Comprehensive Study Notes on Naturally Occurring Food Constituents: Carbohydrates and Proteins

Definition and Fundamental Chemistry of Carbohydrates

Carbohydrates are naturally occurring food constituents defined as hydrates of carbon. They are organic compounds characterized by the basic chemical structure $C_x(H_2O)_y$. Structurally, they are classified as polyhydroxyl aldehydes or ketones and their various derivatives. Carbohydrates are the primary products of photosynthesis in plants and serve as a vital source of energy for living organisms. The most significant sources of these compounds in the human diet include roots, tubers, and sugar cane. In an ideal diet, carbohydrates should provide approximately $60-70\%$ of total calories consumed. Common examples of carbohydrates found in various food sources include fructose (found in fruit), maltose (found in malt), xylose, and lactose (found in milk).

Classification and Structural Units of Carbohydrates

Carbohydrates are classified based on their molecular structure into simple and complex sugars. Monosaccharides are the simplest sugar units, serving as the building blocks for more complex carbohydrates. They provide quick energy to the body. Examples of monosaccharides include glucose and fructose. Oligosaccharides consist of two to ten monosaccharide units linked together by glycosidic bonds. Examples include sucrose and lactose. Polysaccharides are long chains of hundreds to thousands of sugar molecules connected through glycosidic bonds. Major examples include starch and cellulose.

Characteristics and Profiles of Monosaccharides

Monosaccharides are simple sugars that represent the fundamental building blocks of all complex sugars. Examples include glucose, fructose, and galactose. Their chemical structure can be represented using either the Haworth cyclic structure or the Fischer projection. In terms of functional properties, monosaccharides are essential for storing and producing energy. Physically, they are crystalline in nature, water-soluble ($H_2O$), and typically colorless. Chemically, they often act as reducing sugars because they possess a free aldehyde or ketone group. Specific structural forms include D-glucose and L-glucose as represented in Fischer projections (open-chain form) and cyclic forms as represented in Haworth Projections.

Oligosaccharides and Glycosidic Linkages

Oligosaccharides are defined as carbohydrates composed of 2 to 10 monosaccharides joined together by glycosidic linkages. They are categorised as disaccharides ($2$ units), trisaccharides ($3$ units), or tetrasaccharides ($4$ units) depending on the number of monosaccharide components present. Notable examples include sucrose, lactose, and maltose. The formation of these compounds occurs through specific combinations: Glucose + Fructose results in Sucrose; Glucose + Galactose results in Lactose; and Glucose + Glucose results in Maltose. Structurally, they are represented via Haworth cyclic structures.

Oligosaccharides possess several critical functional properties in food chemistry. They act as humectants, meaning they have the ability to absorb moisture from the air. They also function as plasticizers and sweeteners. In food processing, they serve as flavoring agents and are integral to non-enzymatic browning reactions, specifically the Maillard reaction and caramelization.

Non-Enzymatic Browning Reactions in Food

Browning reactions are non-enzymatic processes responsible for the development of color and flavor in various food products. These reactions are caused by the presence of reactive reducing sugars which contain a free aldehyde or ketone group. Heat-induced changes during these reactions include dehydration, fragmentation, and ring opening. There are two primary pathways for non-enzymatic browning. Caramelization occurs when sugars are subjected to heat in the absence of nitrogenous compounds. The Maillard reaction occurs when amino acids react with reducing sugars under the influence of heat.

Polysaccharides: Structural and Functional Overview

Polysaccharides are large polymers consisting of $100-1000$ monosaccharides joined together. These are typically composed of hexose and its derivatives arranged in linear or branched chains. Examples include dextrin and starch in plants, and glycogen in animals. Structurally, they are represented by Haworth cyclic structures. Their functional properties include being insoluble in water, the ability to form gels, and a globular nature. They can appear translucent or opaque and utilize hydrogen bonding ($H$-bonding) as well as ionic interaction bonding to maintain their structure.

Chemical reactions involving polysaccharides include various forms of hydrolysis. Glycans can be hydrolyzed into glucose, which can then yield glucans, xylose, or xylans. Homoglycans can be hydrolyzed into mannan, fructan, galactan, or araban. Heteroglycans can be hydrolyzed into compounds such as galactomannan and glucomannan.

Starch: Amylose, Amylopectin, and Physical Transformations

Starch is a major polysaccharide that can exist in natural, modified, or refined forms. It is composed of two primary fractions: amylose and amylopectin. Amylose typically makes up $25\%$ of starch and consists of linear chains with $1-4$ glycosidic linkages; it is primarily responsible for the gelling properties of starch. Amylopectin makes up approximately $75\%$ of starch and features branched chains with $1-6$ glycosidic linkages; it provides thickening properties.

Starch undergoes two critical physical changes: gelatinization and retrogradation. Gelatinization occurs when starch granules are heated in the presence of water. The granules absorb water, increasing their moisture content from an initial $12/14\%$ to as much as $30\%$, causing them to swell and thicken. At high temperatures (the gelatinization temperature), intermolecular bonds between starch molecules break, and water is taken into hydrogen bonding sites ($H$-bonding). This process results in a gel-like, less crystalline structure. Retrogradation occurs at low temperatures (e.g., $0^{\circ}C$) during cooling. This involves the realignment of gelatinized starch molecules, resulting in the expulsion of water from the polymer network and the formation of a more crystalline structure.

