Chemical and Physical Properties of Food
In this lesson we will discuss the chemical properties of food constituents and how those constituents affect the physical properties of foods. You will learn about the various classes of carbohydrates, ranging from monosaccharides to polysaccharides, and their functional properties important to food science and technology. You will learn about the caramelization and Maillard browning reactions and their importance as a determinant of food quality.
Important properties of proteins such as foaming, emulsion stabilization and gelation will be explored. The importance of proteins in the production of selected food commodities will be explained, and you will learn about enzymes, which are proteins that function as biological catalysts, and their importance in food technology.
We will discuss the basic properties of fats and differentiate between saturated and unsaturated fats. You will learn about triacylglycerols or triglycerides and their functional properties, and about emulsions and emulsifiers and the many foods that are created through the formation of emulsions.
The importance of organic acids, pigments and water in foods and their role in determining the properties of foods will be described.
Through a discussion of the production of flour from wheat and the production of bread, you will begin to appreciate how various constituents interact to produce the desirable physical and flavour characteristics of bakery products.
The overall objective of this lesson is to give you an appreciation that foods are mixtures of chemicals that interact to produce the particular characteristics of the food from sensory, chemical and physical stimuli. At the conclusion of this lesson, you will be able to:
- describe food-colloidal dispersions;
- list the functional properties of carbohydrates, proteins and fats in foods;
- differentiate between caramelization and the Maillard browning reaction and state the importance of these reactions in food;
- describe the important roles that enzymes play in food systems;
- describe the function of emulsifiers and stabilizers in emulsions;
- understand the importance of water, pH, and minor constituents in quality and safety of foods
Position of the Academy of Nutrition and Dietetics: Use of Nutritive and Nonnutritive Sweeteners. (2012). J Acad Nutr Diet.112:739-758.
American Dietetic Association (2007). Position of the American Dietetic Association and Dietitians of Canada: Dietary Fatty Acids. Journal of the AMERICAN DIETETIC ASSOCIATION 107(9), 1599-1611.
- 1 Food-Colloidial Dispersions
- 2 Carbohydrates
- 3 Fats
- 4 Proteins
- 5 Water
- 6 Organic Acids
- 7 Colours and Pigments in Foods
- 8 Aroma Constituents
- 9 Vitamins and Minerals
- 10 References
Foods are essentially mixtures of chemical compounds arranged in specific organizations that give rise to the particular chemical, physical and sensory properties of each food system. A knowledge of the chemical and physical properties of the constituents of foods is important to your understanding of how food systems behave under the various conditions encountered in preservation, storage and preparation for consumption. Food systems vary in their chemical composition and physical properties, ranging from chemically simple systems such as sugar syrups to the chemically complex food systems such as milk, muscle food systems, and plant tissue systems.
In Colloidal Dispersions, the particles of one substance are distributed, dispersed, in another substance without dissolving.
The substance that is dispersed within another is called the dispersed phase.
The substance that extends throughout the system and surrounds the dispersed phase is called the continuous phase.
Table 2.1 shows the dispersed and continuous phases of some typical colloidal dispersions.
Table 2.1 Types of Colloidal Dispersions and Examples
|Name of Dispersion||Dispersed Phase||Continuous Phase||Examples|
|Sol||Solid||Liquid||starches, proteins and some plant polysaccharides in water|
|Solid Emulsion||Liquid||Solid||butter, margarine|
|Gel||Liquid||Solid||starch, pectin or gelatin gels|
|Foam||Gas||Liquid||beaten egg white, whipped cake frostings|
|Solid Foam||Gas||Solid||meringue, ice cream, bread|
For example, a sol is a suspension of large molecules dispersed in a liquid, generally water.
An emulsion is a suspension of liquid droplets (fat or water) within a liquid medium (fat or water). Food emulsions can be either oil in water (o/w) or water in oil (w/o). Homogenized milk is a dispersion of milk fat droplets in a liquid medium (skim milk portion of milk), while skim milk itself is a suspension of milk protein particles, the casein micelles, within a water-based medium. A solid emulsion is a dispersion of liquid droplets within a solid phase. Margarine and butter are examples of water in oil emulsions, in which the continuous phase is solid under refrigerator or low ambient temperatures.
Figure 2.1 Examples of emulsions, (A) Oil in water emulsion (milk); (B) Water in oil emulsion (margarine).
A gel is a dispersion of water held within a continuous matrix of polysaccharides (starch gels) or proteins (gelatin gels). Some scientists consider the water in gels to be a second continuous phase rather than a dispersed phase.
