# Carbohydrates¶

As with other lectures in this unit, these notes are going to continue on from Food Science - Carbohydrates

Carbohydrates can be defined by a few criteria:

• Hydrates of carbon, with the empirical formula $$\ce{C_{n}(H2O)_{n}}$$
• polyhydroxyaldehydes or polyhydroxyketones or substances that yield such compounds on hydrolysis

Uses include :

• A reservoir of chemical energy ($$15.8\:KJ/g$$ of energy)
• Proteins have slightly less energy per gram and fats have twice as much
• Essential component of RNA and DNA
• Are constituents of supportive structures
• In plants as cellulose
• In crustacea and insects as chitin
• Are constituents of connective tissue in animals
• Acidic polsaccarides such as hyaluronic acid

Plants are the primary producers of carbohydrates, accounting for 75% of the dry weight of the plant, and as such, animals get their supply by eating plants, however they don’t store much of what they consume. Less than 1% of the body weight of a human is carbohydrate. Animals either consume carbohydrates and utilise them quickly or store them as fat.

## Some Common Monosaccharides¶

### D-Glucose (Dextrose or ‘blood sugar’) D-glucopyranose¶

• Found in fruits, vegetables, honey and corn syrups
• This is the building block of disacchardies ans polysaccharides

### Fructose (Levulose or ‘fruit sugar’) D-fructofuranose¶

• Sweetest carbohydrate (2x sucrose)
• Found in fruit juices and honey
• Is converted to glucose in the bloodstream
• Forms sucrose with glucose

### Galactose (galactopyranose)¶

• Does not occur as a monosaccharide in nature
• Obtained from the hydrolysis of lactose
• Important in the cellular membranes of the brain and nervous system

## Properties of ‘Simple Carbohydrates’ (monosaccharides)¶

• They are the sweetest of the sugars and can serve as an indicator of energy-rich foods
• Water soluble, due to their high oxygen content
• Crystallise when evaporated from solution
• The are an energy supply
• The can be fermented by micro-organisms (converted into simple alcohols)
• They are hypertonic and can serve as a preservative at high concentrations
• They caramelise upon heating

## Some Common Disaccharides¶

### Sucrose (‘table sugar’)¶

• Obtained from the evaporation of sugar cane juice (20% by mass) or sugar beets (15% by mass)
• Both raw and refined sugars are sucrose

### Lactose¶

• Found in milk and milk products
• Human milk contains 6-8% and cows milk contains 4-5%
• It’s added to baby formula to mimic mother’s milk

### Maltose (‘malt sugar’)¶

• Obtained from starch
• Used in cereals, candies and the brewing of beverages

## Carbohydrates and human health¶

Most of this you already know, but the glycemia tolerances are as follows:

• Hypo < 40 mg/mL
• Normal 70-60 mg/mL
• Hyper >140 mg/mL

### Galactosemia¶

• Is a condition in which the enzyme that converts galactose to glucose is impaired causing galactose to accumulate in the body
• can cause mental retardation, cataract formation and liver damage
• Infants with the condition have to be given a lactose free diet.

## Polysaccharides¶

Are primarily a storage form for glucose in plants. Thy are stored by plant cells in granules and are easily hydrolysed in water and acid. In the human body, starch is digested by α-amylases in salava.

### Starch¶

Starch is made up of amylose and amylopectin, are not readily soluble in cold water, are not sweet and require cooking to improve digestibility and palatability

#### Amylose¶

• Consists of continuous unbranded chains of ~4000 glucose units
• The units are joined together by α-1,4-glycosidic linkages
• Most starches are made of 20-25% amylose
• Forms a helical coil due to the bond rotation

#### Amylopectin¶

• Consists of branched chains of ~10000 glucose unit
• They are joined together by α-1,4-glycosidic linkages
• They have considerable branching, at every 24-26 glucose units with new chains of 24-30 units that are joined by α-1,6-glycosidic linkages
• Most starches contain 75-80% amylopectin

#### Gelatinisation¶

When the starch granules are heated in water, the granules take up the water and swell. This increases the viscosity of the liquid and forms a gel upon cooling. When these are frozen or aged, they can convert back into the starch granules

#### Dextrinisation¶

When the starches are dry-toasted or roasted, the outer layers will partially decompose to form dextrins. the aldehyde and ketone groups react to form new flavour compounds and give the product more complexity.

### Glycogen (‘animal starch’)¶

• Has the same branched structure as amylopectin (α-1,4-glycosidic bonds with α-1,6-branches), but is much more branched than amylopectin.
• It’s stored in the liver (3-6%) and in muscles (1-2%)
• Small amounts may be stored in the kidneys, glial cells (brain) and white blood cells
• During pregnancy, some will be stored in the uterus
• Acts as an energy reserve of carbohydrates for animals
• In an adult, there will only be a total of ~350g of glycogen
• After slaughter, the glycgen is converted into lactic acid which improves the meat’s tenderness

### Cellulose¶

• Is the major structural material of wood and plants
• Like amylose, it consists of long, unbranded chains of glucose units
• The glucose units are joined with β-1,4-glycosidic bonds
• Humans lack the β-amylase that’s required to break these bonds
• These strands don’t form cils like amylose, but instead form long sheets that are very strongly hydrogen bonded together (to neighbouring subunits and neighbouring strands)
• This makes them water insoluble and rigid
• Makes them difficult to hydrolyse

## Acidic Polysaccharides¶

These contain carboxylic acid groups and sulfuric ester groups ($$\ce{OSO3}$$) and are found only in animals and bacteria. The contain amino sugars and are therefore called glycosaminoglycans. They play an important role in the structure an function of connective tissue and bind with collagen (protein) to form tight, or loose networks

### Hyaluronic Acis¶

• Simplest acidic polysaccharide
• Abundant in embryonic tissues, synovial fluid and vitreous of the eye
• Commonly used in lotions, moisturisers and cosmetics, due to their connective tissue boosting properties

### Heparin¶

• Heterogeneous mixture of variably sulfonated polysaccharides
• synthesised and stored in mast cells of various tissues, particularly in the liver, lungs and gut
• Plays an important role in many biological activities
• Is a prominent anticoagulant so it is commonly used in medicine.