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Questions and Answers
Which factor does NOT influence the mechanism of moisture removal during the drying process?
Which factor does NOT influence the mechanism of moisture removal during the drying process?
What is NOT a critical parameter in controlling the drying process?
What is NOT a critical parameter in controlling the drying process?
Which type of drying method is characterized by being slow and having no control over environmental factors?
Which type of drying method is characterized by being slow and having no control over environmental factors?
What effect does higher air temperature have on the drying process?
What effect does higher air temperature have on the drying process?
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Which statement regarding hot air drying is true?
Which statement regarding hot air drying is true?
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In the context of drying, what does psychrometry study?
In the context of drying, what does psychrometry study?
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What happens to the drying process when a larger load size is used in a dryer?
What happens to the drying process when a larger load size is used in a dryer?
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Which factor does NOT directly relate to air's capacity to remove moisture from food?
Which factor does NOT directly relate to air's capacity to remove moisture from food?
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What is the primary purpose of improved drying technologies?
What is the primary purpose of improved drying technologies?
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Which factor primarily influences the loss of volatile components during drying?
Which factor primarily influences the loss of volatile components during drying?
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Which method is NOT used to prevent flavor changes in dried fruits?
Which method is NOT used to prevent flavor changes in dried fruits?
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What causes browning in dried fruits and vegetables during storage?
What causes browning in dried fruits and vegetables during storage?
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What is a common consequence of drying foods at high temperatures?
What is a common consequence of drying foods at high temperatures?
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How can flavor be preserved in dried meat powders?
How can flavor be preserved in dried meat powders?
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Which pigment is most affected by heat and oxidation during the drying of fruits and vegetables?
Which pigment is most affected by heat and oxidation during the drying of fruits and vegetables?
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What process can help retain the color of dried fruits?
What process can help retain the color of dried fruits?
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What is the primary benefit of drum drying compared to hot air drying?
What is the primary benefit of drum drying compared to hot air drying?
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Which drying method is specifically designed for liquids?
Which drying method is specifically designed for liquids?
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What is one consequence of using the counter-current system in tunnel drying?
What is one consequence of using the counter-current system in tunnel drying?
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Which drying method is considered the most expensive but offers the best nutrient quality?
Which drying method is considered the most expensive but offers the best nutrient quality?
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In the process of puff drying, what gas is injected into the reaction vessel?
In the process of puff drying, what gas is injected into the reaction vessel?
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What type of drier consists of large containers with a mesh base allowing hot air to pass through a bed of food?
What type of drier consists of large containers with a mesh base allowing hot air to pass through a bed of food?
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A major drawback of the concurrent system in tunnel drying is:
A major drawback of the concurrent system in tunnel drying is:
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What is a key advantage of using heating systems in puff drying?
What is a key advantage of using heating systems in puff drying?
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Study Notes
Drying
- Drying is the removal of liquid from a solid product using heat.
- The first step involves heating the solid to raise the liquid's temperature to form a vapor.
- The next step is transferring the vapor from the solid's surface into the air.
- Drying and evaporation are similar, but drying is used when the liquid amount is small, whereas evaporation is used for large amounts.
- Drying is a crucial process in food processing.
Learning Outcomes
- Students should be able to define and explain drying, reasons for drying, types of drying, equipment used in drying, and parameters controlling the drying process.
Use of Heat
- Heat significantly impacts food processing in various ways, particularly in extending shelf life by inhibiting enzymatic and microbial activity and by reducing water content.
- It influences the nutritional and sensory qualities of foods, though it also increases processing costs.
Dehydration
- Dehydration (drying) applies controlled heat to remove most of the water in a food, either by evaporation or sublimation (freeze drying).
- The goal is to extend shelf life by reducing water activity and inhibiting microbial and enzymatic activity.
- However, the processing temperature is usually insufficient to fully inactivate these processes.
- Drying causes deterioration in the eating quality and nutritional value of the food.
- Common examples include coffee, milk, raisins, fruits, pasta, flours, beans, nuts, breakfast cereals, and tea.
Drying using Heated Air
- Three factors control air's capacity to remove moisture from food: the amount of water vapor already in the air, air temperature, and the amount of air passing over the food.
- Absolute humidity and relative humidity (RH) values express the amount of water vapor in air.
- Psychrometry studies the inter-related properties of air-water vapor systems.
Mechanism of Drying
- Drying air moves over food cells carrying moisture away from the food's surface.