A foam is a dispersion of gas bubbles distributed within a liquid phase. A typical foam is created when egg whites are beaten and air is incorporated within the liquid by the action of the egg white proteins which stabilize the bubbles by forming a film around the gas. A solid foam is created when gas bubbles are dispersed within a solid phase (ice cream). In the case of whipped egg whites, the foam is converted to a solid foam by the action of heat on the egg proteins. Some of these phenomena are discussed in more detail later in this lesson.
The major food components of food systems are carbohydrates, fats, proteins and water. There are also minor food components, organic acids, pigments, aroma compounds, vitamins and minerals. In this section we will discuss the important functional properties of these components and how those properties influence the chemical and physical properties of foods
Carbohydrate is an example of major food components of food systems. Other major components are fats, proteins and water. There are also minor food components, organic acids, pigments, aroma compounds, vitamins and minerals. In this section we will discuss the important functional properties of these components and how those properties influence the chemical and physical properties of foods
Carbohydrates are one of the three main classes of nutrients (the other two being fats and proteins). They occur in foods as sugars and starches. Digestible carbohydrates contribute 4 Calories (kilocalories) of metabolized energy per gram. Carbohydrates are an efficient, generally inexpensive source of energy and should contribute about 50% of our caloric intake per day; most of the carbohydrates that we consume should be in the form of complex carbohydrates (polysaccharides) such as starch rather than as simple sugars (mono or disaccharides) such as table sugar.
The main monosaccharides found in foods are glucose, fructose and galactose. These are referred to as simple sugars because one of their main functions is their ability to impart a sensation of sweetness; however, sugars vary in their sweetening power. The sweetness of various sugars in comparison to sucrose (table sugar) is shown in Table 2.1.
Disaccharides are formed by the union of two monosaccharide molecules. Disaccharides are also considered as "simple" sugars. Disaccharides are split into their component monosaccharides by enzymes or by boiling with dilute acids. The most important disaccharides in foods are sucrose, lactose and maltose. These disaccharides differ from one another in solubility, sweetness, and other properties.
Table sugar, obtained from sugar-cane or sugar-beet, is mainly pure sucrose. It is formed from glucose and fructose linked together. Sucrose can be found in a variety of fruits, grasses and roots.
One of the recent trends in the food industry, particularly for carbonated beverages, is the use of invert sugar in place of sucrose because of the inherently greater sweetening power per unit weight of the fructose containing sweetening systems (see Table 2.1). Invert sugar is produced by hydrolyzing sucrose with the enzyme invertase or with acid, to produce a mixture of glucose + fructose (1:1).
Incidentally, the primary sugars in honey are glucose and fructose in a 40:60 ratio. Most of the nectar collected by the honey bee contains sucrose which is hydrolyzed by invertase in the saliva of the honey bee. Some of the glucose is converted to gluconic acid and hydrogen peroxide by glucose oxidase, another enzyme secreted into the collected nectar by the honey bee. The gluconic acid and hydrogen peroxide act as preservatives in the nectar. Honey also contains minute quantities of other complex sugars.
Table 2.1 . Relative sweetness of carbohydrate sweeteners *Perceived sweetness of a sweetener compared to sucrose as a reference. Adapted from: Desrosier, N. W. 1976. Elements of Food Technology. AVI Publishing Company. Westport, CT. Pomeranz, Y. 1985. Functional Properties of Food Components. Academic Press Inc., Orlando, Fl.
|Glucose (dextrose)||70 - 80|
|Invert Sugar||100 - 130|
|Corn Syrup (mixture of glucose, maltose and higher oligo-saccharides)||50|
|Lactose||10 - 20|
|High Fructose Corn Syrups (42% fructose)||100|
|High Fructose Corn Syrups (55% fructose)||100 +|
|High Fructose Corn Syrups (90% fructose)||120 - 160|
Lactose, also known as milk sugar, occurs in the milk of all animals. Cow's milk contains about 4-5%, whereas human milk contains 6-8% lactose. Lactose is formed by linking glucose and galactose together. The hydrolysis of lactose found in dairy products into its component monosaccharides is catalyzed by the enzyme lactase. Breaking down lactose substantially increases the sweetness. Lactose can also be fermented by lactic acid-producing bacteria, into lactic acid. This is the acidulant and preservative agent in yogurt and numerous cheeses.
Have you seen lactose-free products in the market or have you met someone who is "lactose intolerant"?
Lactose intolerant people are those who do not have the enzyme lactase necessary to digest (breakdown) lactose (milk sugar). People who are lactose intolerant can suffer from minor cramps to extreme intestinal discomfort. Lactose-free products have had the enzyme lactase (usually isolated from yeast) added to them. Alternatively, lactose intolerant individuals can take tablets containing the enzyme, prior to eating or drinking dairy or other food products with lactose or milk solids.
