The Chemistry of Food

Jan Velisek

• Publisher: Wiley-Blackwell
• Year: 2013
• Print ISBN: 9781118383810, 9781118383841
• eBook ISBN: 9781118384961
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Food Science
• Index: Technology & Engineering

THE CHEMISTRY OF FOOD

THE CHEMISTRY OF FOOD

This advanced textbook covers all the main macro- and micronutrients and the essential nutritional factors that determine the nutritional and energy value of foods and raw food material. It includes chapters devoted to amino acids, peptides and proteins, fats and other lipids, carbohydrates, vitamins, mineral substances and water, and in addition to chapters devoted to antinutritional, toxic and other biologically active substances, food additives and contaminants.

Each chapter addresses one of the main individual components of food, reviewing its important properties and functions. Detailed descriptions and explanations of the changes and chemical/biochemical reactions that occur under different conditions are also covered. The book provides a comprehensive overview of the chemical composition of foods and the changes that take place during food production, processing and storage. With an extensive list of tables and its comprehensive coverage, this almost encyclopaedic volume will be ideal for students at the Masters level and beyond, and is a vital all-in-one reference for professional food chemists, researchers and the food industry.

The Chemistry of Food is supported by a website of online resources, including web links to relevant news and journal articles, references and further reading, glossary of key terms, and revision notes for all topics/chapters.

Chapter 1

Introduction

The term food means material (eaten or drunk) that contains or consists of nutrients (proteins, fats, carbohydrates, vitamins and minerals) and many other chemical substances. Food is usually of plant or animal origin but, to a lesser extent, it may come from other sources, such as algae or microorganisms. Organisms use food to perform one or more of the four essential functions: to supply energy, to build, repair and maintain body tissues, to supply chemical substances that regulate body processes and to supply chemical substances that protect the organism. Traditionally food was provided through agriculture, but today most of the food consumed by the world's population is either supplied by the food industry or home-grown.

Human food, including such items as meat, poultry, fish, milk, vegetables, fruit and beer for example, is chemically very complex. According to rough estimates, fresh foods contain about half a million different chemical compounds. A much larger number of compounds are the result of biochemical (enzymatic) and chemical (non-enzymatic) reactions that occur during the storage of raw materials and foods and during their industrial and culinary processing.

Food science—the scientific study of food—is one of the major branches of the life sciences. Every aspect of food is covered, integrating and incorporating concepts and information from many different fields, including natural, technical and social sciences. It draws primarily on the knowledge of chemical science (biochemistry, organic, inorganic, physical and analytical chemistry), some areas of physics (such as the mechanics of solids and liquids), biology (especially microbiology), biotechnology, some of the branches of medical science (human nutrition, human physiology, pharmacology, toxicology) and agricultural science (e.g., crop and livestock production, post-harvest plant physiology and post-mortem muscle physiology). It also uses a range of experiences from technical engineering disciplines, such as agricultural and food engineering, particularly in the form of genetic food engineering. Food science may also call upon some experience of economics, sociology, psychology and other branches of social science.

The most important building blocks of food science are food chemistry and food technology. Food chemistry deals not only with the composition of food raw materials and end food products, but also with the behaviour, interactions and reactions of food components and the changes occurring in these food raw materials and end food products under various conditions during production, storage, processing, preparing and cooking. Food technology covers the entire gamut from procuring food raw materials to processing them into food products to preserving, packaging and delivering the food products to the consumer market. The common interest of both disciplines is examination of the possibilities of enhancing the positive changes and preventing the unwanted ones, elimination of antinutritive and toxic food components, and prevention of possible contamination with substances that can pose health risks and guarantee food safety. Producing safe and nutritious food for human and animal consumption is one of the principal aims of both food chemistry and food technology.

