or grill. Provenance is incredibly important, as is understanding the breed of cattle, plus the environment in which the animals have been bred, reared and slaughtered. The quality of the pasture is key to grass-fed beef, along with the age and total weight of the carcass at slaughter. Yearling beef (18–22 months) will not have the same depth of flavour or marbling as beef at 24–30 or more months of age. Animal care and ethics are also important to me, along with supporting local farmers, and at Pope Joan we purchase beef farm-direct to have complete knowledge and traceability. There are two ways to age meat: wet and dry. Dry ageing is preferred by chefs, as it results in optimal changes in depth and complexity of flavour plus tenderness of the meat. Technically, meat can be called dry aged if hung for 7 days; however, longer periods, a month or more, are desirable. There are around 18 different beef cuts suitable for steak, each with its own flavour, texture and levels of tenderness. The most well known include the primal cuts, tenderloin, Scotch fillet, sirloin and rib eye on the bone. Rump cuts include the rump, rump centre steak, medallion and tri-tip. Subprimal cuts include the hanger steak (the intercostal cut that hangs from the last rib and attaches to the diaphragm) and the flat iron steak (the bottom part Matt Wilkinson, cookbook author, gourmet food producer and chef-owner, Pope Joan, Melbourne of the oyster blade). Subprimal cuts are often referred to as poor man’s steaks, but they are perfect for marinating or quick pan frying and a juicy lunchtime steak sandwich. Before you start cooking it is important to take the steak out of the fridge and bring it to room temperature. My preference is a cooking method that doesn’t overpower the flavours, such as a gas barbecue (not charcoal coals) and neutral wood for fires (not red gum). There is great debate about methods to cook steak, including constant flipping versus only cooking once each side; however, temperature control is what matters most. A high heat environment is vital at the start, to add rich colour and sear on all sides to help prevent moisture loss. The next stage is to cook the steak to the desired doneness (medium rare for primal 209 EBSCOhost - printed on 8/6/2023 11:54 PM via EDITH COWAN UNIVERSITY (ECU). All use subject to https://www.ebsco.com/terms-of-use Native Files.indd 209 2/08/2017 11:14 AM cuts) on a medium to high heat with the lid down on the barbecue or by transferring the pan to an oven. After searing, a 1.2 kg rib eye on the bone will require around 12–15 minutes at 150–160°C. Resting the cooked steak is the final vital step, and for at least half of the total cooking time. Next comes the seasoning, as adding salt or pepper earlier on leads to burned notes. You also need to respect the flavours that will develop in a beautiful piece of beef and season at the end. Always cut across the grain and serve with paired accompaniments like horseradish, mustard, relish or sauces from homemade tomato to spicy chimichurri. Lately, I am loving a whole hanger steak, 1.4–2.0 kg, Argentinian style, marinated in olive oil, lemon, herbs like parsley, oregano or thyme and pepper, and served with steamed baby potatoes, salads and mustard. Chop and serve it on a board and mop up the juices. However, fond memories are evoked by my British heritage and Nan’s bowler blade steak with horseradish or gravy and Yorkshire pudding. This year at Pope Joan we are having ‘summer camp cookouts’ using half-drums to cook outside with fire and bring people together to mingle and dine alfresco, with dishes including Warialda beef, mustard and salsa verde; cauliflower shawarma salad; mussels with smoked tomato romesco; plus miso and chilli radishes. It’s my twist on the quintessential Aussie barbecue party. Caramelisation Caramelisation is the breakdown of sugars during heat treatment or at a high or low pH. The first stage is the isomerisation of sugars such as glucose, mannose and fructose (known as the Lobry de Bruyn–Alberda van Ekenstein rearrangement) to an intermediary, enediol (see Figure 7.19). From this point the degradation reactions are irreversible. Dehydration, or