The sugar maltose contains two glucose units linked together. It is obtained when starch (e.g. corn starch) is hydrolysed by the enzyme amylase or by heating with dilute acid (Figure 2.3). Maltose can be further hydrolysed by the enzyme maltase into its component D-glucose units, which are then enzymatically isomerized by the enzyme glucose isomerase to produce a liquid syrup composed of 42% fructose, commercially known as high fructose corn syrup (HFCS 42). HFCS has 42% fructose, 52% glucose and 6% starch. Subsequent technological improvements in which the syrup is passed through an ion-exchange column that retains fructose, allow for the production of a 90% fructose syrup. Today, HFCS 90 is blended with HFCS 42 to create HFCS 55, which has a sweetness profile similar to sucrose (Table 2.2). Many soft drinks are now sweetened with HFCS especially when cost of these syrups is lower than the cost of sucrose or even invert sugar.
Functional Properties of Simple Sugars in Foods
READ: Newsome, R.L. 1986 [August]. "Sweeteners: Nutritive and non-nutritive" (A scientific status summary). Food Technology, 40(8):195-206. [Read pp. 195-196, 200-201, and 204.]
- Sugars are widely used for their sweetening power. The sweetness of carbohydrates is determined by their molecular structure and interaction with sensory receptors on the tongue. Simple sugars vary in their sweetness (Table 2.1)
- It is important to note that sweetness has no relation to caloric contribution of a sweetening agent to the diet. Fructose and lactose each produce 4 Calories of metabolized energy per gram when digested and absorbed, but lactose is only one-seventh as sweet as fructose. Thus for an equivalent sweetness intensity, less fructose would be required than lactose. Conversely, a product sweetened with lactose could potentially contain seven times the caloric content compared to a product sweetened with fructose.
- Sugars produce body and mouth feel when they are incorporated into foods at concentrations high enough to affect the viscosity (resistance to flow) of the food product
- Production of hot supersaturated sugar solutions with controlled crystallization during cooling is the basis of formation of many hard candy products, toffees and related products.
- Sugars are readily soluble in water because they contain many hydroxyl (OH) groups, which form hydrogen bonds with water. Solubility of sugars increases as the temperature of water increases. This property is used to produce syrups of varying concentrations for various uses (e.g. pancake syrup, concentrated syrups for use in food processing, cooking or confections.)
- Sugars can be crystallized from solution when water is evaporated. This is the basis of production of table sugar (sucrose) from the juice extracted from sugar cane and sugar beets.
- Sugars, in sufficiently high concentration, can be used to inhibit growth of undesirable microorganisms. They function as a preservative by binding water needed by the microorganisms.
- Sugars are fermented by microorganisms with the concomitant production of acids and/or alcohol as well as flavouring compounds. This is the basis for production of fermented foods and ingredients obtained by means of microbial fermentations.
- Sugars caramelize when exposed to high temperatures. See "Browning reactions" below.
- Reducing sugars react with proteins and amino compounds to produce flavours and colours in foods (Maillard browning). See "Browning reactions" below.
Another important property of the simple sugars is their ability to serve as reactants in non-enzymatic browning reactions, namely caramelization and the Maillard browning reaction.
The caramelization reaction involves reaction of sugars (reducing and non-reducing sugars) when heated at high temperatures (200°C) to produce caramel and butterscotch flavours. The brown pigments formed during the heating of sugars contributes to the colour of caramel candies and toffees. The pigments are not the same as the melanoidins formed during the Maillard reaction
Maillard Browning Reaction
The Maillard browning reaction occurs when reducing sugars react with nitrogenous compounds such as amino acids, proteins or amines.
A reducing sugar contains a free aldehyde or ketone group. Therefore, it will contain a “free” OH on the position next to the O in the ring structure.
Glucose, fructose, galactose and lactose are examples of reducing sugars. Sucrose does not have this "free" OH, therefore is not a reducing sugar.
- Question: Sucrose is not a reducing sugar, will invert sugar be considered as a reducing sugar?
The Maillard browning reaction is responsible for the formation of the brown pigments that appear on bread slices when they are toasted in the toaster.
Many low molecular weight intermediate compounds are formed and these often are aroma and flavour compounds that contribute to the desirable or undesirable flavours produced in a food by the Maillard reaction. Examples of desirable compounds are the aroma and flavour of baked bread, toasted bread and roasted coffee, while undesirable aromas and flavours are those that form in skim milk powder during storage or during the browning of canned peaches during long-term storage. The brown colours are high molecular weight pigments, melanoidins, formed as a result of polymerization of some of the low molecular weight intermediate fractions.
