Mada za sehemu hiiProcess and preserve different types of foodMada 8
- Apply modern methods to process food (milling, canning and bottling, fermentation, pasteurization, sterilization, dehydration, pickling)
- Conduct laboratory analysis to determine the effect of heat and air on selected foods
- Carry out laboratory analysis to determine the actions of raising agents
- Describe the biochemistry of food preservation (principles and modern methods of food preservation)
- Apply modern methods to preserve food (addition of chemicals, freezing, canning and bottling)
- Conduct laboratory analysis to determine the effects of preservation on selected foods
- Conduct laboratory analysis to identify effect of acids and alkali on food
- Carry out laboratory analysis to determine spoilage microorganisms in food
Heat fundamentally changes foods by altering their chemical structure. When foods are heated, carbohydrates gelatinize or dextrinize, proteins denature and coagulate, fats melt and break down, and heat-sensitive vitamins are destroyed. These changes affect texture, flavour, colour, and nutritional value.
Effect of Heat on Carbohydrates
Carbohydrates undergo several important changes when heated:
Gelatinization occurs when starch is heated in water. Starch granules absorb water and swell, breaking hydrogen bonds within the granules. Amylose leaches out, and the mixture forms a viscous paste. This happens at temperatures between 60–70°C. For example, when potatoes are boiled, the starch granules swell and soften the tuber, making it edible.
Dextrinization occurs during dry heat treatment, such as roasting or grilling. Long starch chains break into smaller soluble molecules called dextrins, giving foods a golden-brown colour and nutty flavour. This is observed when wheat flour is heated in a dry pan until light brown.
Caramelization happens when sugars are heated above 160°C. Sugar melts and breaks down into compounds that produce a golden-brown colour and rich flavour. This is commonly seen when sugar is heated to make caramel for sweets.
Maillard Reaction involves reducing sugars reacting with proteins under heat. This produces browning, flavour development, and aroma in foods like roasted meat, bread, and coffee. The reaction gives a golden-brown colour and creates appealing flavours and aromas.
Effect of Heat on Proteins
Proteins denature when heated—the heat breaks bonds maintaining the protein's natural structure, causing it to unfold. Further heating leads to coagulation, where denatured proteins clump together and form a solid mass. For instance, when an egg is boiled, the transparent egg white becomes opaque and firm as proteins denature and coagulate. Excessive heat can make proteins tough and reduce digestibility.
Effect of Heat on Fats
At moderate temperatures, solid fats melt into liquids. When heated to their smoke point, fats begin to decompose and release smoke. Butter smokes at around 150°C, while sunflower oil smokes at approximately 227°C. Continued heating breaks fats into glycerol and fatty acids, which can further decompose into acrolein—a pungent compound that makes food unpleasant. Heated fats also react with oxygen to form peroxides and aldehydes, which cause rancid flavours.
Effect of Heat on Vitamins
Heat-sensitive vitamins, particularly vitamin C and B vitamins, are destroyed during cooking. These water-soluble vitamins also leach into cooking water. Fat-soluble vitamins (A, D, E, K) are more stable but can still be reduced by prolonged high heat. Gentle cooking methods like steaming, microwaving, or stir-frying help preserve more vitamins compared to prolonged boiling.
Exposure to air causes biological, physical, and chemical changes in food, affecting colour, quality, safety, and shelf life.
Enzymatic Browning
When fruits and vegetables are cut, bruised, or peeled, they turn brown due to enzymatic browning. The enzyme polyphenol oxidase catalyzes a reaction between phenolic compounds and oxygen, forming brown pigments. This occurs in apples, potatoes, avocados, and bananas when exposed to air.
Factors affecting enzymatic browning include pH, temperature, and oxygen availability. Browning can be reduced by:
- Keeping food in airtight containers
- Storing food in water to limit oxygen contact
- Adding lemon juice or vinegar to lower pH
- Using antioxidants like ascorbic acid
- Blanching (brief heating) to inactivate enzymes
- Cold storage to slow enzyme activity
Oxidation Reactions
Air contains oxygen, which reacts with food components. In fats, oxidation produces peroxides and aldehydes, causing rancidity. In meats, oxidation can cause colour changes and off-flavours. Vitamin C is also easily oxidized when exposed to air.
A proper laboratory analysis involves systematic experimentation, observation, recording, and reporting.
Planning the Experiment
- Select food samples appropriate for testing heat and air effects (e.g., apple slices for air, potato slices for heat)
- Identify variables: independent variable (treatment), dependent variable (observed change), controlled variables (temperature, time, sample size)
- Prepare materials: equipment, reagents, documentation sheets
- Set up control samples for comparison
Conducting the Analysis
For heat effects:
- Prepare food samples in consistent sizes
- Apply different heat treatments (boiling, roasting, frying)
- Maintain consistent timing and temperature
- Observe and record changes in colour, texture, smell, and appearance
For air effects:
- Prepare identical food samples
- Apply different treatments: control (exposed to air), wrapped in plastic, treated with lemon juice, blanched
- Observe over a set period (e.g., 60 minutes)
- Record colour changes using a scoring system or photographs
Recording Observations
Create a data table to record:
- Treatment applied
- Time of observation
- Colour changes (use descriptive terms or colour charts)
- Texture changes (firm, soft, mushy)
- Smell changes
- Signs of spoilage (mould, liquid release, decay)
Writing the Report
A proper laboratory report should include:
- Title of the experiment
- Objective: what the experiment aims to determine
- Materials and equipment: all items used
- Procedure: step-by-step method followed
- Results: observations recorded in tables
- Discussion: explanation of results using scientific principles
- Conclusion: whether objectives were met
- Recommendations: suggestions for improvement
Sharing Findings
After completing the analysis, present findings to the class or group. Discuss:
- Which treatments were most effective
- Why certain methods prevented undesirable changes
- How results relate to food preservation principles
- Practical applications of findings
In Tanzania, understanding the effects of heat and air on food directly helps household food preservation. For example, when drying fish or cassava strips (kitaisi) using solar drying, knowing that air exposure causes oxidation and heat destroys vitamins helps farmers and homemakers choose appropriate drying times and storage methods—storing dried foods in airtight containers prevents reabsorption of moisture and slows spoilage, reducing losses that could otherwise cost a family tens of thousands of shillings in wasted produce during the dry season.
Swali
At what temperature range does starch gelatinization occur during heating in water?
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