Chapter 1 Carbohydrates PDF

Chapter 1: Carbohydrates |1 carbohydrates Carbohydr...

Chapter 1: Carbohydrates |1 carbohydrates Carbohydrates in Food Carbohydrates are one of main components in food. They are for example the major elements in cell Chapter 1 walls (cellulose, pectin) , they are the storage energy to the plant Carbohydrates (starch granules) and they are the small sugars that make fruits so sweet. In processed foods carbohydrates are for example added as thickeners (starch), gelling agents (xanthan), or sweetener (sugar). From a nutrition point of view they provide energy (sugars, starch, lactose), but also serve as dietary fibres (GOS, cellulose). During processing several reactions can take place on carbohydrates, such as Maillard reactions, which cause brown colouring of bread, cookies and meat, and starch gelatinization which thickens sauces and makes the starch digestible. Copyright © 2024 Laboratory of Food Chemistry, Wageningen University. All Rights Reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. 2 1.1 Introduction and learning outcomes Table of contents 1.1 Introduction and learning outcomes................................................................................................................. 4 1.2 Classification of carbohydrates.................................................................................................................... 7 1.2.1 Monosaccharides................................................................................................................................. 7 Classification of monosaccharides................................................................................................................ 7 Reducing sugar and mutarotation.................................................................................................................. 9 Examples of monosaccharides...................................................................................................................... 9 Uronic acids................................................................................................................................................ 12 1.2.2 Disaccharides..................................................................................................................................... 13 Saccharose (sucrose)................................................................................................................................... 13 Lactose........................................................................................................................................................ 14 Maltose........................................................................................................................................................ 14 1.2.3 Oligosaccharides................................................................................................................................ 15 Raffinose family oligosaccharides.............................................................................................................. 15 Lactose-oligosaccharides (GOS)................................................................................................................. 16 Malto-oligosaccharides............................................................................................................................... 17 Fructo-oligosaccharides.............................................................................................................................. 18 1.2.4 Glycosides......................................................................................................................................... 19 N-glycosides............................................................................................................................................... 19 O-glycosides............................................................................................................................................... 19 1.2.5 Polysaccharides................................................................................................................................. 21 Classification of polysaccharides................................................................................................................ 21 Properties of polysaccharides...................................................................................................................... 24 1.3 Properties of carbohydrates in foods.......................................................................................................... 25 1.3.1 Solubility and crystallization............................................................................................................. 25 Temperature and solubility......................................................................................................................... 26 1.3.2 Water binding.................................................................................................................................... 26 Water activity and processes in food........................................................................................................... 27 Humectants................................................................................................................................................. 27 1.3.3 Sweetness.......................................................................................................................................... 27 Temperature and sweetness......................................................................................................................... 28 1.3.4 Digestibility....................................................................................................................................... 29 NDOs.......................................................................................................................................................... 29 Dietary fibre................................................................................................................................................ 29 1.3.5 Viscosity and gelling......................................................................................................................... 29 Viscosity..................................................................................................................................................... 30 Gelling......................................................................................................................................................... 30 1.4 Chemical reactions of carbohydrates in foods............................................................................................ 32 1.4.1 Changes during storage..................................................................................................................... 