Can you think of other food products from the Maillard reaction?
Polysaccharides are high molecular weight, long chains of monosaccharide units (i.e. glucose). They differ from simple sugars by being insoluble in water and generally tasteless. Most of the polysaccharides used in food products are derived from plant or seaweed sources; a few are from microbial origin. They contribute to the thickness or viscosity and textural properties of food products.
Common polysaccharides seen on food ingredient list:
- extracted from seaweed (kelp)
- used as a thickener agent
- extracted from certain types of seaweed
- used as gelling agents
- keep solids and liquids in suspension in fruit juices
- extracted from certain types of seaweed (red algae)
- used as a suspending agent to keep cocoa particles in suspension in chocolate milk
Cellulose and Hemicellulose
- are present in many plant tissues as supporting structures (e.g. the fibres in celery)
- are polymers of glucose that are indigestible
- along with pectin and the other carbohydrate gums form the indigestible portion of our carbohydrate intake that is known as dietary fibre
Gum Arabic or Gum Acacia
- is a plant exudate from the bark of the acacia trees
- used as thickener and stabilizer in products like beer, soft drinks, ice cream
- are structural polymers in plants
- form the cementing material between individual plant cells
- pectin affects texture of plant tissues
- used in jams and jellies as gelling agents in the presence of sufficient sugar and acid¨
- contribute to viscosity of tomato paste and ketchup
- contribute to the mouth feel and maintenance of particles in suspension (e.g. orange juice, unclarified apple juice)
- are polymers of glucose
- digestible when cooked (e.g. rice, potatoes, etc.)
- used as thickening, suspending and gelling agents(read text below for more information on starch)
- produced by bacteria
- first isolated from rotting cabbage, now cultured in large fermentation tanks and purified
- used in salad dressings as a thickening agent, which enables the dressing to cling to the salad components
- used as a suspending agent to maintain pieces of onion, red pepper, spices in a stable suspension.
Starch - An important food ingredient
In foods such as cereals and tubers starch exists in the form of starch granules (Figure 2.4). Starch molecules (amylose, a straight chain starch molecule and amylopectin, a branched starch molecule) are tightly packed within starch granules. The starch granule is not soluble in cold water but will imbibe large quantities of water when it is heated in water.
The starch granules absorb water, swell and become soft and pliable, by a phenomenon known as gelatinization. This is the phenomenon that occurs when puddings are made or when flour is used as a thickening agent when making gravies. Starch gelatinization is the phenomenon that leads to the conversion of hard, unchewable, raw rice kernels to the soft, easily chewed, cooked rice.
Starches can be partially hydrolysed by acids or enzymes to produce products of intermediate chain length (dextrins) that have a numerous uses in food products. Some of the dextrin products are used to create foods that provide the sensation of containing fat but that are low in fat (more of this in Lesson 03).
Starch gels can lose some of their water holding capacity during refrigerated storage. This phenomenon is known as retrogradation, and involves re-association of starch molecules, especially amylose polymers, into an ordered structure. The linear amylose molecules orient themselves in crystalline regions, leading to a squeezing out ("syneresis") of water and a loss of tenderness of the food (e.g. staling of bread) or the development of a gritty texture (e.g. starch based pudding stored in the refrigerator).
- It is interesting to observe that bread stales more quickly in the refrigerator than the freezer or at room temperature. Can you think of why this is the case?
The phenomenon of retrogradation can be avoided to a certain extent through the use of dextrins and/or modified starches, thus reducing the tendency for alignment of linear amylose chains. Retrogradation can be reversed by heating the food (e.g. heating stale bread or buns in an oven or the microwave oven) but once the product cools the starch quickly retrogrades again to produce a tough product.
Instead of reading: Schneeman, 1986: "Fats in the diet: Why and where?" in Lesson 2 Readings. Based on the reading, please answer the following questions:
You should read the following two Wikipedia articles:
Fat: https://en.wikipedia.org/wiki/Fat (Reading the introduction is sufficient)
Chemistry of Trans Fat: https://en.wikipedia.org/wiki/Trans_fat#Chemistry
- What are fats? What is the correct name for fat?
- What is the nutritional value and calories of fat?
- What is the difference between saturated, monounsaturated and polyunsaturated fatty acids?
- What is an omega-3 fatty acid?
- What are cis and trans unsaturated fatty acids?
- Why are some fats solids and others liquid at room temperature?
- What is the term of the process used to convert a liquid oil into solid or spreadable margarine?
- What are some functions of fat in foods?