Mechanism of Drying Curves
- Drying curves illustrate the relationship between moisture content and drying time, with distinct stages like a settling-down period, constant-rate period, and a falling-rate period.
- Consistent temperature and humidity in the drying air, with heat supplied by convection to the food's surface.
Factors Affecting Drying
- Food composition and structure influence moisture removal rates.
- For instance, the orientation of fibers in vegetables (like celery) and protein strands in meat affect the direction of moisture movement.
- The amount of food in a drier relative to its capacity significantly impacts the drying rate, as smaller quantities dry faster.
Process Control Parameters
- Drying parameters like load size, temperature, relative humidity, airflow rate, and drying time must be controlled for efficient processing.
Types of Drying
- Different drying techniques, including sun drying, hot-air drying, drum drying, spray drying, puff drying, and freeze drying, each having varying levels of efficiency, nutrient loss, cost, and control.
Sun Drying.
- This is one of the oldest, and most common traditional methods.
- It's a slow process, and only practical in hot, dry climates.
- It offers low costs but can have problems with contamination by pests and microorganisms, as well as rain.
- High nutrient loss is associated with sun drying
- Inexpensive products, such as grains, acid fruits, and spices.
- Foods, like fish, are vulnerable to pollution and vermin.
Hot Air Drying
- More efficient and has lower nutrient loss than sun drying, but is more expensive.
- Common products of this method are dried vegetables, pasta, and some fruits.
Drum Drying
- More efficient than hot-air drying and has lower nutrient loss.
- Common products include potato pastes and slurries.
Spray Drying
- Has low nutrient loss.
- It's expensive compared to drum or air drying.
- Good control and efficiency are achievable using this method, making it suitable for liquids.
- Its products include milk, instant tea, and coffee.
Puff Drying (Pressure Drop)
- Uses heating systems to remove free water from fruits and vegetables.
- The process involves washing, putting the produce in a reaction vessel, vacuumizing, injecting carbon dioxide, maintaining the pressure for 30-60 minutes, then depressurizing over 0.5-4 minutes, to puff-dry the materials.
Freeze Drying
- Best nutrient and product quality (shape, rehydration).
- Its the most expensive method of drying.
- Suitable for controlling product quality, making it applicable for products like coffee and camping foods.
- Ideal for use in military and by NASA.
Types of Driers (Bin Driers, Cabinet Driers, Tunnel Driers, Conveyor Driers, Fluidized-bed Dryers, Drum Dryers).
- Various dryer types exist, each with specific functions and applications, depending on the properties of substances being produced, and cost factors.
Tray Drier
- Air flows over each shelf to dry the material placed on trays.
- Electric or steam-heated pipes position the air to dry the material.
Tunnel Driers
- Hot air passes over the product, quickly drying it, but keeping relatively high moisture content.
- Counter-current systems are slower but produce low moisture content products.
Conveyor Driers (Belt Driers)
- Continuous conveyor driers dry food on mesh belts, with air flow initially directed upwards through the food bed and then downwards in later stages.
- This prevents dried food from falling out of the bed.
Fluidized-bed Drying
- Hot air is blown upwards through the food to cause it to float and remain separated, making it suitable for small items like peas and coffee.
Drum Dryer (Film Drying)
- Drum dryers have a drum that rotates and applies liquid to the surface, forming a thin film as it rotates.
- The drying rate is controlled by the drum speed and temperature.
- It's ideal for a variety of materials, like starch products and ferrous salts.
Advantages of the Drum Dryer
- Rapid drying due to large area and thin film spread.
- Compact equipment compared to other dryers.
- Short heating time, only few seconds.
- Enclosed with vacuum jacket, allowing low drying temperature
- Product is in flake form, convenient for many purposes
- Can handle a variety of materials in solution or suspension forms.
Spray Drying
- Suitable for products like dried milk and coffee
- Fine liquid spray injected into a hot air blast resulting in quick vaporization.
- Some moisture may be added to make larger particles, improving the wettability of the product.
Freeze Drying
- Preservation method for food that combines freezing with drying.
- First, food is frozen and then processed to remove solvents, primarily water.
- This method, known as lyophilisation, effectively removes water through sublimation of ice.
- Results in dried food that can be stored for many months.
- Has high quality preservation, making it applicable for coffee and camping foods.
Advantages of Freeze Drying
- Minimizes chemical decomposition (like hydrolysis)
- Product is light and porous due to maintaining volume.
- Avoids concentrating solutions which can help proteins.
- Has minimal contact with air preventing oxidation.