β-elimination, results in α-dicarbonyl compounds. Further degradation leads to a range of products (as described above), including 5-hydroxymethylfurfural, which depends on the sugar source, pH and temperature. Hydrogen ions are released during the reaction, reducing the pH, which can result in some sourness. Sugar syrup is often made from a heated solution of sucrose and water. Water is brushed down the sides of the pan to prevent recrystallisation of the sugar. Temperature and time determine how quickly a syrup moves through the different stages, but recipes dictate a steady increase to a temperature above 160°C, at which point the sugar melts and then caramelises. Chefs and confident cooks simply melt sugar over heat. Recent advances in the understanding of 210 EBSCOhost - printed on 8/6/2023 11:54 PM via EDITH COWAN UNIVERSITY (ECU). All use subject to https://www.ebsco.com/terms-of-use Native Files.indd 210 2/08/2017 11:14 AM LIZZIE O'HALLORAN Nutrition News 7.1 Maillard reaction and effects on health The Maillard reaction produces a complex mix of compounds which impart charac­ teristic brown colours, aromas and flavours and are being studied for both positive and negative health implications. One example of positive benefits are the melanoidins generated at the late stage of the Maillard reaction and consumed in cooked, fried and roasted foods. Melanoidins appear to have prebiotic properties, and in one study fractions from bread crust appeared to selectively enhance the growth of beneficial Bifidobacteria in the gut. However, in recent years concerns have been raised about another class of Maillard reaction products, the acrylamides. When the amino acid asparagine is heated to a high temperature (over 120°C) by frying, baking or grilling in the presence of reducing sugars, acrylamides can be formed. Potatoes have a high asparagine content, so potato chips and French fries can be major sources of acrylamides in the diet, along with coffee and commercial cereal-based products like baked biscuits (Food Standards Australia and New Zealand, 2014). Studies in animal models have shown that acrylamide exposure poses a risk for several types of cancers; Maillard reactions are seen on the surface of roast meat however, the evidence from human studies is incomplete, and for now acrylamide is described only as a probable human carcinogen. According to FSANZ, new farming and processing techniques are being investigated to produce lower levels of acrylamide in the food supply, to lower the cooking temperature, to use enzymes to minimise formation and to alter raw materials to have lower reducing sugar levels. (FSANZ, 2014; Wang, Qian & Yao, 2011) sucrose caramel found that there is no set temperature at which it transforms from solid to liquid (a state in which it retains its chemical characteristics). It can break down at the same time as liquefying. A slow increase in temperature allows breakdown and melting to occur simultaneously, thus lowering the 211 EBSCOhost - printed on 8/6/2023 11:54 PM via EDITH COWAN UNIVERSITY (ECU). All use subject to https://www.ebsco.com/terms-of-use Native Files.indd 211 2/08/2017 11:14 AM UNLOCKING KEY FOOD CHEMISTRY REACTIONS Figure 7.19 Lobry de Bruyn–Alberda van Ekenstein rearrangement Source: Adapted from Nursten (2005) and Vistoli (2013). temperature at which the melanoidins form (McGee, 2014). This is referred to as apparent melting. The lower temperature allows better control of the progression of the syrup and prevents too many acrid and bitter flavours from developing. The balance of sweet and bitter in true caramel syrup is key to desserts such as tarte tatin and crème caramel: stopping too early results in a sickly sweet flavour, and too late, in a bitter, thin flavour. (See Scientist’s Secret 7.1.) Sugar syrups, crystalline and non-crystalline sweets Sucrose dissolves easily in water to form a molecular solution, but only a certain amount of sugar will dissolve in a measurement of water. As the temperature rises, the ability of water to dissolve sucrose increases. Heating the solution allows more solute to dissolve in the solvent. A supersaturated solution has more solute dissolved