Functional Properties of Fats in Foods
Fats play many important nutritional functions in the human body. Fats contribute energy, aim in the absorption of fat soluble vitamins, and are precursors of many essential hormones. Fats play important functional properties in foods too. The required reading talked about fats the important functional properties they have by influencing the flavour and texture of foods. These functional properties of fats (and oils) in foods are summarized below:
- Fats act as a lubricant in food making the food more palatable and easier to chew and swallow.
- Fats have tenderizing power because they coat the flour particles (protein and starch) in baked goods, creating a flaky, lighter texture that makes them easy to tear apart. Fats work best as shortening when the crystals are in the beta prime form, which produces a fine texture in the baked goods. Cakes will have a crumbly texture without the moistness given by fats.
- Another function of fats in baked goods is the one called "'aeration'". Fats add air (gas) to batter and doughs. The fat surrounds the air molecules that are being incorporated into the batter. They contribute to the formation of the dispersion by decreasing the viscosity in the batter, thus making it easier to flow and rise.
- Fats and oils are carriers of many aroma constituents in foods that are usually fat-soluble. Thus, fats contribute to the overall flavour of food (we will review aroma and flavour in Lesson 3- Sensory perception of foods)
- Fats and oils can be heated to very high temperatures before they begin to smoke and vaporize. Foods fried in hot fats and oils (deep fat frying) cook very fast because of the temperatures that can be attained.
- Fats gradually soften when heated. This contributes to the desirable features such as chocolates that melt in your mouth and butter and margarines that are spreadable.
- Fats form part of emulsions (review colloidal dispersions) by acting as the dispersed phase or continuous phase. Some fats can also act as '"emulsifiers"', assisting in keeping the emulsion stable (see "Fats and their role in emulsions" below).
Fats and Their Roles in Emulsions
You will recall that there are two types of emulsions (see colloidal dispersions and Figure 2.1): oil in water [e.g. mayonnaise, homogenized milk] water in oil [e.g. butter, margarine].
Homogenization or other high-energy mixing processes may be used to disperse one liquid phase into another. Nevertheless, after two liquid phases such as oil and water are mixed and then left to stand, the natural tendency is for the two phases to separate. Emulsifiers are compounds that promote the formation of emulsions, i.e., the dispersion of one phase in the form of small droplets, in the second continuous phase.
Certain type of fat molecules called phospholipids, can function as emulsifiers. Phospholipids are structurally similar to triglycerides, except that only two fatty acids are linked to the glycerol (making it a diglyceride), and a charged group (negatively charged phosphoric acid esterified with positively charged choline group) is linked to the third position of glycerol.
- Lecithin is an example of a phospholipid. Lecithin is a naturally occurring emulsifier commonly found in egg yolk and soybean oil. Other naturally occurring emulsifiers include the proteins from milk, egg yolk or other foods. Sometimes, synthetic emulsifiers (e.g. Polysorbate 60) are used to assist in forming food emulsions.
Emulsifiers are amphiphillic molecules that have a hydrophilic [water loving] portion and a hydrophobic [water hating] portion. Emulsifiers assist in formation of an emulsion by orienting themselves at the interface between the two phases, with their hydrophilic and hydrophobic portions facing water and oil, respectively, thereby reducing the interfacial tension between oil and water phases. This can also help to stabilize the emulsion by preventing the dispersed oil droplets or water droplets from coalescing together. Other factors that affect emulsion stability are droplet size, and the viscosity of the continuous phase. Droplet size must be such that, the downward pull of gravity, is balanced by the upward forces of buoyancy. This will reduce the tendency for "creaming" (floating to the top) of the less dense (oil) phase.
Note the difference between Emulsifiers and Stabilizers. Stabilizers are compounds that increase the viscosity of the continuous phase, keeping the droplets suspended or dispersed and thus reducing the rate of creaming. Some of the polysaccharides discussed earlier in this lesson are commonly used as stabilizers to thicken the continuous phase (water); some examples are “xanthan gum” and “propylene glycol alginate”.
Something to think about: Are any emulsifiers or stabilizers added to assist in the formation of a stable emulsion in:
Check out the label of these foods to find the answer!
The major function of milk homogenization is the formation of a stable emulsion to prevent fat separation, such as that which occurs in unhomogenized milk.
Protein molecules are made up of long chains of hundreds or even thousands of amino acid units joined together. Amino acids are made up of an amino group (NH2) and a carboxyl group (COOH) attached to the same carbon atom. See figure on right:
There are 20 different amino acids in proteins found in food systems and in the human body. Eight of the amino acids cannot be synthesized by human tissues and must be obtained via food. These essential amino acids are isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine; histidine (essential for infants only).