32 1.4.2 Changes during processing................................................................................................................ 32 1.4.2.1 Hydrolysis of disaccharides, oligosaccharides and polysaccharides........................................ 33 1.4.2.2 Enolization of reducing sugars................................................................................................. 33 1.4.2.3 Example of a follow-up reactions after enolization: Formation of HMF................................. 34 1.4.2.4 Caramelization reactions (non-enzymatic browning).............................................................. 34 1.4.2.5 Maillard reactions (non-enzymatic browning)......................................................................... 35 1.5 Some important polysaccharides................................................................................................................ 43 1.5.1 Pectin................................................................................................................................................. 43  Chapter 1: Carbohydrates |3 Origin of pectin........................................................................................................................................... 43 carbohydrates Structure of pectin....................................................................................................................................... 43 Gelling properties of pectin......................................................................................................................... 44 1.5.2 Starch................................................................................................................................................. 45 Origin of starch........................................................................................................................................... 45 Structure of starch: Amylose and amylopectin........................................................................................... 45 Physical characteristics of starch................................................................................................................. 47 Starch gelatinization.................................................................................................................................... 48 Starch modification..................................................................................................................................... 50 1.6 Analysis of carbohydrates............................................................................................................................... 53 Literature.............................................................................................................................................................. 54 4 1.1 Introduction and learning outcomes 1.1 Introduction and learning outcomes Besides fats and proteins, carbohydrates belong to one of the three macronutrients of foods. They play a role as supplier of calories, as sweetener or as part of biochemical important molecules (i.e. nucleic acids, glycoproteins, etc.). In food technology and food chemistry carbohydrates are important compounds due to a series of reactions they can be involved in, which influence the main properties (taste, texture and colour) of a food. Even small quantities of carbohydrates in a food can cause chemical reactions that considerably change its sensory properties and nutritional value (i.e. Maillard reactions). Learning outcomes for the chapter carbohydrates §1.2.1  Recognize and explain the structural differences between aldose/ketose, hexose/pentose, pyranose/furanose, α/β.  Recognize the difference between D/L-enantiomers.  Recognize the anomeric carbon in a saccharide.  Explain what a reducing sugar is.  Recognize the structure of a reducing end.  Explain the importance of reducing ends.  Explain what mutarotation is.  Recognize the difference between the structures of the different monosaccharides.  Recognize the structures of sugar alcohols.  Recognize and explain the general structure of uronic acids.  Explain the effect of pH on the charge of uronic acids. §1.2.2  Recognize and be able to draw glycosidic linkages.  Explain the systematic name of oligosaccharides.  Recall the monomeric units of saccharose, lactose, maltose.  Explain what an invert sugar is. §1.2.3  Define the words oligo-, di-, tri-, tetra- and pentasaccharide.  Recall the monomeric composition of the raffinose family oligosaccharides/α-galacto- oligosaccharides, the lactose-oligosaccharides (GOS, transgalactosyl-oligo), the malto- oligosaccharides, and the fructo-oligosaccharides (FOS).  Explain what starch syrups are.  Describe what Dextrose Equivalent (DE) is and be able to make calculations with it.  Explain what happens with the DE during hydrolysis. §1.2.4  Recognize N-glycosides (glycosylamines) and explain how they are formed.  Recognize O-glycosides and explain how they are formed. §1.2.5  Explain what degree of polymerization (DP) is.  Classify polysaccharides in homo/heteroglycans and linear/branched.  Classify polysaccharides in storage polysaccharides and plant cell wall polysaccharides.  Recall the factors that influence the properties of polysaccharides.  Chapter 1: Carbohydrates |5  Recall the differences in structure between amylose and cellulose, and explain the differences in carbohydrates properties. §1.3.1  Recall when crystallization takes place.  Recall that solubility depends on the kind of sugar and temperature. §1.3.2  Explain the effect of sugars on the water activity.  Explain the role of humectants in food. §1.3.3  Classify the relative sweetness of sugars, sugar alcohols, and intense sweeteners.  Recall the effect of temperature on the relative sweetness of sugars. §1.3.4  Recall what NDOs are and what their prebiotic function is.  Recall what dietary fibres are. §1.3.5  Explain which structural properties of polysaccharides play a role in viscosity.  Explain how a polysaccharide can form a gel. §1.4.1  Explain what happens to the texture of vegetable products during ripening. §1.4.2  Explain the hydrolysis reaction and under which conditions it takes place.  Explain under which conditions enolization of sugars takes place and what products are formed.  Explain the follow-up reactions after enolization to the formation of HMF.  Recognize the structures of intermediate reaction products: α-dicarbonyl and HMF.  Explain what caramelization is (substrates, reactions, reaction products, conditions).  Recall the basic reacting compounds and conditions for a Maillard reaction.  Explain the results (i.e. effects) of the Maillard reactions.  Recognize and be able to draw the reactions in the initial phase of the Maillard reaction.  