Analyze the chemical changes that occur using different cooking ingredients for different foods

    Contents:-

v Introduction

• What is cooking?
• Why we cook food?
• What are the main types of cooking?

v What happens to food when it is cooked?

Introduction:-

At present, conclusive evidence eludes us as to the origins of cooking. Some researchers believe that cooking was invented over 2.3 million years ago, whilst others argue it is a more recent concept, being invented only 40,000 years ago. Despite these opposing views, it is clear that cooking has been around for a long time and continues today to play a fundamental role in daily life across the globe. Cooking was first used for preservation but it has evolved and now it is a form of entertainment and creativity for many people. Currently using the search term "cooking" in Google produces a search result containing over 40 million blog posts and news alerts. These include articles ranging from cooking recipes and creative cooking classes to games using a cooking theme as a platform.

What is cooking?

Cooking is the process of producing safe and edible food by preparing and combining ingredients, and (in most cases) applying heat. Cooking is a means of processing food, without which many foods would be unfit for human consumption.

Why do we cook food?  

1.     Safety

Raw foods such as meat, fish and eggs, may harbour food poisoning bacteria, which if consumed are likely to cause illness. The optimum temperature for the multiplication of most food poisoning bacteria is between 5 - 63^oC, whilst, at temperatures over 70^oC most bacteria are killed and below 5^oC most food poisoning bacteria can only multiply slowly or not at all. Most cooking methods if performed properly will heat foods to over 70 ^oC, so applying such a temperature for a carefully calculated time period (along with correct food preparation and storage procedures) will prevent many food borne illnesses that would otherwise manifest if the raw food was eaten.  

Campylobacter, Salmonella and Listeria monocytogenes are three of the most common food poisoning bacteria and together are reported to affect over 380,000 European Union (EU) citizens each year. Table 1 lists the foods these bacteria are most likely to be found in and the symptoms they commonly cause. 

Table 1: Common food poisoning bacteria and their likely food sources and symptoms

2. Digestibility

The fundamental reason we consume food is to extract the vital nutrients that different foods contain to allow our bodies to function properly. This is achieved by digestion, where foods are broken down in the body into a form that can be readily absorbed. However, many of the nutrients contained in foods are not readily accessible prior to cooking and thus, cannot be easily digested by the body. For example, the enzyme amylase (found in the mouth and intestine) breaks down the polysaccharide starch into its monomer glucose constituents, which can easily be digested by the body. Cooking foods containing starch (e.g., cereals and vegetables), prior to consumption initiates the breakdown of the polysaccharide, thus, aiding the action of amylase and the consequent digestibility of the carbohydrate component of the food.

3.Edibility

The desire to eat is primarily driven by the body's need for nutrition, with the intake of essential nutrients being indispensable for life. This fundamental reason to eat is challenged by the psychological needs of enjoyment and pleasure. Cooking can cause changes in the colour, flavourand texture of foods that allow us to create foods that we derive pleasure from eating. For example, roasting potatoes initiates a series of changes that makes them edible, as well as attractive in colour and taste by generating a golden brown colour, invoking a natural sweetness and producing a crisp shell and a soft internal texture.

For many foods, the cooking process gives them the characteristics we associate with edible food, which are generated through an intricate series of physical and chemical changes that occur when foods are heated. Therefore, without cooking, these changes could not occur and many foods would be deemed inedible.

What are the main types of cooking?

The fundamental types of cooking from which cooking methods stem across Europe and indeed, the world, are listed below.

1.Frying

Frying is the cooking of food in oil or fat. Usually, foods that have been fried have a characteristic crisp texture. This is because oils and fats can reach higher cooking temperatures than water, which results in the food being seared. Common types of foods that are fried include; battered or breaded fish or vegetables, crisps, chips and doughnuts.

2.Baking

Baking is the process of cooking foods in the dry heat of an oven. During baking, moisture within the food is converted to steam, which combines with the dry heat of the oven to cook the food. Common types of foods that are baked include; bread, cakes, jacket potatoes, and pastries.

3. Boiling

Boiling is the cooking of foods in a liquid (e.g., water, milk or stock), which is at boiling point. Common types of foods that are boiled include; vegetables, rice and pasta.

What happens to food when it is cooked?

Heating causes a complex series of physical and chemical changes to occur. These changes vary depending on the type of food being cooked and the method used to cook it. The changes may be advantageous e.g., improving the flavour, texture and colour of the food, or they may be disadvantageous e.g., reducing the nutrient value of the food, or the generation of undesirable compounds. The main chemical changes that occur during the cooking of foods are discussed below.

1.Caramelisation

Caramelisation produces the desirable flavours and colours (see 'colour' section), that are characteristic of many food products such as dark beer, coffee, confectionery and peanuts. The caramelisationreaction occurs when foods containing a high concentration of carbohydrates are cooked at high temperatures using a dry-heat e.g., roasting peanuts, setting-off a chain of chemical reactions. In actual fact, caramelisation generates hundreds of flavour compounds. One of the most important flavour compounds produced is diacetyl. Diacetyl is generated during the initial stages of caramelisation and has a butterscotch flavour, which provides one of the characteristic flavours of caramelised foods.

2.Maillard reaction

The Maillard Reaction is one of the most complex chemical reactions that occurs in the food we eat and is responsible for generating many of the flavour and colour (see 'colour' section) characteristics that we associate with a wide range of cooked foods e.g., toast, meat, coffee and baked goods.
The Maillard Reaction is essentially a chemical reaction between an amino acid and a sugar such as glucose, fructose or lactose. Usually, heat is required to start the reaction that causes a cascade of chemical changes, which, ultimately, result in the formation of a range of flavour and colour compounds.

3. Starch degradation

The polysaccharide starch is present in all plant seeds and tubers, which meanit can be found in many foods such as pasta, rice, bread, potatoes and oats. It is a common form of carbohydrate, composed of several thousand glucose units, linked together by glycosidic bonds. When foods containing starch are cooked, the heat can break the glycosidic bonds linking the glucose units together and effectively break-up the polysaccharides to release the glucose monosaccharides. This imparts a natural sweetness to the cooked food.

4.Loss of pigmentation

The main foods containing pigments and therefore likely to be prone to losing pigmentation during cooking, are fruits and vegetables. There are three families of pigments found in fruit and vegetable plants, which vary depending on the ripening stage of the plant and the cooking method: chlorophyll - green pigment, carotenoids -pigments ranging from yellow to deep red, and the flavonoids; anthocyanins - red, blue or purple pigments according to the pH, and anthoxanthin - white pigment.  

5.Protein denaturation

Many foods contain proteins, such as meat, fish, eggs, vegetables, nuts and pulses. Proteins are large molecules, composed of strands of amino acids, which are linked together in specific sequences by the formation of peptide bonds. Proteins form different 3-dimensional structures, by the folding and subsequent bonding of the amino acid strands. Generally, the bonds which link the folded amino acid strands together (mostly hydrogen bonds), are much weaker than the strong peptide bonds forming the strands.

During cooking, the heat causes the proteins to vibrate violently, which results in the breakage of the weak hydrogen bonds holding the amino acid strands in place. Ultimately, the protein unravels to re-take its initial form of amino acid strands. 

6. Polysaccharide degradation

Many plant foods, in particular vegetables, maintain their rigidity by the incorporation of polysaccharides such as cellulose and pectin in the plant walls. As with the degradation of starch, cellulose and pectin can also be broken down into their monosaccharide constituents during cooking, resulting in the substantial softening of foods containing these polysaccharides.

TABLE 2: Summary table of the effects of different cooking methods on the properties of cooked foods