Carbohydrates (Leaving Cert Home Economics): Revision Notes
Carbohydrates
Carbohydrates are one of the main macronutrients and serve as the body's primary source of energy. They are organic compounds made up of carbon, hydrogen, and oxygen atoms, typically in a 1:2:1 ratio. Understanding carbohydrates is essential for nutrition and food preparation.
Composition and structure
Basic building blocks
Monosaccharides are the simplest units of carbohydrates, often called single sugars. They contain carbon, hydrogen, and oxygen atoms and can exist as either straight chains or ring structures.
Monosaccharides form the foundation of all carbohydrate structures and are the end products of carbohydrate digestion.
Common examples of monosaccharides include:
- Glucose - found in fruits, vegetables, and honey
- Fructose - abundant in fruits and honey
- Galactose - typically found as part of lactose in milk
How larger carbohydrates form
More complex carbohydrates develop when simple sugars join together through a process called dehydration synthesis. During this reaction:
- Two monosaccharides combine
- A water molecule is removed
- A glycosidic bond forms between the sugar units
This process creates disaccharides (two sugar units) and eventually polysaccharides (many sugar units linked together).
The reverse process, called hydrolysis, breaks down complex carbohydrates by adding water molecules back, which is essential during digestion.
Classification of carbohydrates
Monosaccharides
These are the simplest form and include glucose, fructose, and galactose. They dissolve easily in water and provide quick energy to the body.
Disaccharides
Disaccharides form when two single sugars bond together, creating double sugar molecules.
Disaccharides must be broken down into monosaccharides before the body can absorb and use them for energy.
Key examples include:
- Sucrose (glucose + fructose) - common table sugar from sugar cane and beets
- Lactose (glucose + galactose) - milk sugar found in dairy products
- Maltose (two glucose units) - produced when starch breaks down during seed germination
Polysaccharides
These complex carbohydrates contain many sugar units joined together and serve different purposes:
Energy storage polysaccharides:
- Starch - plants store energy in this form, found in potatoes, grains, and legumes
- Glycogen - animals store energy this way, mainly in liver and muscles
Structural polysaccharides:
-
Cellulose - provides structure to plant cell walls, found in all plant foods
-
Non-starch polysaccharides (dietary fibre) - includes cellulose, pectins, and gums from fruits, vegetables, and whole grains
Properties and behaviour
Sweetness characteristics
Simple carbohydrates like glucose and fructose taste sweet, with fructose being the sweetest. The level of sweetness decreases as carbohydrates become more complex - starch, for example, has no sweet taste.
Water solubility
- Monosaccharides and disaccharides dissolve readily in water
- Polysaccharides like starch have limited solubility in cold water but can absorb water and swell when heated
Important cooking processes
Gelatinisation happens when starch granules absorb water and swell during heating, creating thicker mixtures essential for sauces and baked goods.
Understanding gelatinisation is crucial for successful cooking - it's the process that transforms liquid batters into solid cakes and thin sauces into thick gravies.
Other key processes:
- Hydrolysis - breaking down complex carbohydrates into simpler forms using water and enzymes
- Caramelisation - browning of sugars when heated, creating new flavours and colours
- Crystallisation - formation of sugar crystals from concentrated solutions
- Dextrinisation - breakdown of starch during toasting, causing colour and flavour changes
Worked Example: Gelatinisation in Cooking
Step 1: Starch granules in flour are mixed with cold liquid (no thickening occurs)
Step 2: Mixture is heated to 60-70°C, starch granules begin to absorb water and swell
Step 3: At 80-85°C, granules burst and release starch molecules, creating a thick gel
Result: Transformation from thin liquid to thick sauce or solid structure
Functions in the body and food
Biological roles
Carbohydrates serve several vital functions:
- Quick energy source - sugars provide immediate fuel for brain function and muscle activity
- Energy storage - starch in plants and glycogen in animals store energy for later use
- Digestive health - dietary fibre aids digestion, prevents constipation, and supports weight management
The brain requires approximately 120g of glucose daily and relies almost exclusively on carbohydrates for energy under normal conditions.
Culinary applications
In food preparation, carbohydrates:
- Add sweetness and enhance flavour
- Provide structure in baking through starch gelatinisation
- Act as thickening agents in sauces and gravies
- Serve as gelling agents (like pectin in jams)
- Contribute to food preservation
Energy value and digestion
Nutritional contribution
Carbohydrates provide 4 kilocalories per gramme and represent the primary energy source in most diets.
Carbohydrates should make up 45-65% of total daily caloric intake according to dietary guidelines, making them the most important macronutrient by volume.
The main dietary sources include grains, fruits, vegetables, and dairy products. Health guidelines recommend consuming 25-30 grammes of dietary fibre daily through varied plant foods.
How the body processes carbohydrates
The body systematically breaks down carbohydrates through a coordinated digestive process:
Digestion process:
- Mouth - salivary amylase begins breaking down starch
- Small intestine - pancreatic amylase continues starch breakdown
- Intestinal lining - final enzymes convert disaccharides to monosaccharides
Absorption and use:
- Simple sugars absorb directly into the bloodstream
- The liver processes these sugars for immediate energy or storage
- Excess glucose converts to glycogen or, if storage is full, to fat tissue
Worked Example: Carbohydrate Digestion Timeline
Step 1: Starch consumption (e.g., slice of bread)
Step 2: 0-30 seconds - salivary amylase begins starch breakdown in mouth
Step 3: 1-4 hours - pancreatic amylase completes breakdown in small intestine
Step 4: Final enzymes convert remaining disaccharides to glucose
Result: Glucose absorption into bloodstream within 15-30 minutes
Health considerations
Disease prevention benefits
Adequate carbohydrate intake, particularly dietary fibre, links to reduced risk of chronic diseases including diabetes and heart disease. Fibre helps regulate blood sugar levels and supports cardiovascular health.
Physical activity support
Carbohydrates provide essential fuel for muscle contraction and endurance during physical activity. Athletes and active individuals require sufficient carbohydrate intake to maintain performance levels.
Balanced approach
Maintaining a healthy balance between carbohydrate intake and physical activity helps prevent lifestyle-related diseases and supports optimal body weight management.
The glycaemic index can help guide food choices - low GI carbohydrates provide more sustained energy release compared to high GI options.
Summary
Key Points to Remember:
- Structure matters - carbohydrates range from simple sugars to complex polysaccharides, each serving different purposes
- Energy powerhouse - providing 4 kcal per gramme, carbohydrates are the body's preferred fuel source
- Cooking applications - gelatinisation, caramelisation, and other processes are crucial for food preparation
- Digestion sequence - enzymes systematically break down complex carbs into absorbable simple sugars
- Health balance - adequate fibre intake (25-30g daily) supports digestive health and disease prevention