Disadvantages & Usages of Freeze Drying
- Packing requires special containers to prevent loss of quality.
- Process has slower rates and uses sophisticated, expensive equipment, limiting it to high-value products.
Using Freeze Dried Foods
- Freeze-dried foods can be stored for months or years.
- They must be stored in airtight packaging.
- Examples are found in fruit, cereals, and instant soups.
Effect of Drying on Food Characteristics
- Drying and storage affect food quality, reducing qualities like texture, flavor, and aroma— though color and nutritional value can vary according to the food.
- Technologies aim to reduce these negative impacts and maximize efficiency
Flavor and Aroma
- Heat causes volatile compounds that contribute to taste and aroma to evaporate during drying.
- The extent of loss depends on the temperature, moisture, and the volatiles' solubility, and diffusivity. Foods with high economic value due to characteristics flavors are dried at low temperatures to retain flavor.
- Additional processes to maintain flavors include recovery of volatiles and/or mixing with flavor fixing compounds for preservation,. and/or activation of naturally occurring enzymes to retain those characteristics.
Color
- Drying alters food surface characteristics, changing reflectivity and color.
- In fruits and vegetables, chemical changes to carotenoids and chlorophyll, or residual polyphenoloxidase enzyme activity during drying causes browning during storage which can be prevented by blanching or treatment with ascorbic acid or sulfur dioxide.
Nutritional Value
- Varying reported nutritional values are due to preparation methods, drying times, temperatures, and storage conditions.
- Preparation processes, such as slicing apples, blanching, or pureeing, typically have more significant impact on nutritional values than the drying operation alone in instances involving fruits and vegetables.
Rehydration
- Rehydration is not the reverse of drying, as water is not simply replaced in the same way.
Preservation Operations - Chilling
- Chilling reduces the rate of biochemical and microbial processes to extend the shelf life of fresh and processed foods.
- Chilling causes minimal changes to sensory characteristics and nutritional properties.
- Chilling is often done with fermentation or pasteurization for longer shelf life.
Theory on Chilling
- The rate of biochemical changes caused by microorganisms or enzymes increases exponentially with temperature.
- Chilling slows enzymic and microbiological changes and retards respiration.
Factors that Control Shelf Life of Fresh Crops in Chill Storage
- Type of food and variety.
- Part of the produce (fastest growing parts have shortest shelf life)
- Harvest conditions (mechanical damage, contamination, maturity)
- Temperatures of harvest, storage, distribution, and retail display.
- Relative air humidity in storage affecting dehydration losses.
Botanical Function, Respiration Rate & Storage Life (Table)
- A table summarizing the respiration rate and typical storage life of various botanical products.
Optimum Storage Conditions for Fruits and Vegetables (Table)
- A table showing ideal temperature and humidity ranges for various fruits and vegetables in storage.
Processed Foods, Categories of Micro-organisms
- Four categories of micro-organisms are determined by the temperature range for growth (thermophilic, mesophilic, psychrotrophic, psychrophilic)
Chilling, Prevention of Microbial Growth
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Chilling generally prevents the growth of thermophilic and mesophilic micro-organisms.
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Main microbiological concerns with chilled foods often involve a number of pathogens that can grow during extended refrigerated storage below 5°C or if the temperature increases.
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Some examples of pathogens that can survive chilled food conditions include Aeromonas hydrophilia, Listeria spp., Yersinia enterocolitica, and strains of Bacillus cereus, and enteropathogenic Escherichia coli.
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A risk assessment often factors in types of foods and contamination potential.
Pathogenic or Spoilage Bacteria in High-Risk Chilled Foods (Table)
- Table presenting microbiological characteristics (including source, minimum growth temperature, and incubation period ranges).
Shelf life and Determinant factors of chilled, processed foods
- Key factors determining shelf life for chilled processed foods (food type, microbial and enzyme inactivation during processing, hygiene control in handling and packaging, barrier of packages, temperature during processing, distribution and storage)
Range of chilled foods, Microbial Risk Classification
- Classes of foods with varying types of microbial risk to consumers (Class 1, Class 2, Class 3, Class 4) are outlined
Chilled Foods (Preparation and Storage)
- Cooked chilled foods need to be proportioned and chilled within 30 minutes, cooling finished products to 3°C within a ninety-minute timeframe, storing at 0-3°C following.
- Cook-pasteurise-chill system method describes procedures needed in packaging, vacuumizing, heating, and cooling processes to extend shelf-life.
Chilling Equipment
- Descriptions of different chilling equipment (mechanical refrigerators, cryogenic freezers).
