Analytical Chemistry of Foods

Analytical Chemistry of Foods

Analytical Chemistry of Foods c. s. JAMES Seale-Hayne Faculty of Agriculture, Food and Land Use Department of Agriculture and Food Studies University of Plymouth SPRINGER-SCIENCE+BUSINESS MEDIA, B.v.

First edition 1995 1995 Springer Science+Business Media Dordrecht Origina\ly published by Chapman & HalI in 1995 Typeset in 10112pt Times by Iulia Stevenson of Hove ISBN 978-1-4613-5905-0 ISBN 978-1-4615-2165-5 (ebook) DOI 10.1007/978-1-4615-2165-5 Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the UK Copyright Designs and Patents Act, 1988, this publication may not be reproduced, stored; or transmitted, in any form or by any means, without the prior permission in writing ofthe publishers, or in the case of reprographic reproduction only in accordance with the terms of the licences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of licences issued by the appropriate Reproduction Rights Organization outside the UK. Enquiries conceming reproduction outside the terms stated here should be sent to the publishers at the Glasgow address printed on this page. The publisher makes no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility or Iiability for any errors or omissions that may be made. A catalogue record for this book is available from the British Library Printed on acid-free text paper, manufactured in accordance with ANSIINISO Z39,48-1992 (permanence ofpaper)

Preface Food laws were first introduced in 1860 when an Act for Preventing the Adulteration of Articles of Food or Drink was passed in the UK. This was followed by the Sale of Food Act in 1875, also in the UK, and later, in the USA, by the Food and Drugs Act of 1906. These early laws were basically designed to protect consumers against unscrupulous adulteration of foods and to safeguard consumers against the use of chemical preservatives potentially harmful to health. Subsequent laws, introduced over the course of the ensuing century by various countries and organisations, have encompassed the features of the early laws but have been far wider reaching to include legislation relating to, for example, specific food products, specific ingredients and specific uses. Conforming to the requirements set out in many of these laws and guidelines requires the chemical and physical analysis of foods. This may involve qualitative analysis in the detection of illegal food components such as certain colourings or, more commonly, the quantitative estimation of both major and minor food constituents. This quantitative analysis of foods plays an important role not only in obtaining the required information for the purposes of nutritional labelling but also in ensuring that foods conform to desired flavour and texture quality attributes. This book outlines the range oftechniques available to the food analyst and the theories underlying the more commonly used analytical methods in food studies. Details of specific procedures for undertaking the routine analysis of the major food constituents are provided and, where appropriate, reference is made to official methods. The latter should be referred to in the case of disputes and legislative requirements in order that full details regarding apparatus design, product specifications and technical procedures may be obtained. C.SJ.

Contents Abbreviations xi Part 1 Theory 1 1 2 3 Introduction Assessment of analytical methods and data 2.1 Requirements and choice of analytical methods 2.2 Presentation of data 2.3 Quality of data 2.3.1 Procedures to improve quality of data 2.4 Statistical assessment of quality of data 2.4.1 Precision 2.4.2 Reproducibility 2.4.3 Accuracy Principles of techniques used in food analysis 3.1 Classical methods 3.1.1 Titrimetric analysis 3.1.2 Gravimetric procedures 3.1.3 Solvent extraction methods 3.1.4 Refractometry 3.1.5 Polarimetry 3.2 Instrumental and modem methods 3.2.1 Spectroscopic methods 3.2.2 Chromatography 3.2.3 Electrophoresis 3.2.4 Immunochemical methods 3 5 5 6 7 7 8 9 10 11 13 13 13 16 16 16 17 18 18 26 30 31 4 Theory of analytical methods for specific food constituents 37 4.1 Sampling 37 4.2 Moisture and water activity 37 4.2.1 Methods of measuring moisture 38 4.3 Protein 40 4.3.1 Kjeldahl method 41 4.3.2 Direct distillation methods 43 4.3.3 Thermal combustion methods 43 4.3.4 Dye binding methods 44 4.3.5 Formol titration 45 4.3.6 Spectroscopic methods 45 4.4 Fats 46 4.4.1 Gravimetric solvent extraction procedures 46 4.4.2 Volumetric methods 50 4.4.3 Instrumental methods 51 4.4.4 Study of the nature offats and oils 51 4.5 Carbohydrates 53 4.5.1 Determination of available carbohydrates 56 4.5.2 Estimation of dietary fibre in foods 59

