CCC-Haard/Sim FM (Ii-Xviii)
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Preface It has been over 100 years since Eduard Büchner showed that molecules with catalytic activity could be isolated from yeast cells. Following decades of inten- sive research on these molecules (enzymes), we now know they are ubiquitous in living systems and are the agents that make chemical reactions possible in a diversity of life forms, albeit sometimes under vastly different environmental conditions. In 1961, when the International Commission on Enzymes of the In- ternational Union of Biochemistry established a system to classify enzymes, the committee listed only 712 enzymes. The total number of enzymes identified has since grown to more than 3000. Most of the known enzymes have been extensively studied in land mam- mals, such as rats, and microorganisms, such as Escherichia coli. So why devote a book to enzymes from aquatic animals? Although most of the enzymes dis- cussed in this book are also found in terrestrial life forms, homologous enzymes from different sources, which have the same name and Enzyme Commission (EC) number, may exhibit vastly different properties with respect to stability, temperature optimum, secondary substrate specificity, and others. These differ- ences are based on adaptation and are magnified when the cellular milieu of the source organisms varies because of habitat conditions or other reasons. Aquatic organisms occupy unique and often extreme environments, such as the deep ocean where pressure is high and light is absent; temperature ranges from –2ºC in Polar saline gradients to 103ºC at thermal vents; salinity ranges from very low vii Foreword v fish processing byproducts as processing aids. These discussions of the spe- cial roles of seafood enzymes in postmortem fish metabolism and the quality changes they effect are critical pieces of knowledge in achieving the goal of obtaining maximum value from the available species. We have a strong oblig- ation to use seafood resources wisely and responsibly so future generations may also enjoy their benefits. Herbert O. Hultin, Professor Department of Food Science University of Massachusetts/Amherst Gloucester Marine Station Gloucester, Massachusetts iv Foreword produce high-value materials (e.g., enzymes) and developing techniques to re- cover all the flesh and use it for human food. Approximately half of all the species caught in the world today go into the production of fish meal and oil. Development of procedures that will make these resources directly available for human food would greatly improve their efficiency of use. The chemical instability of both the protein and lipid frac- tions, the presence of high concentrations of unstable dark muscle, seasonal fluctuations in catch, unfavorable sizes and shapes, strong flavors, and skeletal structure that does not permit easy removal of bones are all factors limiting the use of these species. Advances in our knowledge of the chemistry and biochem- istry of the unstable components and improvement in processing procedures will be necessary to adapt these species for human food. We must face and ac- cept this challenge. A thorough understanding of the nature of the critical components of seafood tissues and how they respond to processing, storage, and handling procedures is an absolute necessity to achieve these goals. No class of compo- nents of seafood tissues is more important than their enzymic systems. The number of books or reviews devoted exclusively to seafoods is small com- pared to those dealing with the muscle tissue of land animals. Indeed, in many cases land animals or birds have been used as models to discuss fishery prob- lems. Although there are many similarities between the muscles of fish and land animals, some important considerations for specific seafoods are often overlooked or downplayed. One point often overlooked in discussions of enzymes in foods is that most of the important commercial species are caught in cold water. In fact, 95% of the ocean has a temperature of less that 5ºC year-round, leading to species with lipids that contain a large percentage of the highly unsaturated fatty acids eicosapentaenoic and docosahexaenoic acids (which maintain fluidity of the lipids at low temperature). This makes the lipid fraction susceptible to oxidation. Likewise, proteins (enzymes) of seafoods must have a greater inherent flexibility to be able to function at low temperatures—a flexibility that also makes them less stable. Thus, seafood enzymes function well at low temperatures and refrigeration might not have the same inhibitory effect on postmortem changes that it would for warm- blooded land animals. Seafood Enzymes covers a myriad of topics on how enzymes are impor- tant to improve uses of seafood raw materials. These topics include the nature of the enzymes themselves and biological factors that affect them; the role of native enzymes in postmortem effects on quality attributes such as texture, flavor, and color; the use of products of enzymic breakdown as quality in- dices; control of enzymic activity by