than its predicated solubility. At room temperature (20°C) 100 mL of water will normally dissolve 203.9 g of sucrose (67.1% solubility). Adding 250 g sucrose to 100 mL of water will mean that 203.9 g will dissolve and the remaining 46.1 g will remain undissolved. Heating the solution to 60°C will allow the remaining sucrose to dissolve. At the new temperature, the solubility limit in 100 mL water is 287.3 g (74.2% solubility) so the solution 212 EBSCOhost - printed on 8/6/2023 11:54 PM via EDITH COWAN UNIVERSITY (ECU). All use subject to https://www.ebsco.com/terms-of-use Native Files.indd 212 2/08/2017 11:14 AM ISTOCK Scientist’s Secret 7.1 Crème caramel, part 2: the caramel Making caramel seems a simple enough process: just heat ordinary table sugar in a pan. The sugar melts, then starts to turn brown and develop a deliciously rich aroma. In reality there is highly complex chemistry happening that is still not fully understood. All the recipes state that some water is added to the sugar before heating. However, this really isn’t necessary, as the water is boiled off before the caramel starts to form. Some recipes suggest brown sugar, but this is not necessary either; it actually can make it harder to tell when the caramel is ready. Heat a cupful of ordinary white sugar in a thick-bottomed saucepan. The sugar will eventually start to melt, initially to a clear, colourless liquid. This quickly starts to turn brown, and after further heating the molten sugar starts to bubble, as water is released as steam. There will soon be a rich, golden-brown, gooey foam with a strong caramel aroma. This is a good time to stop, as further heating makes the caramel bitter. The sucrose first decomposes into fructose and glucose. A series of complex condensation reactions then follows as the individual sugar molecules lose water and react with each other. The major caramelisation products are caramelan (C24H36O18), caramelen (C36H50O25) and Caramel syrup from heating sugar over 160°C eventually the polymer caramelin (C125H188O80). There are also minor reaction products such as diacetyl (2,3-butandione), which are responsible for the buttery or butterscotch flavour, esters and lactones, which add a sweet rum-like flavour, furans, which have a nutty flavour, and maltitol, which has a toasty flavour. A final tip when making crème caramel: it’s best made the day before you want to serve it and kept in the fridge. This allows plenty of time for the caramel flavours to diffuse into the custard. (McGee, 2014; ScienceGeist, 2011) 213 EBSCOhost - printed on 8/6/2023 11:54 PM via EDITH COWAN UNIVERSITY (ECU). All use subject to https://www.ebsco.com/terms-of-use Native Files.indd 213 2/08/2017 11:14 AM UNLOCKING KEY FOOD CHEMISTRY REACTIONS is now unsaturated. At 100°C the solubility limit is 487.2 g sucrose (82.8% solubility) in 100 mL water (Lowe, 1937). As the solution cools, it usually takes more time for the sucrose to reform into its solid form than it does for the temperature to decrease, so for a period, the solution is supersaturated. Rock candy and fudge are made from supersaturated solutions. In the case of rock candy, a string is usually suspended in the solution and left to cool slowly. As the sucrose molecules move, slowly trying to regain the original order to allow crystal formation, some adhere to the surface of the string. Other sucrose molecules collide with the solid forming on the string and over time, results in large, well-formed sugar crystals known as rock candy. Other sweets and candies are non-crystalline, and avoiding crystallisation during production is critical. Interfering with the formation of crystals can be achieved through the addition of glucose or fructose, and recipes include glucose or invert syrup. Fat also interferes with crystal formation, and butter is a typical ingredient in caramels. Table 7.2 lists sugar syrup temperatures, sugar concentrations, uses and characteristics. Table 7.2 Sugar syrups, with temperatures, uses and characteristics Temp. (°C) SC (%) 100a 101–112 80 Use in sweet making Stage Water, simple sugar syrup Sugar syrup, fruit liqueur, Thread some icing 104–105 Jelly, candy, fruit liqueur, some