UL TRA·HIGH· TEMPERATURE PROCESSING OF MILK AND MILK PRODUCTS ULTRA-HIGH-TEMPERATURE PROCESSING OF MILK ANO MILK PROOUCTS H. BURTON Formerly Head, Process Technology Department, National Institute for Research in Dairying, Shinfield, Reading, UK SPRINGER SCIENCE+BUSINESS MEDIA, LLC First edition 1994 © 1994 Springer Science+Business Media New York Origina11y published by Blackie Academic & Professional in 1994 Typeset in 10/12pt Times by The Universities Press (Belfast) Ud. ISBN 978-1-4613-5901-2 ISBN 978-1-4615-2157-0 (eBook) DOI 10.1007/978-1-4615-2157-0 Apart from any fair deaIing for the purposes of research or private study, or criticism or review, as permitted under the UK Copyright Designs and Patents Act, 1988, this pubIication may not be reproduced, stored, or transmitted, in any form or by any means, without the prior permission in writing of the pubIishers, 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 Iicences issued by the appropriate Reproduction Rights Organization outside the UK. Enquiries conceming reproduct.ion outside the terms stated here should be sent 10 the pubIishers at the G1asgow 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 liability for any errors or omissions that may be made. A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data available El Printed on acid-free text paper, manufactured in accordance with ANSIJNISO Z39.48-1992 (Permanence of Paper). Dedication This book has its origins in work on UHT milk at the National Institute for Research in Dairying, Shinfield, England. The Engineer ing Department, of which I was a member, was central to most of that work. The NIRD, as it was known world-wide, ceased to exist in March 1985. The process engineering group continued its work within another Institute until March 1987 when to the surprise of many in Britain and overseas it was disbanded and the staff made redundant. It seems unlikely that any work on UHT processing will ever be done at Shinfield again. I therefore dedicate this book to all those who worked on UHT processing and aseptic filling at the NIRD between 1948 and 1985, and particularly to my former colleagues in the process engineering group. H.B. Preface This book attempts to explain the scientific basis for UHT sterilization and aseptic filling, as well as describe the processes and equipment used. I have tried to avoid producing merely a catalogue of sterilizers and aseptic fillers. Instead I have attempted to explain the principles on which the different types of plant operate, and discuss the factors which influence performance, so that information given by manufac turers may be assessed by readers in relation to their own processing requirements. Statements are generally supported by references. Where no re ference is given, personal experience or my interpretation of the work of others is my justification. Although the book deals mainly with milk and milk products, I hope that the information it contains will be useful to those dealing with other products, since the principles of processing are in general the same. The book is based on more than 30 years' involvement with research into UHT processing and aseptic filling. During this time I have been fortunate to work with and to talk to many people from whom I have learned a great deal. I benefited from conta.cts with Dr T. R. Ashton (England) and Professor H. Hostettler. (Switzerland), who were pioneers in the commercial development of UHT milk. More recently I have been privileged to know and work with research workers in many countries having a common interest in UHT processing. Of these, I should mention particularly Professors E. L. Thomas, V. A. Jones, H. E. Swaisgood and K. R. Swartzel of the USA; Professor B. Hallstrom, Dr Irene von Bockelmann and Dr Bernard von Bockel mann of Sweden; Professor Dr H. Reuter and Professor Dr H. G. Kessler of Germany; Dr O. Cerf of France; and Dr J. Mottar of viii Preface Belgium. All of these have contributed in some way to this book, although they may not realize it. The photograph of the Nielsen sterilizer reproduced as Fig. 1.5 was kindly given to me many years ago by Professor P. Solberg of Norway. The work of many of my immediate colleagues is referred to here, but I must acknowledge the particular influence in the early days of Dr C. C. Thiel and Professor L. F. L. Clegg, and more recently of Mr A. G. Perkin. Although I have identified no processing equipment by the manufacturer's name, preferring to deal with types of plant rather than individual makes, this book could not have been written without the information provided by many manufacturers. They are too numerous to be mentioned individually, but I am most grateful to them all. I must, however, particularly thank Mr D. P. Fisher of Tetra Pak Ltd, and Tetra Pak Rausing SA, for the statistics used in the first chapter. The staff of the library of the Reading Laboratory of the Food Research Institute, and particularly Mrs Anna Cox, have been a great help in my study of the literature. On a more personal level, I am very much indebted to my long-standing friend and former colleague Dr John Ford, and to my son Dr John Burton, who have both read the text in its entirety, and to Mr Albert Hersom for his comments on Chapter 11. They have saved me from many errors: any faults which remain are entirely my own. Finally, I must thank my wife, who has been extremely tolerant and understanding when she might reasonably have expected that, in retirement, I would be doing other things. H. BURTON Contents Dedication v Preface vii Chapter 1. Introduction and Historical 1 Chapter 2. Bacteriology of the Heat Sterilization of Milk 13 Chapter 3. Chemical and Physical Changes in Milk at High Temperatures 44 Chapter 4. Types of UHT Processing Plant 77 Chapter 5. Performance of UHT Processing Plant 130 Chapter 6. Aseptic Filling 181 Chapter 7. Cleaning and Sterilization of UHT Processing Plant and Aseptic Fillers 228 Chapter 8. Connection of Aseptic Fillers to UHT Processing Plant 243 Chapter 9. Properties of UHT- Processed Milk . 