VIII ANALYTICAL CHEMISTRY OF FOODS 4.6 Micronutrients 4.6.1 Mineral elements and ash 4.6.2 Vitamins 4.7 Food energy 4.8 Additives 62 62 63 64 66 Part 2 Experimental 69 5 Experimental procedures - estimation of major food constituents 71 5.1 Laboratory safety 71 5.2 Sampling 72 5.3 Determination of moisture and total solids 73 5.4 Determination of ash content 75 5.5 Determination of mineral elements in foods by atomic absorption spectrophotometry (ashing process) 76 5.6 Determination of mineral elements in canned food products by atomic absorption spectrophotometry (non-ashing process) 78 5.7 Determination of calcium in foods by permanganate titration 80 5.8 Determination of phosphorus by the vanadate colorimetric method 82 5.9 Determination of phosphorus by the molybdenum blue colorimetric method 84 5.10 Determination of iron by the bipyridyl colorimetric method 86 5.11 Determination of nitrogen and protein by the Kjeldahl method using the Kjeltec instrument 88 5.12 Determination of protein content by the formol titration 90 5.13 Determination of fat by the Soxhlet and Soxtec methods 91 5.14 Determination of the fat content of dairy products by the Gerber method 93 5.15 Determination of fat by the Mojonnier method 96 5.16 Determination off at by the Rose-Gottlieb method 98 5.17 Determination of fat by the Werner-Schmid method 100 5.18 Determination of the fat content of cheese by the modified SBR (Schmid-Bondzynski-Ratzlaft) method 102 5.19 Determination of fat by the Weibull-BerntroplWeibull-Stoldt method 104 5.20 Determination of dietary fibre in foods by the neutral detergent fibre method 106 5.21 Determination of dietary fibre by the Englyst enzymatic instrumental method 108 5.22 Determination of dietary fibre in foods by the AOAC enzymatic gravimetric method 114 5.23 Volumetric determination of sugars by copper reduction (Lane and Eynon method) 117 5.24 Volumetric determination of sugars by copper reduction (Lane and Eynon method - constant volume modification) 120 5.25 DNS colorimetric determination of available carbohydrates in foods 124 5.26 Determination oflactose in milk by the Chloramine-T method 126 5.27 Determination of the lactose content of milk by polarimetry 128 5.28 Determination oflactose in cheese by the phenol colorimetric method 130 5.29 Identification and determination of sugars in milk products by HPLC 132 5.30 Calculation of the calorific value of foods 135 6 General food studies 137 6.1 Determination of ascorbic acid by titration 137 6.2 Gas chromatographic study of the fatty acid composition offats 140 6.3 Determination of the iodine value of fats and oils 142 6.4 Determination of the saponification value offats 144 6.5 Determination of sulphur dioxide by iodine titration 146 6.6 Determination of total sulphur dioxide (free and combined) using distillation methods 148 6.7 Determination of the salt content of dairy products (Volhard method) 150 6.8 Determination of the salt content of brine (Mohr titration) 152

CONTENTS IX 6.9 Titrimetric determination ofthe chloride content of meat products 153 6.10 Colorimetric determination of nitrates and nitrites in meat products and brine 155 6.11 Spectrophotometric determination of antioxidants 159 6.12 Extraction and colorimetric estimation of gallates 161 6.13 Determination of alcohol in beverages by gas chromatography 162 6.14 Determination of alcohol by the distillation method 163 6.15 Titratable acidity of fruit juices 165 6.16 Determination of the acetic acid content of vinegar 167 6.17 Acidity measurements in dairy products 168 6.18 Determination ofl-lactic acid in cheese by an enzymatic method 170 7 Additional reading material Index 173 175

Abbreviations AOAC BS BSI FAO FDA GC/GLC HPLC ISO NIR NMR ppm SI TLC Association of Official Analytical Chemists British Standard British Standards Institution Food and Agriculture Organisation Food and Drug Administration (USA) Gas chromatography/gas-liquid chromatography High performance liquid chromatography International Organisation for Standardisation Near infrared Nuclear magnetic resonance spectroscopy parts per million Statutory Instrument, and also Systeme International d'unites Thin-layer chromatography