Food Science and Nutrition: Lipids

Lipids

Introduction

Lipids are a diverse group of organic compounds, essential to human health and integral to numerous biological functions.

Classification of Fatty Acids

Saturated Fatty Acids

Definition: Saturated fatty acids have no double bonds between carbon atoms. They are "saturated" with hydrogen atoms.

Characteristics: These acids are typically solid at room temperature and found in animal fats, dairy products, and some plant-based oils like coconut and palm oil.

Example: Stearic acid, found in beef fat.

Monounsaturated Fatty Acids

Definition: These contain one double bond in the fatty acid chain.

Characteristics: Liquid at room temperature but start to solidify when chilled. They are found in a variety of foods and oils.

Example: Oleic acid, predominant in olive oil.

Polyunsaturated Fatty Acids

Definition: Characterised by the presence of more than one double bond in the molecule.

Characteristics: Generally liquid at room temperature and remain liquid when refrigerated. Found in various plant-based oils.

Example: Linoleic acid, commonly found in sunflower oil.

Essential Fatty Acids

Definition: These are polyunsaturated fats that the human body cannot produce and must be obtained through diet.

Examples: Alpha-linolenic acid (an omega-3 fatty acid) and linoleic acid (an omega-6 fatty acid).

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Food Science and Nutrition: Lipids

Cis and Trans Fatty Acids

Cis Fatty Acids: The hydrogen atoms are on the same side of the double bond, causing a bend or a "cis" formation.

Trans Fatty Acids: Hydrogens are on opposite sides of the double bond, resulting in a straighter chain. These are often found in partially hydrogenated oils and are associated with various health risks.

Composition and Structure of Lipids

Structure of Saturated, Monounsaturated, and Polyunsaturated Fatty Acids

Saturated Fatty Acids: Consist of long chains of carbon atoms with single bonds. Each carbon atom is saturated with hydrogen atoms.

Monounsaturated Fatty Acids: Have one double bond within the carbon chain, causing a kink at the point of the double bond.

Polyunsaturated Fatty Acids: Feature multiple double bonds in the carbon chain, causing more kinks and bends.

Chemical Composition and Molecular Structure of a Triglyceride

Definition: A triglyceride is a type of lipid molecule formed from three fatty acids and one glycerol molecule.

Composition: The glycerol backbone is the same in all triglycerides, while the fatty acids can vary, determining the properties of the fat.

Formation: Triglycerides are formed by a condensation reaction where three fatty acid molecules bind to one glycerol molecule, releasing water.

Variability: The nature of the fatty acids (saturated, monounsaturated, or polyunsaturated) impacts the physical properties of the triglyceride, such as melting point and health implications.

Exam Focus: Questions and Answers

1. Explain the difference between saturated and unsaturated fatty acids.

Saturated fatty acids have no double bonds between carbon atoms and are typically solid at room temperature. Unsaturated fatty acids (including monounsaturated and polyunsaturated) have one or more double bonds and are usually liquid at room temperature.

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Food Science and Nutrition: Lipids

2. Describe the molecular structure of a triglyceride.

A triglyceride consists of three fatty acid chains attached to a single glycerol backbone. This attachment occurs through a condensation reaction where each fatty acid releases a water molecule as it binds to the glycerol.

Conclusion

Understanding the classification of fatty acids and the composition and structure of lipids is vital for a comprehensive grasp of human nutrition and biochemistry. These components play crucial roles in cell membrane structure, energy storage, and insulation, underscoring their importance in our diet and health.

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Food Science and Nutrition: Lipids

Lipids–Properties

Melting, Smoke, and Flash Points of Lipids

Melting Point

Definition: The temperature at which a solid lipid turns into a liquid.

Example: The melting point of butter is typically around 32-35°C (90-95°F). This property is crucial in baking, where the melting of butter contributes to the texture and richness of baked goods.

Smoke Point

Definition: The temperature at which lipid begins to produce a continuous bluish smoke, indicating the decomposition of glycerides.

Example: Olive oil has a smoke point ranging from 190-220°C (374-428°F). This makes it suitable for medium-heat cooking methods like sautéing but not for high-temperature frying.

Flash Point

Definition: The temperature at which lipid vapours ignite in air.

Example: Vegetable oils, such as canola oil, generally have flash points around 327°C (620°F), which is significantly higher than their smoke points. This high flash point indicates a higher safety margin in cooking processes.

Emulsions

• The process where two immiscible liquids (like oil and water) are mixed to form a stable blend.
• Emulsifying Agents: Substances that stabilise emulsions.
• Stabilisers: Additives that help maintain the emulsified state and improve the shelf-life and texture of the product.
• Example: Mayonnaise is a classic example of an oil-in-water emulsion, stabilised by the emulsifying properties of lecithin found in egg yolk.

Plasticity

• The ability of fat to be shaped or moulded without breaking.
• Example: Margarine demonstrates plasticity, allowing it to be spread on bread at room temperature.

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Food Science and Nutrition: Lipids

Rancidity

The degradation of lipids leading to unpleasant flavours and odours. There are two types:

Hydrolytic Rancidity: Occurs when water causes the breakdown of triglycerides into fatty acids.

Oxidative Rancidity: Caused by the reaction of fats with oxygen, leading to off-flavours and odours.

Hydrogenation

• A chemical process where unsaturated fats are turned into saturated fats.
• Hydrogen gas is forced through the double bond in an unsaturated oil, in the presence of a nickel catalyst, converting the unsaturated oil into a saturated solid fat.
• Example: Margarine is often produced by hydrogenating vegetable oils, solidifying them at room temperature.