254 Chapter 10. Fouling of Heat Exchangers 292 Chapter 11. Laboratory and Legislative Control. 310 x Contents Appendix 1. Use of UHT Processing Methods for General Foods 337 Appendix 2. Selected Publications 342 Appendix 3. Kinetic Parameters to Describe the Temperature Characteristics of Reactions 345 Index 347 Chapter 1 Introduction and Historical Milk and milk products may be heat treated by a range of processes, according to the properties required after processing. For example, in order of increasing severity of heat treatment, thermization is intended to increase the keeping quality of milk under refrigeration by giving a minimum of heat treatment sufficient to destroy psychrotrophic vegetative contaminating microorganisms; pasteurization is intended to make the milk or product safe by destroying all the vegetative pathogenic organisms; and sterilization is intended to destroy all the microorganisms present, both vegetative forms and spores, or at least make them incapable of growth in the product, so that a long keeping quality is obtained without refrigerated storage. For many years, the only important commercial processes for sterilization were those deriving from the early work of Appert, in which the product was sealed in a container and heated by steam maintained under pressure in a suitable vessel. Commercial sterilized milk has been produced in this way since the beginning of the 20th century, particularly in Europe, and is still an important product (Burton et al., 1965). It must be realized that the term 'sterilized' here does not imply that the product is sterile in the strict microbiological sense. This was probably the sense in which the description was originally used, but it has been clear for many years that absolute sterility cannot be obtained by any heat treatment process. If the treated product is suitably incubated, a proportion of spoilage is almost certain to be found. This proportion of spoilage can be reduced by increasing the severity of the heat treatment process, but the extent to which this is possible is limited by the chemical changes which are brought about in the product, modifying its colour and flavour, and changing its nutritional value, normally for the worse. 2 UHT Processing of Milk and Milk Products A commercial processor is therefore faced with a difficult choice: whether to strive for a low level of spoilage in his product, or to produce a product with the minimum amount of chemical change brought about by the heat treatment. Some processors supplying 'sterilized' milk to a local market with frequent delivery either to shops or direct to the consumer find that the time before the product is consumed is so short that spoilage does not occur, although laboratory incubation shows a high proportion of potential spoilage: a relatively mild 'sterilization' process can therefore be used to give a better colour and flavour in the product. This alternative is not available to the exporter of sterilized milk, where the time before consumption will be prolonged and, if the milk is to be exported to the tropics, the incubation conditions during transport severe: the product will then have to be almost truly sterile, and the adverse effects of a severe heat treatment will have to be accepted. Typical time-temperature conditions for in-container sterilization processes are !oS-120°C with effective processing times of 10-30 min, the higher temperatures being associated with the shorter times. Similar bacteriological effectivenesses can be obtained by using much higher processing temperatures, e.g. 13S-1S0°C, with correspondingly shorter holding times of the order of a few seconds. For reasons which will become clear later, these processes cause much less chemical change as a result of the heat treatment than do conventional sterilization processes. The differences are so marked that the higher temperature processes are frequently considered as different proc esses, which have been given the description 'ultra-high-temperature', commonly abbreviated to UHT. This abbreviation has become very generally accepted because it is the abbreviation for the full descrip tion in the three languages English, French and German. The use of high processing temperatures combined with shorter times therefore avoids the dilemma faced by processors, of not being able to improve the bacteriological quality of the product without making the chemical quality unacceptable. However, in general a sufficient improvement cannot be obtained with in-container proc esses, because the limited heat transfer rates which are possible across the container surfaces do not allow a sufficiently rapid change of product temperature. Consequently UHT processing involves the treatment of the product in continuous flow in a heat exchanger. After the sterile product leaves the heat exchanger, it needs to be filled into suitable containers, large or small, for distribution and sale. This