Mechanical Refrigerators (Components)
- Mechanical refrigerators have four basic components (evaporator, compressor, condenser, expansion valve)
- Evaporator: cools the refrigerant to create a low-pressure vapor that absorbs heat.
- Compressor: increases the pressure to release heat.
- Condenser: releases heat and condenses the refrigerant to a liquid form
- Expansion Valve: reduces the pressure to lower the temperature of liquid refrigerant required to start the process again.
Refrigerants
- Key refrigerant properties include low boiling points with high latent heats of vaporisation which is essential, to generate dense vapor for the compressor. Refrigeration properties also include low toxicity, low mix compatibility with oil (lubricants) and low cost options.
- Common refrigerants in use are Freon-22, ammonia, and propane.
Cryogenic Chilling
- Cryogens, such as liquid nitrogen and solid carbon dioxide, absorb heat from the foods to cool rapidly. - This is done safely and efficiently by controlling, and removing excess carbon dioxide while also addressing safety precautions, like using an exhaust system to remove excess CO2. - Hazards related to cryogenic materials include cold burns, frostbite, and hypothermia associated with exposure to intensive cold.
Chill Storage
- Maintain consistent temperatures via refrigerated storage with mechanisms for circulation of cold air produced and managed by mechanical refrigeration.
- Foods are stored using strategies facilitating air circulation.
Control of Storage Conditions
- Maintaining air circulation throughout the storage area allows maintaining suitable storage temperatures, humidity, and composition.
- This is crucial when handling produce requiring higher humidity content for fresher product and quality.
Temperature Monitoring
- Temperature monitoring is essential throughout the food production and distribution chain for quality maintenance and safety.
- Microelectronic advancements allow storage of data for efficient computerized management systems.
Effect on foods
- The significant effect of chilling mostly concerns the hardening of foods that have fats or oils with rapid cooling.
- Other issues include enzymic browning, lipolysis, altered colour and flavour, deterioration or retrogradation of starch (in products).
- Lipid oxidation (WOF) can be a quality issue in cook-chilled products.
Freezing
- Freezing is a food preservation method where the product's temperature drops below its freezing point.
- Water in the food changes its state to form ice crystals when this process occurs, reducing the water activity of the frozen food.
- Preservation can happen using low temperatures, reduced water activity, and pre-treatment techniques, such as blanching.
Major Groups of Commercially Frozen Foods
- Fruits, vegetables, fish fillets/seafood, meats (beef, lamb, poultry), baked products, prepared foods, and complete meals.
Freezing Theory
- Sensible heat is first removed to lower the item to the freezing point, and then the latent heat of components is reduced (similar to other freezing processes), enabling rapid crystallization.
- In fresh foods, respiratory heat removed is significant, impacting production rates and determining freezing needs of equipment.
Types of Freezing Equipment
- Freezers (mechanical refrigerators, cryogenic freezers).
Cooled-air Freezers
- Chest freezers with stationary (natural-circulation) air are common but less efficient compared to other commercial freezing methods because they have lower freezing rates.
- A major concern relating to cold store maintenance is ice formation on surfaces, which can be caused by moisture in the surrounding air or from unpackaged products in the store.
Blast Freezers
- Food is placed on trays to be frozen in continuous equipment such as trolleys.
- High air velocities speed up the cooling process and heat transfer in air systems.
- Continuous systems, involving conveyor belts or tunnels, can hold/handle varying food qualities, enabling freezing/cooling.
Fluidized-bed Freezers
- Modified blast freezers that use high-velocity air to freeze food in a fluidized bed.
- Two-stage freezing designs prevent ice glaze on the surface of the food while enabling freezing.
Cooled-liquid Freezers
- Foods are passed through a bath of refrigerants like glycerol, calcium chloride, or brine.
- Traditional methods like saltwater (brine) immersion for food freezing are less common now.
Cooled-surface Freezers (Plate Freezers)
- Stacked plates with pumped refrigerant, used for flat, thin foods, such as fish fillets or burgers.
- Pressure is applied between plates to force the food for rapid freezing.
Scraped heat Exchangers
- Frozen food products like ice-cream are formed simultaneously with products being stirred using scrapers.
Cryogenic Freezers
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Freeze food products using refrigerant change of state through absorption.
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Cryogen is placed in contact with the food, enabling quick heat transfer for fast freezing rates.
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Liquid nitrogen or solid carbon dioxide spray is commonly used onto small foods for rapid freezing, such as soft fruit or prawns
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