Antioxidants

Substances that prevent oxidative rancidity in lipids. They work by donating an electron to free radicals, thereby neutralising them

Example: Vitamin E is a natural antioxidant found in vegetable oils, nuts, and seeds, protecting these foods from rancidity.

Exam Focus: Questions and Answers

1. Describe how the smoke point of an oil affects its culinary use.

The smoke point of an oil determines the types of cooking it is best suited for. Oils with high smoke points, like avocado oil (smoke point around 271°C or 520°F), are ideal for high-heat cooking methods such as deep-frying. In contrast, oils with lower smoke points, like extra virgin olive oil, are better for lower-heat methods or dressings.

2. Explain the role of antioxidants in maintaining the quality of lipids.

Antioxidants in lipids, such as Vitamin E in olive oil, prevent oxidative damage. They neutralise free radicals, thereby protecting the oil from rancidity, which can adversely affect flavour and nutritional value.

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Food Science and Nutrition: Lipids

Lipids-Formation of Emulsions

Introduction

An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable). Common examples include mayonnaise, milk, and vinaigrettes.

Basic Components of an Emulsion

Dispersed Phase

This is the liquid that is distributed in the form of small droplets within another liquid.

Continuous Phase

This is the liquid in which the droplets are dispersed.

The Role of Emulsifiers

Emulsifiers are substances that help stabilise emulsions by reducing the surface tension between the two liquids.

Function

They have both hydrophilic (water-loving) and hydrophobic (water-fearing) parts which allow them to interact with both water and oil.

• Hydrophilic Part: This part of the molecule is attracted to water. It usually consists of polar groups like hydroxyl (-OH) or carboxyl (-COOH).
• Hydrophobic Part: This part is attracted to oils and fats. It typically consists of long carbon chains or non-polar groups.

Mechanism of Emulsification

Dispersion

When an emulsifier is added to a mixture of oil and water, the hydrophobic end of the emulsifier aligns towards the oil, while the hydrophilic end aligns towards the water.

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Food Science and Nutrition: Lipids

Stabilisation

As a result, the emulsifier forms a protective barrier around the oil droplets, preventing them from coalescing (coming together) and separating from the water. This action stabilises the droplets in the continuous phase, forming an emulsion.

Types of Emulsions

There are mainly two types of emulsions:

• Oil-in-Water: Where oil droplets are dispersed in water (e.g., milk).
• Water-in-Oil: Where water droplets are dispersed in oil (e.g., butter).

Factors Influencing Emulsion Formation

• Amount of Emulsifier: Adequate emulsifier is needed to cover all the dispersed droplets.
• Mixing: Vigorous mixing or agitation is required to disperse the droplets and allow the emulsifier to do its job.
• Temperature: Some emulsions are temperature sensitive and may break or separate at high or low temperatures.

Applications in Food

• Mayonnaise: An example of an oil-in-water emulsion where egg yolk is used as the emulsifier. The lecithin in egg yolk is a common natural emulsifier.
• Salad Dressings: Often vinaigrettes, where vinegar (water phase) is emulsified with oil using emulsifiers like mustard.

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Food Science and Nutrition: Lipids

Lipids-Functions, Energy Value, Digestion, and Absorption

Introduction

Lipids play a vital role in the human diet, offering numerous health benefits and functioning as a major source of energy.

Functions of Lipids in the Diet

Energy Storage

Lipids are a concentrated source of energy, providing 9 kcal/g, which is more than double the energy provided by carbohydrates or proteins.

Cell Structure

Lipids are key components of cell membranes, providing structural integrity to cells.

Insulation and Protection

Fats act as insulators, maintaining body temperature, and cushioning vital organs against shock.

Transport of Fat-Soluble Vitamins:

Lipids aid in the absorption and transportation of fat-soluble vitamins (A, D, E, K) throughout the body.

Hormone Production

Lipids are essential in the synthesis of various hormones, including sex hormones and corticosteroids.

Energy Value of Lipids

High Caloric Density

Lipids provide about 30-35% of the total daily caloric intake in a balanced diet.

Satiety and Flavour

Fats contribute to the feeling of fullness after eating and enhance the flavour and texture of foods.

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Food Science and Nutrition: Lipids

Digestion and Absorption of Lipids

Hydrolysis of Lipids

• Begins in the mouth with lingual lipase, continuing in the stomach with gastric lipase.
• The major digestion occurs in the small intestine, where bile emulsifies fats, and pancreatic lipase breaks them down into fatty acids and glycerol.

Digestion Sequences

Emulsification: Bile salts emulsify large fat globules into smaller droplets. Enzymatic Action: Pancreatic enzymes further break down these droplets into free fatty acids and mono-glycerides.

Absorption Mechanism

The end products of lipid digestion are absorbed through the intestinal lining into the lymphatic system before entering the bloodstream.

Utilisation of Triglycerides

Triglycerides are reassembled in the body's cells and either used for energy or stored in adipose tissue for later use.

Exam Focus-Questions and Answers

1. Describe the process of lipid digestion in the human body.

Lipid digestion begins in the mouth and stomach with minor contributions from lingual and gastric lipases. The majority of lipid digestion occurs in the small intestine, where bile emulsifies fats into smaller droplets, and pancreatic lipase breaks them down into free fatty acids and mono-glycerides for absorption.

2. What are the primary functions of lipids in the diet?

Lipids provide a concentrated source of energy, are essential for the structure of cell membranes, offer insulation and protection to the body, assist in the transport of fat-soluble vitamins, and are necessary for hormone production.