Non-Starch Polysaccharides and Dietary Fibre

Several other polysaccharides play critical roles in food structure and nutrition. Cellulose is a major constituent, making up $25\%$ of the plant cell wall. It is insoluble in alkali and is embedded in hemicellulose and pectic substances. Cellulose is found in fruits, vegetables, and wooden tissues. While it cannot be digested by humans, it serves as the roughage component of dietary fiber. Hemicellulose is insoluble in water but soluble in alkali. It is composed of pentoses such as xylose and occurs mostly in land plants, forming undigested carbohydrates when combined with cellulose. Pectic substances are constituents of plant cell walls that act as cementing materials. They are insoluble in water, and commercial pectin is often derived from citrus peels or pomace.

Gums are hydrophilic polysaccharides used in applications such as baked foods and foam stabilizers. Common examples include guar, gum Arabic, agar, algin, dextran, and carrageenans. Dietary fiber, also known as roughage, refers to components derived from plant cell walls (cellulose, hemicellulose, pectin, starch, etc.) that are not digested by humans. Dietary fiber is nutritionally important as it helps lower cholesterol levels and controls blood sugar levels.

Protein Chemistry and Structural Levels

Proteins are complex molecules built from amino acids and possess four distinct structural levels. The primary structure is the specific sequence of amino acids in a polypeptide chain. The secondary structure refers to the localized folding of the chain into alpha ($\alpha$) helices and beta ($\beta$) sheets. The tertiary structure is the overall three-dimensional folding of the units of secondary structure into a specific spatial arrangement, often driven by the positioning of non-polar residues. The quaternary structure involves the spatial arrangement of multiple tertiary structures or peptide chains coming together to form a native protein molecule.

Denaturation is a process where proteins lose their native conformation, leading to changes in their properties and identity. This can be caused by heat, changes in $pH$, or mechanical action. A common example of denaturation is the change of egg whites from clear to white during cooking. Proteins serve critical functional roles in food products. Enzymes like amylase are proteins that break down starches during digestion and brewing. In bread-making, gluten proteins (specifically glutenin and gliadin) form elastic networks when hydrated and kneaded. This matrix traps $CO_2$ and provides bread with its characteristic chewy texture.

Amino Acids: The Building Blocks of Proteins

Amino acids are the constituents of proteins, containing both a basic amino group ($-NH_2$) and an acidic carboxyl group ($-COOH$) attached to a central carbon atom. They principally consist of carbon, hydrogen, oxygen, and nitrogen, and sometimes sulfur. Amino acids are classified by the nature of their R-groups. Negatively charged R-groups are hydrophilic, such as aspartic acid. Non-polar R-groups are hydrophobic and include proline (and its derivative hydroxyproline), alanine, valine, leucine, and isoleucine. Polar uncharged R-groups include glycine, serine, threonine, and cysteine. Positively charged R-groups include lysine, arginine, and histidine.

Peptide Bond Formation and Protein Classification

Peptides are formed when the amino group of one amino acid reacts with the carboxyl group of another. This reaction forms a peptide bond through the elimination of a molecule of water ($H_2O$). Chains of amino acids are categorized as oligopeptides, polypeptides, or full proteins depending on their length.

Proteins are classified based on their composition. Simple proteins yield only amino acids upon hydrolytic degradation; examples include albumin, globulin, glutelin, prolamine, histones, and protamin. Conjugated proteins yield both amino acids and a non-protein prosthetic group upon hydrolysis. Examples include hemoglobin (protein + heavy metals/metalloprotein), lipoproteins (protein + lipids like triglycerides, phospholipids, or cholesterol, e.g., HDLP, LDLP, VLDLP), glycoproteins (containing hexosamine, galactose, mannose, or sialic acid, e.g., ovomucoid in egg white), nucleoproteins (protein + nucleic acids like DNA or RNA), and phosphoproteins (conjugated with inorganic phosphate, e.g., casein or pepsin). Derived proteins include products like proteans, metaproteins, peptones, and peptides.

Functional Properties and Food Sources of Proteins

Proteins exhibit specific physical properties based on their amino acid arrangement. Hydrophilic R-groups typically orient toward the exterior of the molecule, while hydrophobic R-groups orient toward the interior. Proteins behave as electrolytes and can be hydrolyzed by acids, alkalis, or enzymes. Putrefaction refers to the deamination of proteins by microbial proteases, which alters flavor and texture. Protein gels are formed when dry proteins absorb water; these gels can immobilize water up to ten times the weight of the hydrated protein. This process is associated with syneresis. Examples include gelatin and casein.

Food proteins are derived from both plant and animal sources. Plant proteins include lysine and tryptophan in potatoes; prolamines, gliadins, and glutelins in endosperm; globulins and albumins in the germ of seeds; and prolamine and glutelins in rice. Oil seeds and nuts provide lysine and methionine, while pulses provide methionine. Animal protein sources include myoglobin in red meat, casein and whey in milk, lipoproteins in egg yolk, and various enzymes in fish.

Nutritional Importance of Proteins

Proteins play both functional and nutritive roles in human health. Functionally, they contribute to the formation of gels and emulsions, which influence the color and flavor of foods. Nutritively, proteins supply nitrogen and amino acids. They serve as an energy source, supplying $10-14\%$ of total dietary energy, with $1\,g$ of protein providing $4.1\,kcal$.

There are approximately $20$ amino acids required by the human body. Nine of these are considered essential amino acids because they cannot be synthesized by humans in adequate amounts to sustain growth and health and must be obtained through the diet. These nine essential amino acids are: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.