The most important natural constituents of foods are the nutrients that all organisms need in order to live and grow. The main (primary or basic) nutrients are proteins, fats (mainly triacylglycerols and phospholipids, but also numerous other lipids) and carbohydrates (such as some poly-, oligo- and monosaccharides) that determine the nutritional (nutritive) value of food, as they provide structural material (e.g., proteinogenic amino acids used for protein synthesis and lipids from which cell membranes are built) and energy. They may also have other functions. Nutrients are said to be essential if they must be obtained from an external source, either because the organism cannot synthesise them or it cannot produce them in sufficient quantities. Essential main nutrients include not only essential amino acids, but also lipids that are precursors of biologically active substances (such as essential fatty acids and some signalling molecules) and thus have protective functions. Also, some oligosaccharides are biologically active substances (e.g., breast milk oligosaccharides that act as receptor analogues to inhibit the adhesion of pathogens on the epithelial surface).

The energy value of consumed food depends on the content of nutrients and on such factors as their digestibility, utility, content of some other substances, dietary regime, mental health status and other factors. Energy value is mainly related to proteins, fats and carbohydrates.

Organic nutrients also include vitamins. Inorganic chemical compounds, such as dietary minerals, water (and oxygen), may also be considered nutrients. Vitamins and dietary minerals are collectively known as accessory (additive) nutrients and are often referred to as essential nutritional factors. With the exception of some vitamins that humans cannot synthesise, these accessory nutrients must be obtained from foods. They are therefore known as exogenous factors. Another essential nutrient is water, which is obtained in small amounts by oxidation of primary nutrients, but in much larger quantities from foods, and especially from beverages. Most foods contain a mixture of some or all of the nutrient classes, together with many other substances. Some nutrients can be stored internally (such as the fat soluble vitamins), while others are required more or less continuously. Poor health may be caused by a lack of required nutrients or, in extreme cases, too much of a required nutrient.

Some nutrients, such as proteins, peptides and amino acids, lipids, carbohydrates, vitamins and minerals, are also sensorially active substances (e.g., sweet or bitter). The main nutrients along with other substances (certain organic acids, such as acetic and citric acids, and sugar alcohols, such as glucitol, known as sorbitol) can simultaneously also be a source of energy, for example, ethanol has a relatively high energy value.

In human nutrition, nutrients are often inaccurately classified in a way that reflects the amount that the body requires. These nutrient classes can be categorised as either macronutrients (required in relatively large amounts) or micronutrients (required in smaller amounts). The macronutrients include proteins, fats, carbohydrates and water; the micronutrients are minerals and vitamins. A third class of dietary material, known as fibre (such as non-digestible polysaccharide cellulose), is also necessary, for both mechanical and biochemical reasons. Other micronutrients include antioxidants and various phytochemicals, which are said to influence or protect some body systems. Their necessity is not well established.

Besides nutrients, foods contain many substances that influence the food sensory impression and its organoleptic properties. These food constituents are known as sensorially active compounds. They determine the sensory value (quality) of foods, inducing an olfactory sensation (perception), which is described as the aroma, odour and smell, gustative perception, which is the taste, visual perception, which is the colour, haptic (tactile) perception, which is the touch and feel, and auditorial perception, which is the sound. The olfactory sensation is derived from odour-active compounds and the gustative perception from taste-active compounds. Flavour is the sensory impression determined by the chemical senses of both taste and smell and is caused by flavour-active food components. Haptic sensation is the texture, which is affected mainly by high molecular weight compounds, such as proteins and polysaccharides, often referred to collectively as hydrocolloids. Geometric aspects of texture that evoke both haptic and visual sensations symbolise the terms appearance and shape. Aspects of texture related to mechanical properties of food are called consistency. Auditorial sensations are associated with a range of textural characteristics (such as crispiness).