Adults require 0.8 grams of protein per kilogram of body weight. Protein consumed in excess of body requirements is converted to energy or is converted to fat for storage. Proteins produce 4 Calories per gram when they are digested and the amino acids are metabolized for energy.
Functional Properties of Proteins in Foods
Proteins in tissue systems such as meats and fish contribute to the texture of the products. The difference between a tender steak and a tough steak can often be related to the types and relative abundance of various types of protein molecules within the muscle structure. Proteins from various sources (cereal grains, milk, meat, fish, legumes) can be used in various states of purity as food ingredients with differing functional properties. Some of those functional properties are described below.
Many proteins are amphiphillic molecules as they contain hydrophilic and hydrophobic portions (from amino acids) allowing them to act as emulsifiers. One part of these amino acids is attracted to water, forming hydrogen bonds, while the other part avoids water and binds with oil.
Egg yolk and mustard proteins in mayonnaise function as emulsifiers.
Proteins have the ability to trap air in bubbles and this leads to the formation of foams.
Egg white proteins function as foaming agents in the making of whipped egg whites. Whipping introduces air and denatures (unfolds) the protein molecules. The protein molecules then coagulate to form a fine film around the air pockets.
Solid foams such as meringue are formed when the whipped egg whites are heated causing the protein to denature and form a more rigid three-dimensional structure, which won't collapse when the air escapes.
Bread and ice cream are also examples of solid foams. Ice cream is also a solid emulsion.
Gelatin (from the animal protein: collagen) forms a gel by trapping large volumes of water within a semi rigid three dimensional protein matrix.
Heating of meat proteins during the manufacture of luncheon meats, such as bologna and frankfurters, leads to gelation and formation of the textures characteristic of cured meats.
Products such as frankfurters and bologna are also emulsions.
Milk protein also forms a gel when it is acidified, such as in making of yogurt and cheese. The gel holds water and has a smooth texture.
The Amazing Functionalities of Milk Proteins
Proteins as Enzymes
Enzymes are proteins that function as biological catalysts. In some cases enzymes are added to food as an ingredient (invertase in candy making). Some enzymes are used to promote food-processing operations while others are causative agents in food spoilage (amylase can ruin a starch gel; lipases can cause lipolytic rancidity which is the release of free fatty acid from glycerides).
Enzymes in living tissue food systems such as fruits and vegetables are responsible for the reactions associated with ripening. Those same enzymes will continue the ripening process after harvest and unless they are inactivated the enzymes will eventually cause spoilage of the product (e.g. loss of crispness of stored apples; loss of sweetness of apples during storage; loss of colour in the skin of apples during storage).
Many heating processes in food processing are designed to inactivate enzymes in addition to inactivating undesirable microorganisms in order to extend storage life of foods. These processes will be discussed later in Lesson 6. Microorganisms, when added to food systems, to produce fermented foods, are essentially sources of desirable enzymes required to catalyse the desired chemical reactions needed to produce fermented food products (yogurt, sauerkraut, soy sauce). This will be discussed in more detail in Lesson 9.
Enzymes are also extracted from a variety of sources (plants, animal by-products, microorganisms) and purified for use as aids in food processing (e.g. proteases used for milk coagulation during cheese making; pectinases to enhance juice recovery and for clarification of apple juice; invertase for conversion of sucrose to invert sugar; isomerase to produce high fructose corn syrup).
Food Proteins and Food Allergies
What is the relation between food proteins and allergies?
Intolerance to certain proteins in foods is the basis for many food allergies. You will often see statements on labels for ice cream, cereals and candy bars that warn of the possibility that the product may contain traces of peanuts or other nut products.
Allergies to peanut protein can be very severe, such that even with proper cleaning and sanitation, all residues of peanut allergens cannot be removed from processing equipment. Thus products that do not contain nuts, but that have been processed with equipment that was used for nut containing products may contribute enough allergen to cause some problems in very sensitive individuals.
For recent food recalls in Canada due to potential allergy risks, see http://www.inspection.gc.ca/english/corpaffr/recarapp/recaltoce.shtml
What are the priority food allergens in Canada?
Check this website out:
Water is an extremely important component of food systems. Water is related to all aspects of food ranging from our perception of quality, to the ability of microorganisms to be metabolically active in food systems.
Water exists in food in two forms: Free Water and Bound Water.
Some water may be present within intergranular spaces, within pores of the food matrix and as a thin film of water on the surface of many foods.
Free water can be found in tissue food systems and in dispersions.
Water that is free and not bound by food components generally retains its usual physical properties, can also function as a dispersing agent for colloidal substances, can function as a solvent and can be used by microorganisms.
