Carbohydrates : FOOD ANALYST EXAMINATION SEREIES

 Introduction

Carbohydrates are one of the three macronutrients that are essential for human health and are found in many foods we consume. They are a source of energy and play a crucial role in many biological processes in the body.

Carbohydrates are made up of carbon, hydrogen, and oxygen atoms, and are classified into two types: simple and complex. Simple carbohydrates, also known as sugars, are found in foods such as fruits, candy, and soda. Complex carbohydrates, on the other hand, are found in foods like whole grains, vegetables, and legumes.

The primary function of carbohydrates is to provide energy to the body. When we consume carbohydrates, they are broken down into glucose, which is used by the body to produce ATP, the primary source of energy for the body's cells. Glucose can also be stored in the liver and muscles as glycogen, which can be converted back into glucose when the body needs energy.

Carbohydrates also play a critical role in maintaining healthy gut bacteria, which can aid in digestion and absorption of nutrients. Additionally, they can help regulate blood sugar levels and reduce the risk of certain chronic diseases like type 2 diabetes, heart disease, and certain types of cancer.

It is important to choose carbohydrate sources that are nutrient-dense and provide fiber, vitamins, and minerals, such as fruits, vegetables, whole grains, and legumes. It is also important to limit the consumption of processed and refined carbohydrates, such as candy, white bread, and sugary drinks, as these can contribute to health issues when consumed in excess.

Nomenclature

The nomenclature of carbohydrates refers to the systematic way in which these molecules are named and classified. Carbohydrates are classified based on their size and structure, with smaller molecules called monosaccharides, and larger molecules called oligosaccharides and polysaccharides.

·         Monosaccharides are named based on the number of carbon atoms they contain, with the most common monosaccharides being triose, tetrose, pentose, hexose, and heptose. They are further classified based on the spatial arrangement of their functional groups, with either D- or L- configuration.

·         Oligosaccharides and polysaccharides are named based on the number and type of monosaccharide units they contain, and the way in which they are linked together. For example, a disaccharide like sucrose is made up of glucose and fructose linked together by a glycosidic bond, while a polysaccharide like cellulose is made up of many glucose units linked together by beta-1,4 glycosidic bonds.

Classification:

Carbohydrates are classified into several categories based on their size, structure, and function. Here are the main categories of carbohydrates:

·         Monosaccharides: These are the simplest form of carbohydrates and cannot be broken down further. They are made up of a single sugar molecule and are classified based on the number of carbon atoms they contain. For example, glucose, fructose, and galactose are all monosaccharides with six carbon atoms and are classified as hexoses.

·         Disaccharides: These are composed of two monosaccharide units linked together by a glycosidic bond. Examples include sucrose, which is made up of glucose and fructose, and lactose, which is made up of glucose and galactose.

·         Oligosaccharides: These are composed of 3 to 10 monosaccharide units linked together by glycosidic bonds. Examples include raffinose and stachyose, which are found in legumes.

·         Polysaccharides: These are composed of many monosaccharide units linked together by glycosidic bonds. They are classified based on the type of monosaccharide units they contain and the way they are linked. Some examples include:

·         Starch: This is a polysaccharide composed of many glucose units linked together by alpha-1,4 and alpha-1,6 glycosidic bonds. It is the main carbohydrate storage molecule in plants.

·         Glycogen: This is a polysaccharide composed of many glucose units linked together by alpha-1,4 and alpha-1,6 glycosidic bonds. It is the main carbohydrate storage molecule in animals.

·         Cellulose: This is a polysaccharide composed of many glucose units linked together by beta-1,4 glycosidic bonds. It is the main structural component of plant cell walls.

·         Chitin: This is a polysaccharide composed of many N-acetylglucosamine units linked together by beta-1,4 glycosidic bonds. It is the main component of the exoskeletons of arthropods.

Functional carbohydrates: These are carbohydrates that have a specific biological function. Examples include:

·         Glycoproteins: Glycoproteins are proteins that have one or more carbohydrate molecules attached to them. These carbohydrates are usually attached to the protein through a process called glycosylation. Glycoproteins are found on the surface of cells and are involved in a wide range of biological processes, such as cell signaling, immune response, and cell adhesion. They also play a role in protein folding and stability. Examples of glycoproteins include antibodies, which are involved in the immune response, and hormones, such as follicle-stimulating hormone, which regulates reproductive function. Glycoproteins are important in the development and function of various tissues and organs in the body.

·      Proteoglycans: Proteoglycans are large molecules that consist of a core protein attached to long chains of sugar molecules called glycosaminoglycans (GAGs). These molecules are found in the extracellular matrix of connective tissues, such as cartilage, bone, and skin. Proteoglycans play a role in maintaining the structure and function of these tissues, as well as in regulating cellular behavior.

The GAG chains of proteoglycans are highly negatively charged, which allows them to attract and bind positively charged ions and water molecules, forming a hydrated gel-like matrix. This matrix provides resistance to compression, which is important for the function of tissues that experience mechanical stress, such as cartilage in joints.

Proteoglycans are involved in a wide range of biological processes, including cell adhesion, cell signaling, and tissue repair. They also play a role in diseases such as osteoarthritis, where degradation of proteoglycans in cartilage leads to joint damage and pain.

·         Glycolipids: Glycolipids are lipids that have one or more carbohydrate molecules attached to them. They are found on the surface of cells and play a role in cell-to-cell recognition and adhesion. Like glycoproteins, glycolipids are involved in various biological processes, such as immune response, cell signaling, and tissue development.