modification of environmental condi- tions, processing, or use of inhibitors; and the use of enzymes isolated from Foreword Gathering food from the oceans represents the last major hunting effort of hu- mans. As has happened in other fields, however, our technological abilities have outstripped our capacity to deal with the social, cultural, and economic conse- quences of these technologies. Where national and international regulations, ex- hortations, and pleas have failed, the common property status of sea resources had led to overexploitation, which has reduced the number of animals available to be used as foods. This decline has been accompanied by an increase in human populations and, more importantly, rising expectations for improved diets in newly developing areas of the world. These factors have increased demands on fish and shellfish stocks and increased strains on the ability of the fish and shell- fish to maintain their numbers. Quite naturally, this has had the most impact on those species that have the greatest attraction as food for humans. Traditionally, the fisheries have been a wasteful industry, in that often no more than 50% of the flesh of even desirable species is converted to high-value human food, and often much high-value seafood protein and lipid is dumped back, unused, into the oceans. No one denies the great need to utilize our aquatic resources more efficiently. It not only makes economic sense, but on a planet with limited resources, there is a moral imperative not to needlessly destroy valuable raw materials. First, we must use traditional commercial species to the fullest extent. This means adding value by utilizing processing byproducts to iii Library of Congress Cataloging-in-Publication Data Seafood enzymes : utilization and influence on postharvest seafood quality / edited by Norman F. Haard and Benjamin K. Simpson. p. cm — (Food science and technology ; 97) ISBN 0-8247-0326-X (alk. paper) 1. Seafood—Composition. 2. Fishery processing—Quality control. 3. Enzymes. I. Haard, N. F. II. Simpson, Benjamin K. III. Series. TX556.5.S42 2000 664'.9497—dc21 99-087831 This book is printed on acid-free paper. Headquarters Marcel Dekker, Inc. 270 Madison Avenue, New York, NY 10016 tel: 212-696-9000; fax: 212-685-4540 Eastern Hemisphere Distribution Marcel Dekker AG Hutgasse 4, Postfach 812, CH-4001 Basel, Switzerland tel: 41-61-261-8482; fax: 41-61-261-8896 World Wide Web http://www.dekker.com The publisher offers discounts on this book when ordered in bulk quantities. For more in- formation, write to Special Sales/Professional Marketing at the headquarters address above. Copyright © 2000 by Marcel Dekker, Inc. All Rights Reserved. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming, and recording, or by any information storage and retrieval system, without permission in writing from the publisher. Current printing (last digit): 10987654321 PRINTED IN THE UNITED STATES OF AMERICA viii Preface to saturated; and daily fluctuations in oxygen availability for tidal organisms. For this reason, the topic of seafood enzymes is of interest to the comparative bio- chemist. There are also practical reasons for publishing a book on seafood enzymes. As a general rule, fish and shellfish are more perishable than other food myosys- tems, e.g., beef and poultry. Several studies have shown that the initial rate of food quality deterioration in microbiologically sterile and nonsterile fish is the same. Thus, it is evident that so-called “autolysis” or endogenous biochemical reactions, rather than spoilage microorganisms, are responsible for the loss of prime quality in seafood. Furthermore, it has been observed that tropical fish, coming from a habitat temperature similar to the body temperature of mammals, keep much better than seafood from the more typical cold (<5ºC) environment of the oceans. Fish are poikilotherms, which means that their body temperature is, in most cases, a direct reflection of the water temperature and their enzymes are accordingly cold adapted. Moreover, methods developed to extend the shelf life of chilled seafood, such as low-dose ionizing radiation, modified atmospheres, and most chemical treatments, have been ineffective in preserving prime qual- ity—and in some cases accelerate loss of prime quality shelf life. This is because these methods target spoilage microorganisms rather than endogenous tissue en- zymes. Accordingly, future development of methods to preserve the quality of chilled fish must evolve from an understanding of seafood enzymes and aim to target key enzyme-catalyzed reactions that contribute to loss of appearance, fla- vor, texture, nutrition, and functional properties. Seafood enzymes are also becoming more important as industrial process- ing aids. Although enzymes were inadvertently used as processing aids for cen- turies prior to Büchner’s discovery, the wide-ranging use of enzymes as seafood processing aids described in this book is relatively new. Enzymes recovered from fishery products’ wastes have recently been obtained as “value added” products for use as industrial enzymes.