Explain and recognize the reactions in the intermediary phase (enolization, deoxysones, reductones).  Recognize and be able to draw the Strecker degradation reaction.  Explain the formation of acrylamide.  Explain what melanoidins are.  Explain how pH, temperature, water activity, sugar properties and sulphite influence the Maillard reaction. §1.5.1  Recognize the difference in structure between high methylated pectin and low methylated pectin.  Explain what DM (Degree of Methyl-esterification) is.  Explain how high methylated pectin and low methylated pectin form gels. §1.5.2 6 1.1 Introduction and learning outcomes  Explain how iodine gives a blue colour with starch.  Explain why and when a hilum is observed in a starch granule by a polarization microscope.  Recall and recognize the monomeric units and the glycosidic linkages in amylose and amylopectin.  Explain the process of starch gelatinization.  Explain what retrogradation is and the difference between amylose and amylopectin in this process.  Explain how gelatinization behaviour can be measured (Brabender and microscope).  Explain how starch physically and chemically can be modified and know what the new properties of these modified starches are. §1.6  Explain the reducing sugar assay and the total sugar assay.  Chapter 1: Carbohydrates |7 1.2 Classification of carbohydrates carbohydrates Carbohydrates can be classified into monosaccharides, oligosaccharides, polysaccharides, glycosides and sugar derivatives. In the following paragraphs these different classes of carbohydrates are described in more detail. 1.2.1 MONOSACCHARIDES Food product example: Sugars in fruit The main monosaccharides in fruits are glucose and fructose, of which the ratios can vary greatly. For apple the glucose and fructose content is respectively 1.8% and 5.7%. For grapes the content is respectively 7.2% and 7.4%. Other monosaccharides occur only in trace amounts. Also honey mainly consists of the monosaccharides glucose (38%) and fructose (31%). The other components are mainly water (17%) and some saccharose (2.4%) and maltose (7.3%). Classification of monosaccharides Aldose and ketose O O Monosaccharides are polyhydroxy-aldehydes (R C H ) or -ketones (R C R). These compounds are known as aldoses (the aldehydes) and ketoses (the ketones). The following aldoses occur frequently: D-glucose, D- galactose and D-xylose. D-fructose is the prevailing ketose (Figure 1). Figure 1. Projection formulas of the aldoses D-glucose, D-galactose, D-xylose and the ketose D-fructose. Hexose and pentose Monosaccharides are further classified according to the number of carbon atoms. For instance, a monosaccharide with six carbon atoms is a hexose. If this compound also carries an aldehyde function (such as for instance D-glucose and D-galactose), it is called an aldohexose. In the same way, aldotriose, aldotetraose and aldopentose refer to aldoses with respectively three, four and five carbon atoms. So D-xylose is an aldopentose. An identical nomenclature, analogous to that of aldoses, also applies to ketoses. So D-fructose is a ketohexose. D- and L-enantiomer Each D-sugar has an L-enantiomer, which is the mirror image of the whole D-molecule (Figure 2). By definition, in a D-enantiomer the hydroxyl group on the highest numbered chiral carbon atom (for hexoses this is C5) is 8 Classification of carbohydrates positioned on the right side in the Fisher projection, while in an L-enantiomer this group is on the left. Note that the highest numbered chiral carbon atom is always the next-to-last carbon. Most monosaccharides mainly exist in the D-form, for example D-glucose, D-galactose and D-fructose. An example of a monosaccharide that mainly consist in the L form is L-arabinose. Figure 2. Fisher projection and ring structures of D-glucose and L-glucose. The hydroxyl group on the highest numbered chiral carbon atom is marked. Pyranose and furanose In a sugar the hydroxyl group (-OH) and the carbonyl group (=O) are present in the same molecule. As a result of an intramolecular reaction between a hydroxyl and the carbonyl group, a cyclic hemiacetal can be formed. A cyclic hemiacetal is a ring containing an oxygen atom and C-atom next to that atom contains an OH-group. The structure on the right shows an example of a cyclic hemiacetal. The hemiacetal is indicated with the dotted circle. In this example the OH on C5 reacted with C1. Monosaccharides mostly occur in the ring (cyclic hemiacetal) form (>99%), rather than the open form (120˚C) foods such as French fries, potato crisps, bread, breakfast cereals and coffee. Acrylamide is neurotoxic in animals and humans, and is probably also carcinogenic to humans, but the real impact of acrylamide on human health is still unclear. In 2002 the potential carcinogenicity of acrylamide and the relatively high amounts (up to mg/kg) that could be formed in processed foods, led to great concern. Since then, much research is performed towards controlling and minimizing acrylamide formation.  C h a p t e r 1 : C a r b o h y d r a t e s | 41 carbohydrates Figure 46: Asparagine (left) and acrylamide (right). 6. Reactions in the final phase (reaction G) In the final phase so called melanoidins are formed (rreaction G, Figure 37). Various low-molecular weight intermediary products that are formed via the different reactions start to condense and polymerize and consequently they form high molecular weight brown-coloured substances: melanoidins. Amino groups are involved in the reaction, which includes that in this stage a strong decrease of available (sometimes essential) amino acids can occur. Due to the fact that the intermediary products may differ in composition (e.g. sugar-like substances, unsaturated carbonyl products and cyclic compounds), and can react in different ways, the composition of the final products is very divers. As yet it is only partly known how the melanoidins are formed and how their structures are exactly. All kinds of saturated, unsaturated and aromatic nitrogen-containing compounds have been demonstrated in the melanoidins. Inhibition of the Maillard reaction Sometimes Maillard reactions give desired, but often also undesired colour and aroma changes in foods. Moreover, they can affect the nutritional value of a product. Lysine-residues in proteins often are involved in this reaction and this results in a decrease of the nutritional value of these proteins. So in a number of cases it is advisable to prevent Maillard reactions. Factors that influence Maillard reactions: 1. pH. The effect of pH on Maillard reactions is complex. In the pH-range of 3-7, the Maillard reactions do not proceed very fast. Below pH 3 Maillard reactions are slow, but still more brown-colouring reactions take place then at pH 3-7. This is because there are more reactants present. In acid conditions (pH