Food also contains a range of substances with beneficial or, vice versa, with a negative impact on human health. There is now increasing evidence to support the hypothesis that some foods and food components have beneficial physiological and psychological effects over and above the provision of the basic nutrients. Food that does not have a significant history of consumption or is produced by a method that has not previously been used for food is called novel food. This was first termed by the European Union (EU) in Regulation (EC) No. 258/97 of the European Parliament and of the Council of 27 January 1997 concerning novel foods and novel food ingredients. An example of a novel food is margarine containing phytosterols that help to reduce the blood cholesterol level. Other examples include canola oil produced from rapeseed with low levels of both erucic acid and glucosinolates and exotic fruits and vegetables, which have a long history of safe use. Functional foods are defined as foods (beverages) having disease preventing and/or health promoting benefits in addition to their nutritive or processing value. A food can be regarded as functional if it is satisfactorily demonstrated to affect beneficially one or more target functions in a body, beyond adequate nutritional effects. Functional food can be a natural food, a food to which an existing ingredient has been added, or a food from which a harmful component has been removed by technological or biotechnological processes. The term functional food (officially not used by the EU) was first used in Japan in the 1980s. (FOSHU, Ministry of Health, Labor and Welfare, Japan. Available at: http://www.mhlw.go.jp/english/topics/foodsafety/fhc/02.html.) Examples of functional foods with a long history are table salt fortified with iodine or margarine fortified with vitamin D. Another example is yoghurts with live microorganisms beneficial to the host organism (bacteria of the genus Bifidobacterium), which have advantageous effects on health and are called probiotics. Prebiotics are non-digestible food ingredients (usually oligosaccharides and other substances classified as soluble dietary fibre) that stimulate the growth and/or activity of beneficial bacteria in the small intestine. Symbiotics contain both prebiotics and probiotics. Nutraceuticals are foods and food products, in some countries physiologically active compounds isolated from foods, which reportedly have a physiological benefit or provide protection against chronic diseases. Such products may range from nutrients (essential amino acids, essential fatty acids, vitamins and minerals), to dietary supplements (also known as food supplements or nutritional supplements) and specific diets, to genetically modified foods and other products. Dietary supplements provide nutrients, fibre and various biologically active substances from foods, usually classified according to their effects as antioxidants and anticarcinogens, which may be missing or may not be consumed in sufficient amounts in food. Some countries define dietary supplements as foods, while in others they are defined as drugs or natural health products. Supplements containing vitamins or dietary minerals are included as a category of food in the ‘Codex Alimentarius’, a collection of internationally recognised standards, codes of practice, guidelines and other recommendations relating to foods, food production and food safety. Organic food is food that is produced using methods that do not involve modern synthetic inputs, such as synthetic pesticides and chemical fertilisers, and does not contain genetically modified organisms or food additives and is not processed using irradiation.

The food components that impair utilisation of nutrients (proteins, lipids, carbohydrates, vitamins and minerals) by biochemical mechanisms are called antinutritional substances. An example of such an antinutritional substance is oxalic acid, found in spinach and rhubarb, and which binds up calcium to insoluble calcium oxalate and prevents its absorption. Many foods (especially foods of plant origin) contain various natural toxic substances. These may be toxic only to certain individuals causing an allergy, which is related to the immune system response of the organism, or a food intolerance, which occurs when food irritates a person's digestive system or when a person is unable to properly digest or breakdown the food. The most common food allergies are peanuts, tree nuts (such as walnuts, pecans and almonds), fish and shellfish, milk, eggs, soy products and wheat. Intolerance to lactose in milk and other dairy products is the most common food intolerance.

Substances toxic to all individuals are called toxic substances or toxins. Once a toxic substance has contacted the body it may have either acute (immediate) or chronic (long term) effects. Most of food-born toxins are substances with low acute toxicity (such as the pungent alkaloid piperine in black pepper), although some may present chronic effects, such as hepatotoxic pyrrolizidine alkaloids in plants (such as comfrey and coltsfoot species) or cause pathological changes to the respiratory system (e.g., tobacco smoke).

Many chemical compounds in foods (as well as in feeds for livestock) are added intentionally as food additives. The purpose is to protect foods against spoilage, oxidation and increase some aspects of the food quality (e.g., nutritional value, aroma, taste, colour or texture). Preservatives, colours and flavours are the best-known additives, but there are in fact many categories of additives, each tailored to a specific purpose.