Some water can be adsorbed on surfaces of macromolecules such as starches, pectins, proteins through forces such as van der Waals forces and hydrogen bond formation.
This water does not display all of its normal physical properties and it is not readily available for use by microorganisms and chemical or enzymatic reactions.
Another form of bound water is that water that is associated with food matrices as water of hydration. That water is also not readily available for use by microorganisms and enzymatic and chemical reactions in the food matrix, whether it is a tissue based food system or a dispersion.
Sugars and salts (sodium chloride)) can bind substantial amounts of water and are often added to foods for the purpose of decreasing the amount of free water in the food system. In that context, sugars and salts can be used to control or prevent growth of certain microorganisms in foods.
Water activity is a measurement that is frequently used in monitoring the availability of water (free water) in foods for the support of:
- microbial growth
- chemical reactions
- enzymatic reactions
Water activity can be measured as the ratio of the vapour pressure of water in the food to the vapour pressure of pure water, both measured at the same temperature.
- Aw = (Vapour Pressure of Water in Food at X °C) / (Vapour Pressure of Pure Water at X °C)
Water activity can range from 0 (no free water) to 1.0 (all the water is free, such as in distilled water).
Water activity values of a number of food products are shown below. Water activity of foods can be adjusted by physically removing water from foods during concentration and dehydration processing operations, also when the water in the food is frozen (the free water is in a solid state, in the form of ice crystals), or by adding substances that bind water thus lowering the proportion of water in the free form. The most commonly used water-binding agents are sugars and salt.
Is water activity the same as moisture content?
The answer is no. Measurement of the moisture content of a variety of food systems is a frequently conducted quality assurance measurement. However, measurement of water content of foods does not indicate whether the water is bound or free.
The data in the above table shows that water content of foods cannot be used as a reliable indicator of the water activity.
|Food'||Water Activity||Water Content (%)|
|Fruits, Vegetables||0.97||90 to 95|
|Jams and Jellies||0.80 to 0.95||32|
|Dried Fruit||0.60 to 0.70||20|
A number of other food constituents are present in foods in much smaller quantities than the major constituents (carbohydrates, proteins, fats). These minor constituents are very important in their influence on our perception of the quality attributes of the food. The minor constituents are: organic acids, pigments, aroma compounds, vitamins, and minerals.
Fruits contain natural acids which give the fruits tartness and slow down bacterial spoilage. Organic acids also impart flavour and acidity to food. Examples of organic acids and foods in which they are occur are:
|Organic Acids||Foods Containing These Acids|
|Citric Acid||Citrus Fruits, Tomatoes, Strawberries|
|Lactic Acid||Yogurt, Cheese, Olives, Cottage Cheese, Sauerkraut|
The major uses of organic acids are to adjust pH or to acidify food, and to impart flavour. For example, acetic acid provides flavour and decreases pH; phosphoric acid provides flavour and tartness in beverages.
What is pH?
pH is a measure of the acidity of a food. Foods and beverages differ in pH because of their content of acids, which produce hydrogen ions (Table 2.3). We are able to detect these ions by using a hydrogen sensitive electrode in a device called a pH meter.
|Food Item||pH Value||Acidity Classification|
|Meat, Fish, Poultry||7.0||Low Acid (pH > 4.6)|
|Milk||6.5||Low Acid (pH > 4.6)|
|Corn||6.3||Low Acid (pH > 4.6)|
|Wheat Flour||6.0||Low Acid (pH > 4.6)|
|Potatoes, Peas||5.8||Low Acid (pH > 4.6)|
|Carrots||5.1||Low Acid (pH > 4.6)|
|Figs||5.0||Low Acid (pH > 4.6)|
|Apples||3.7||Acid (pH = 0 to 4.6)|
|Cherries||3.6||Acid (pH = 0 to 4.6)|
|Oranges, Pears, and Tomatoes||3.5||Acid (pH = 0 to 4.6)|
|Pickles||3.0||Acid (pH = 0 to 4.6)|
|Lemon/Lime Juice||2.3||Acid (pH = 0 to 4.6)|
Is pH important to the Food Industry? Yes. An important pH for the food industry is pH 4.6; this is the borderline between an acid food and a low acid food (Table 2.3)
- Acid foods have pH of 4.6 or less
- Low-acid foods have pH greater than 4.6
Acid foods will not support growth of disease causing microorganisms. This aspect will be discussed in more detail later in the course.
A number of organic acids are also employed as antimicrobial agents. Organic acids have a wide range of textural effects in food systems due to their reactions with proteins, starches, pectins, and other food constituents.