icing Pearl 110–113 Delicate sugar candy, syrup Blow, soufflé 112–115 85 Fudge, fondant, pralines, Soft ball pâté à bombe (Italian meringue), peppermint creams, classic buttercream Cold water syrup consistency test At this relatively low temperature, there is still a lot of water left in the syrup. The liquid sugar can be pulled into brittle threads between the fingers. Or if a small amount is taken onto a spoon and dropped from about 5 cm above the pan, it spins a long thread, like a spider web. The thread formed by pulling the liquid sugar can be stretched. When a cool metal spoon is dipped into the syrup and then raised, the syrup runs off in drops, which merge to form a sheet. Boiling sugar creates small bubbles resembling snowflakes. The syrup forms a 5 cm thread when dropped from a spoon. A small amount of syrup dropped into chilled water forms a soft, flexible ball but flattens like a pancake after a few moments in the hand. 214 EBSCOhost - printed on 8/6/2023 11:54 PM via EDITH COWAN UNIVERSITY (ECU). All use subject to https://www.ebsco.com/terms-of-use Native Files.indd 214 2/08/2017 11:14 AM Sugar, browning and caramelisation 116–120 87 Caramel Firm ball 121–131 92 Nougat, marshmallow, taffy, gummies, rock candy Hard ball 132–143 95 Butterscotch, toffee apple, Soft crack toffee 148–154 99 Brittle, hard candy (lollipop, toffee) Hard crack 160b 165–182 100 100 Thermal decomposition Glaze, coating agent Caramelising Clear to light brown 171 100 Light caramel for syrup, colour, flavour, dessert decoration, nut coating 179–182 Spun sugar, sugar cage 190–193 210 Colouring agent for sauces None The sugar forms a firm ball that does not flatten when removed from water but remains malleable and flattens when squeezed. The syrup forms thick, ropy threads as it drips from a spoon. The sugar concentration is rather high now, which means there is less moisture in the syrup. Syrup dropped into iced water forms a hard ball, which holds its shape on removal. The ball is hard, but its shape can still be changed if it is squashed. The bubbles on top of the syrup become smaller, thicker and closer together. The moisture content is low. Syrup dropped into iced water separates into hard but pliable threads that bend slightly before breaking. This is the highest temperature likely to be specified in a recipe. There is almost no water left in the syrup. Syrup dropped into iced water separates into hard, brittle threads that break when bent. The syrup no longer boils but begins to break down and caramelise. When some is dropped into cold water from this stage onwards it does not yield any solid form of syrup. Light The syrup turns brown due to brown caramelisation; the sugar is beginning to break down and form many complex compounds that contribute to a richer flavour. Medium The syrup darkens, with a characteristic brown bitter aroma. Dark brown The syrup darkens further. Black Jack The syrup turns black and then decomposes. Note: The temperatures given are for the boiling point at sea level (for each 270 m of elevation, subtract 1°C). The temperatures are for standard sugar syrup and do not consider apparent melting temperature. Sugar syrup should always be measured with a sugar thermometer. During the early stages the syrup can be handled (with care) to check the consistency. SC: sugar concentration. a Water boils at 100°C. b Sugar begins to melt at around 160°C and to caramelise at around 171°C. Source: Adapted with permission from Phillips (2000). 215 EBSCOhost - printed on 8/6/2023 11:54 PM via EDITH COWAN UNIVERSITY (ECU). All use subject to https://www.ebsco.com/terms-of-use Native Files.indd 215 2/08/2017 11:14 AM UNLOCKING KEY FOOD CHEMISTRY REACTIONS Dextrinisation In the kitchen, dextrinisation is the breakdown of starch to dextrins due to exposure to dry heat. This contrasts with the browning in caramelisation and the Maillard reaction, which both involve sugars. A common example of dextrinisation is the browning of toast. The starch in some baked goods undergoes some dextrinisation, as do starches that are toasted to make sauces, gravy and toasted breakfast cereal. In these examples, where heat is responsible for the