Contaminants are harmful chemical substances (chemical contamination) and toxic microorganisms (microbiological contamination) that have not been intentionally added to food but may be present there as a result of human activities in agriculture or industry. They may enter food at various stages of its production, packaging, transport or storage. A separate issue is genetically modified food or the presence in food of ingredients from genetically modified organisms, which is sometimes referred to as a form of food contamination. In relation to food, contaminants are sometimes differentiated into primary or exogenous contaminants that come from the external environment (e.g., residues of agrochemicals such as pesticides) and secondary or endogenous contaminants, which are, under certain circumstances, generated during food processing (e.g., by heating and fermentation) from natural food components (nutrients and other constituents). These contaminants are often called technological, processing or process-induced contaminants. They are absent in the raw materials, and are formed by chemical reactions between natural and/or added food constituents during processing. Typical examples are acrylamide formed in potato chips from asparagine, methanol produced in fruit alcoholic beverages by hydrolysis of pectin and chloropropanols formed from fats and hydrochloric acid in hydrolysed vegetable proteins.

Food contaminants and some additives used inappropriately, and sometimes even natural toxic substances, are collectively termed xenobiotics. The term is primarily understood to describe artificial substances that do not exist in nature and also covers substances that are present in much higher concentrations than are usual. Their presence in foods is related to the hygienic–toxicological quality.

As food chemistry is a dynamic discipline, it also deals with the biochemical and chemical reactions, interactions and physical processes that occur during food processing and storage. Knowledge of these processes is a prerequisite for their eventual regulation and also allows the optimisation of production processes, so that it is possible to produce high value foods in all aspects of quality, foods with high nutritional, sensory and hygienic–toxicological value (healthy foods), while satisfying all the requirements and demands of the consumers. In addition, knowledge of these processes can be applied in ordinary culinary experiences. Finally, food chemistry examines the ways in which to enhance positive changes and to prevent the unwanted ones, eliminate the antinutritive and toxic substances, and prevent possible contamination with substances that can pose health risks to guarantee food safety. Producing safe and nutritious food for human and animal consumption is one of the principal aims of both food chemistry and food technology.

This book takes you on a tour of food chemistry, an incredibly fascinating field of study that will provide you with a constant sense of discovery. For better or for worse, food chemistry is all around us every day and everything in food is chemistry, as chemistry is essential to meet our basic needs of food, energy, health, water and air. Food plays a role in everyone's lives and touches almost every aspect of our existence in some way. Therefore, the magic of food composition and its changes has attracted inquisitive minds for centuries. The understanding of the nature of chemicals and pivotal chemical processes occurring in foods, which the reader will gain, has been divided into 11 chapters, each of which provides insights into different classes of food constituents and related reactions, including proteins, peptides and amino acids, fats and other lipids, saccharides and their derivatives, vitamins, mineral compounds, water, odour- and taste-active substances, colours, beneficial, antinutritive and toxic components, food additives and contaminants and covering all aspects of the subject that are necessary for a comprehensive understanding about how things work. Let us wade into this information soup.

Chapter 2

Amino Acids, Peptides and Proteins

2.1 Introduction

Aminocarboxylic acids (see p. 5) occurring in nature have vital functions in living organisms. Many of these amino acids are found as free substances or higher...

Table of contents

• Cover
• Title Page
• Copyright
• Preface
• About the Companion Website
• Chapter 1: Introduction
• Chapter 2: Amino Acids, Peptides and Proteins
• Chapter 3: Fats, Oils and Other Lipids
• Chapter 4: Saccharides
• Chapter 5: Vitamins
• Chapter 6: Minerals
• Chapter 7: Water
• Chapter 8: Flavour-active Compounds
• Chapter 9: Pigments and other Colorants
• Chapter 10: Antinutritional, Toxic and other Bioactive Compounds
• Chapter 11: Food Additives
• Chapter 12: Food Contaminants
• Bibliography
• Index