Colours and Pigments in Foods
Many food systems are coloured by pigments naturally present in the food system:
- Chlorophyll, the green pigment in plants, is responsible for the green colour in apples, lettuce, celery and broccoli. Chlorophyll a has a blue green hue (e.g. in the florets of fresh broccoli) while chlorophyll b has a yellow green hue (stems of broccoli).
- Carotenoids, a diverse group of pigments, can be subclassified into carotenes and xanthophylls. Carotenes (isoprene polymers), produce red (tomatoes), orange (carrots) and orange-yellow (pink grapefruit) colours. Xanthophylls (isoprene polymers containing some oxygen atoms), produce yellow (pineapple) and orange colours (peaches, salmon, shrimp).
- Anthocyanidins and anthocyanins (anthocyanidin complexed with glucose or other sugars). These pigments are the predominant colour pigments in blueberries, cherries, cranberries, plums and red cabbage. The anthocyanins are particularly sensitive to changes in pH, showing marked changes in colour with pH changes in the food system. See Figure below:
Other pigments in food systems include hemoglobin and myoglobin (the red pigments in blood and muscle). All of the pigments noted above are sensitive to varying degrees to changes in the environment (pH, presence or absence of oxygen, presence of metal ions, enzymatic degradation) in the food system. Thus colour changes often occur in fresh fruits, vegetables, meats and fish during storage, spoilage and as a result of processing and cooking. In addition, there are a number of pigments, either those extracted from plants or microorganisms, or synthetic pigments that are used as colouring agents in fabricated food systems. These will be discussed in more detail in the section on food additives (Lesson 4).
The aroma profile of foods is very complex. The aroma of food is detected when we inhale volatile constituents of foods that react with the receptors in the olfactory regions of our nasal passages.
Most of us have experienced the aroma of freshly brewed coffee; there are actually hundreds of volatile compounds that have been identified in the aroma. The table below is a list of some of the chemical components that have been identified to contribute to the aroma of coffee; you don't have to memorise any of these compounds, it is only an example to demonstrate the amount of constituents that influence the aroma we perceive on a food.
|Acetyl methyl carbinol||Methyl alcohol|
|Cresols||Methyl ethyl acetic acid|
|Diethyl ketone||p-Vinyl guaiacol|
|Ethyl alcohol||Pyridine and homologues|
|Higher fatty acids|
Please note that:
- No single chemical compound can be attributed as being the sole source of the aroma of a particular food product.
- It is the specific mixture of chemicals in a particular concentration that creates the aroma that we associate with a high quality food product.
- Any change in that specific mixture of volatile compounds or their concentrations will alter the aroma that we perceive.
- The volatile constituents that contribute to the aroma of foods are present in very low concentrations but are nonetheless very important constituents of foods.
- The flavour constituents are either present as part of the food matrix (fresh strawberries) or are modified (cooking of strawberries) or created (roasting of coffee) during processing or cooking.
Vitamins and Minerals
Vitamins are mainly classified into water-soluble and fat-soluble vitamins. Vitamins are organic compounds that make up a small portion of food; however, vitamins are very important from a nutritional point of view because they are essential components of the human diet as they carry out some very important tasks in the body.
- Water-soluble vitamins include vitamin C (ascorbic acid), thiamin, riboflavin, niacin, pyridoxine, vitamin B12, and folacin. These vitamins are found within the water (aqueous) phase of foods.
- Fat-soluble vitamins (vitamin A, vitamin D, vitamin E) are found within the fat (oil) portion of foods.
Although Vitamins do not contribute to the physical characteristics of food, some are actually used as "food additives". Below are some examples of the two most common vitamins used as food additives:
|Vitamin||Food Additive Category||Functions|
|Ascorbic Acid (Vitamin C)||Bleaching Agents||Hasten oxidation and aging processes as in flour whitening treatment|
|Ascorbic Acid (Vitamin C)||Preservatives||Act as antioxidant to slow down rancidity and browning reactions|
|Tocopherols (Vitamin E)||Preservatives||Antioxidant|
Minerals (calcium, magnesium, sodium, potassium, iron, zinc) are often associated with various cellular components in tissue food systems and often are active participants in chemical and biochemical reactions that affect the chemical properties and textural characteristics of food systems, both tissue systems and dispersed food systems.
- Benarde, M.A. 1971. Chemicals We Eat. American Heritage Press, N.Y., pp.208
FNH 200 Course content on this wiki page and associated lesson pages was originally authored by Drs. Brent Skura, Andrea Liceaga, and Eunice Li-Chan. Ongoing edits and updates are contributed by past and current instructors including Drs. Andrea Liceaga, Azita Madadi-Noei, Nooshin Alizadeh-Pasdar, and Judy Chan.