production of dextrins, the dextrins are referred to as pyrodextrins. Dextrins are also produced following breakdown of the starch molecule through exposure to alkalis, acids and enzymes. In food manufacturing, dextrinisation of starch has been investigated in relation to increasing resistant starch content in products high in starch (such as maize). SUMMARY Browning of food occurs through enzymatic and non-enzymatic pathways. Non-enzymatic pathways include the Maillard reaction and caramelisation, two complex processes that are not completely understood. A reducing sugar is required for caramelisation, and a reducing sugar and an amino acid for the Maillard reaction to take place. Both reactions yield a range of chemical compounds that react with other breakdown compounds during the cooking of food to produce the colours, aromas and flavours of foods. References Australian Sugar Heritage Centre (2010). The History of the Sugar Industry. www.sugarmuseum.com.au/the-history-of-the-sugar-industry Belitz, H.D., Grosch, W. & Schieberle, P. (2009). Food Chemistry (4th rev. & extended edn). Berlin: Springer Canegrowers (2010). How Sugarcane Is Grown: Paddock to plate. www.canegrowers.com. au/page/archived-pages/About_Australian_Sugarcane Chakraborty, S., Kaushik, N., Rao, P.S. & Mishra, H.N. (2014). High-Pressure Inactivation of Enzymes: A review on its recent applications on fruit purees and juices. Comprehensive Reviews in Food Science and Food Safety, 13, 578–96. doi: 10.1111/1541-4337.12071 Dauthy, M.E. (1995). Fruit and Vegetable Processing. Agricultural Services Bulletin 119. Food and Agriculture Organization of the United Nations. www.fao.org/docrep/ V5030E/V5030E00.htm 216 EBSCOhost - printed on 8/6/2023 11:54 PM via EDITH COWAN UNIVERSITY (ECU). All use subject to https://www.ebsco.com/terms-of-use Native Files.indd 216 2/08/2017 11:14 AM Sugar, browning and caramelisation Food Standards Australia New Zealand see FSANZ FSANZ (Food Standard Australia New Zealand) (2010). NUTTAB 2010 Online Searchable Database. www.foodstandards.gov.au/science/monitoringnutrients/ nutrientables/Pages/default.aspx FSANZ (Food Standards Australia New Zealand) (2014). Acrylamide and Food. www.foodstandards.gov.au/consumer/chemicals/acrylamide/Pages/default.aspx Kroh, L.W., Fiedler, T. & Wagner, J. (2008). α-Dicarbonyl Compounds: Key intermediates for the formation of carbohydrate-based melanoidins. Annals of the New York Academy of Sciences, 1126, 210–15. doi: 10.1196/annals.1433.058 Lowe, B. (1937). Experimental Cookery: From the chemical and physical standpoint (2nd edn). New York: John Wiley & Sons McGee, H. (2014). Food and Cooking: Caramelisation; New science, new possibilities. Chemistry in Australia, July, 32–3 Monash University (2016). Low FODMAP Diet for Irritable Bowel Syndrome. www.med.monash.edu/cecs/gastro/fodmap Nursten, H. (2005). The Maillard Reaction: Chemistry, biochemistry and implications. Cambridge, UK: Royal Society of Chemistry Phillips, S. (2000). Candy: Sugar Syrup Temperature Chart. Crafty Baking. www.craftybaking.com/howto/candy-sugar-syrup-temperature-chart ScienceGeist (2011). The Chemistry of Caramel. www.sciencegeist.net/the-chemistryof-caramel Smuda, M. & Glomb, M.A. (2013). Fragmentation Pathways during Maillard-Induced Carbohydrate Degradation. Journal of Agricultural and Food Chemistry, 61(43), 10198–208. doi: 10.1021/jf305117s Sugar Australia (2014). Our History. www.csrsugar.com.au/csr-sugar/our-history University of Sydney (2016). Glycaemic Index. www.glycemicindex.com/index.php Vistoli, G., De Maddis, D., Cipak, A., Zarkovic, N., Carini, M. & Aldini, G. (2013). Advanced Glycoxidation and Lipoxidation End Products (AGEs and ALEs): An overview of their mechanisms of formation. Free Radical Research, 47(supp. 1), 3–27. doi: 10.3109/10715762.2013.815348 Wang, H.-Y., Qian, H. & Yao, W.-R. (2011). Melanoidins Produced by the Maillard Reaction: Structure and biological activity. Food Chemistry, 128(3), 573–84. doi: 10.1016/j.foodchem.2011.03.075 217 EBSCOhost - printed on 8/6/2023 11:54 PM via EDITH COWAN UNIVERSITY (ECU). All use subject to https://www.ebsco.com/terms-of-use Native Files.indd 217 2/08/2017 11:14 AM