·         Glycolipids consist of a hydrophobic lipid tail and a hydrophilic carbohydrate head. The carbohydrate head can vary in size and composition, which allows for a wide range of specific interactions between cells. Some glycolipids also have a role in intracellular signaling and are involved in the regulation of various cellular processes.

One example of a glycolipid is ganglioside, which is found in nerve cells and plays a role in cell recognition and signaling. Another example is the blood group antigens, which are glycolipids found on the surface of red blood cells and are used to determine blood type.

 

Reducing and non-reducing sugars

Reducing and non-reducing sugars are two types of carbohydrates that differ in their chemical properties and reactivity.

·         Reducing sugars are carbohydrates that contain a free aldehyde or ketone group, which can donate electrons and participate in chemical reactions that involve reduction. In the presence of certain chemical reagents, reducing sugars can undergo a chemical reaction that results in a change in color or the formation of a precipitate. One example of a reducing sugar is glucose, which has an aldehyde group and is commonly found in fruits, honey, and some vegetables. Another example of a reducing sugar is maltose, which is composed of two glucose molecules linked by a glycosidic bond.

·         Non-reducing sugars, on the other hand, do not have a free aldehyde or ketone group and cannot participate in chemical reactions that involve reduction. Non-reducing sugars must first be hydrolyzed into their constituent reducing sugars before they can be detected by certain laboratory tests. One example of a non-reducing sugar is sucrose, which is commonly found in table sugar and is composed of glucose and fructose linked by a glycosidic bond. Another example of a non-reducing sugar is lactose, which is composed of glucose and galactose linked by a glycosidic bond and is found in milk and other dairy products.

One commonly used laboratory test to detect the presence of reducing sugars is the Benedict's test. In this test, a reducing sugar is heated in the presence of Benedict's reagent, which contains copper ions. The reducing sugar reduces the copper ions to form a colored precipitate, indicating the presence of the reducing sugar. Non-reducing sugars, however, do not react in this test and must first be hydrolyzed into their constituent reducing sugars before they can be detected.

In addition to their different chemical reactivity, reducing and non-reducing sugars also have different physiological effects in the body. For example, reducing sugars such as glucose are a major source of energy for the body, while non-reducing sugars such as sucrose and lactose must first be broken down by enzymes in the digestive system before they can be used by the body.

Optical and stereoisomerism

Optical isomerism and stereoisomerism are two important concepts in chemistry that apply to carbohydrates. Optical isomerism occurs when a molecule is non-superimposable on its mirror image, while stereoisomerism occurs when two molecules have the same molecular formula and connectivity but differ in their spatial arrangement.

In the case of carbohydrates, they typically contain chiral centers, which are carbon atoms that have four different groups attached to them. These chiral centers can give rise to different stereoisomers, including enantiomers, diastereomers, and epimers.

Enantiomers are a specific type of stereoisomer that are mirror images of each other and cannot be superimposed. Diastereomers, on the other hand, are stereoisomers that are not mirror images of each other and can be differentiated based on their physical and chemical properties. Epimers are a type of diastereomer that differ in their configuration at a single chiral center.

Carbohydrates can also undergo various color reactions that can be used to detect their presence and identify their structure. For example, the Molisch test involves adding a few drops of alpha-naphthol to a carbohydrate solution, followed by the addition of concentrated sulfuric acid. If a carbohydrate is present, a violet ring will form at the interface between the two layers.

Another color reaction is the Benedict's test, which is used to detect the presence of reducing sugars. In this test, a reducing sugar is heated in the presence of Benedict's reagent, which contains copper ions. The reducing sugar reduces the copper ions to form a colored precipitate, indicating the presence of the reducing sugar.

The iodine test is another commonly used color reaction for carbohydrates. In this test, a carbohydrate solution is mixed with iodine, which reacts with the carbohydrate to form a complex that has a characteristic blue-black color. The intensity of the color can be used to determine the amount of carbohydrate present.

Changes in carbohydrates during processing

Carbohydrates are a common component in many food products and undergo various changes during processing. One important reaction that occurs during food processing is the Maillard reaction, which is a non-enzymatic browning reaction that occurs between reducing sugars and amino acids.

The Maillard reaction begins with the condensation of a reducing sugar, such as glucose or fructose, with an amino acid, such as lysine or arginine, to form a glycosylamine intermediate. This intermediate undergoes a series of complex reactions, including rearrangements, dehydration, and fragmentation, to form a variety of compounds that contribute to the characteristic color, aroma, and flavor of many foods.

The Maillard reaction is responsible for the brown color and rich flavor of baked goods such as bread and pastries, as well as the flavor and aroma of coffee and chocolate. However, excessive Maillard reactions can lead to the formation of undesirable compounds such as acrylamide, which is a potential carcinogen.

Other changes that can occur in carbohydrates during processing include hydrolysis, oxidation, and polymerization. Hydrolysis occurs when carbohydrates are broken down into smaller units, such as glucose or fructose, by the addition of water. This process is commonly used to convert starches into simple sugars, which can then be used as a sweetener or fermentation substrate.

Oxidation can also occur during processing, resulting in the formation of carbonyl groups and other reactive species. This can lead to changes in color, flavor, and nutritional quality. For example, the oxidation of ascorbic acid (vitamin C) in fruits and vegetables can lead to a loss of nutritional value and a decrease in shelf life.

Finally, polymerization can occur when carbohydrates are heated or processed under certain conditions, leading to the formation of complex polysaccharides. This process is used to create food products such as gels, thickeners, and emulsifiers.

Overall, understanding the changes that occur in carbohydrates during processing is important for ensuring the quality and safety of food products. The Maillard reaction, in particular, plays a crucial role in determining the flavor and aroma of many foods, but must be carefully controlled to prevent the formation of harmful compounds.

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