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
Applying Modern Methods to Preserve Food
Food preservation prevents or slows down spoilage caused by microorganisms, enzymes, or chemical reactions, extending shelf life while maintaining nutritional value, safety, and quality. This topic covers four key modern preservation methods: chemical preservation, freezing, canning, and bottling.
Chemical preservation uses specific substances called preservatives that inhibit or retard microbial growth by interfering with cell division, cell membrane permeability, and enzyme activity.
Common Chemical Preservatives
- Salt (sodium chloride): Creates high osmotic pressure that draws water out of microbial cells, causing them to shrink and die. Used in fish, meat, and vegetables.
- Sugar: Similar to salt, high sugar concentration creates an environment where microorganisms cannot grow. Used in jams and jellies.
- Acids (vinegar, citric acid, benzoic acid, sorbic acid, propionic acid): Lower the pH of food, creating acidic conditions that inhibit microbial growth. Vinegar is commonly used in pickling.
- Sodium nitrate: Used in processed meats to preserve colour and prevent bacterial growth.
Advantages of Chemical Preservation
- Improves flavour and colour of food
- Relatively easy and affordable to apply
- Effectively extends shelf life by slowing microbial growth
Disadvantages of Chemical Preservation
- Some preservatives (excessive salts and sugars) may cause health concerns like hypertension
- May trigger allergies in sensitive individuals (benzoic acid, sorbic acid)
- Can cause nutrient loss; for example, sulphites reduce vitamin B1 in dried fruits
Worked Example: Preserving Fish with Salt
In the textbook activity, fish was divided into two portions. One portion was rubbed with salt (Plate A), while the other remained untreated (Plate B). After three hours at room temperature:
- Salted fish (A): The salt drew moisture from the fish surface, creating an environment hostile to bacteria. Microbial growth was significantly slowed.
- Unsalted fish (B): Without preservation, bacteria and enzymes broke down the fish faster, leading to quicker spoilage, odours, and texture changes.
This demonstrates how salt acts as a preservative by creating osmotic pressure that inhibits microbial activity.
Freezing preserves food by maintaining temperatures at −18°C or lower. At this temperature, water in food turns to ice, making it unavailable for microorganisms and slowing enzymatic reactions.
How Freezing Works
The low temperature:
- Stops or slows microbial growth significantly
- Reduces enzyme activity that causes ripening and spoilage
- Preserves the food's original structure, flavour, and nutrients when properly handled
Advantages of Freezing
- Retains nutritional content better than canning or drying
- Extends shelf life for months without chemical preservatives
- Maintains flavour and texture of most foods when correctly frozen and thawed
- Works well for both raw and cooked foods
- Reduces food waste by preserving leftovers and surplus
Disadvantages of Freezing
- Requires constant electricity, which can be costly
- Can alter texture of some fruits and vegetables (ice crystals damage cell structures)
- Freezer space is required
- Initial cost of freezer and packaging materials
- Thawing takes time before preparation
Worked Example: Tomato Preservation Experiment
The textbook describes an experiment where tomatoes were stored under three conditions:
| Storage Method | Temperature | Expected Outcome |
|---|---|---|
| Room temperature (A) | ~25°C | Spoils quickly (mould, softening within days) |
| Refrigerator (B) | 0–4°C | Slows spoilage; remains usable for 1–2 weeks |
| Freezer (C) | −18°C | Stays safe for months; texture changes upon thawing |
This demonstrates that lower temperatures more effectively preserve food by reducing microbial and enzymatic activity.
Pressure canning preserves food by sealing it in jars and heating under high pressure, raising the temperature above water's boiling point (240°F or 116°C). This temperature is essential to destroy harmful bacteria, particularly Clostridium botulinum (which causes botulism), and their spores.
Steps in Pressure Canning
- Prepare the canner: Add 2–3 inches of water and heat until hot but not boiling.
- Fill jars: Pack prepared food into clean jars, leaving appropriate headspace.
- Seal jars: Wipe rims clean, place lids on, and screw bands until fingertip tight.
- Process: Place jars in canner, lock lid, and heat until desired pressure (typically 10 pounds) is reached. Maintain this pressure for the time specified in the recipe.
- Cool: Rapidly cool cans to prevent further cooking and maintain quality.
Advantages of Pressure Canning
- Extends shelf life for months or even years
- Provides convenient ready-to-eat foods
- Preserves seasonal foods for year-round access
- Relatively compact storage
Disadvantages of Pressure Canning
- Some vitamins and minerals may degrade during high-temperature processing
- Time and labour-intensive process
- Requires specialized equipment (jars, lids, pressure canner)
- Can cause flavour and texture changes in some foods
- Requires knowledge of correct procedures to avoid health risks (botulism)
- Risk of accidents (glass jars cracking)
Bottling is similar to canning but typically uses glass containers. It is commonly used for high-acid foods like fruits, jams, jellies, pickles, and sauces. The process involves heating food in glass bottles and creating an airtight seal to prevent recontamination.
Bottling Process
- Prepare and cook the food (e.g., make jam or pickle solution)
- Fill clean, sterilized glass bottles with the food while hot
- Wipe bottle rims clean
- Seal with sterilized lids while the food is hot
- Process in a water bath (for high-acid foods) or pressure canner (for low-acid foods)
- Allow to cool and check seals
Advantages of Bottling
- Glass containers are reusable and environmentally friendly
- Allows visibility of contents
- Maintains flavour and quality of fruits and preserves
- Creates airtight seal preventing contamination
Comparison of Preservation Methods
| Method | Best For | Key Principle | Shelf Life |
|---|---|---|---|
| Chemical (salt/sugar) | Fish, meats, jams | Osmotic pressure inhibits microbes | Months to years |
| Freezing | Vegetables, fruits, meats, prepared foods | Low temperature stops microbial growth | Months |
| Canning | Vegetables, meats, soups | High heat destroys microbes; airtight seal | Months to years |
| Bottling | Fruits, jams, pickles, sauces | Heat + acid + airtight seal | Months to years |
In Tanzania, these preservation methods are highly relevant for household food security and small-scale food businesses. For example, a market vendor in Dar es Salaam selling fresh fish by the beach can use salt preservation to extend the fish's usability from one day to several days, reducing losses during periods of low demand. Similarly, families in rural areas can freeze seasonal fruits and vegetables harvested during harvest season for consumption during the dry season when fresh produce is scarce or expensive, helping to manage household food budgets effectively.
Swali
At what temperature should food be maintained during freezing to preserve it effectively?
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