1. Dehydration Power Point

Principles of Food Preservation by Drying/Dehydration

  • Key Principles:

    • Low moisture content/reduced water activity (aw).

    • Pre-drying treatments.

    • Appropriate packaging material.

Low Moisture Content/Reduced Water Activity (aw)

  • Defined as follows:

    • p: Water vapor pressure of the food.

    • po: Vapor pressure of pure water.

    • ERH: Equilibrium relative humidity of the food.

  • Moisture Content:

    • Consists of bound water and free water (aw).

    • All microorganisms require free moisture for growth and activity.

    • All chemical and biochemical reactions occur in an aqueous medium.

Microbial Activity and Chemical Reactions

  • Both are aw dependent:

    • Decreasing aw lowers rates of microbial activity and chemical/biochemical reactions, and vice-versa.

    • Non-enzymatic browning, hydrolytic reactions, and lipid oxidation peak at aw of approximately 0.70 – 0.85.

Intermediate Moisture Foods (IMFs)

  • Defined as foods where water is immobilized using humectants (e.g., Trichloroacetic acid (TCA), Ferulic acid).

Enzymatic Browning

  • Definition: Color change to brown in food due to enzymatic reactions.

  • Key Enzyme: Polyphenol oxidase, catalyzes oxidation of phenols into quinones.

  • Result: Quinones polymerize into melanins, contributing to browning in fruits (bananas, apples, potatoes) and seafood (shrimp).

  • Control Methods:

    • Heat (destroys enzymes).

    • Acidic substances (e.g., lemon juice, to lower pH and preserve color).

Non-Enzymatic Browning

  • Definition: Involves chemical reactions during food preparation/storage, resulting in brown compounds.

  • Types:

    1. Caramelization: Browning of sugars when heated, leading to flavor development.

    2. Maillard Reaction: Reaction between amino acids and reducing sugars upon heat application, crucial for flavor and aroma in nuts, baked goods, fried foods.

  • Summary: Enzymatic browning involves enzymes; non-enzymatic browning occurs through chemical reactions, both affecting food quality.

Reduced Water Activity (aw) Effects

  • Prolongs the lag phase of microbes and desiccates microbial cells.

  • Table of Approximate Lower Limits of aw for Microbial Growth:

    • Most bacteria: 0.91.

    • Most yeasts: 0.88.

    • Most molds: 0.80.

    • Halophilic bacteria: 0.75.

    • Xerophilic fungi: 0.65.

    • Osmophilic yeast: 0.60.

  • Lower limits for microbial growth determined.

Pre-Drying Treatments

  • Main Treatments:

    1. Blanching

    2. Sulphiting/Sulphuring

    3. Dipping treatments

  • Purpose:

    • Destroy microbes and enzymes to varying extents.

Reasons for Drying/Dehydration

  • Prevent microbial activity.

  • Prevent physical and chemical changes in food.

  • Reduce costs related to packaging, transportation, and storage.

  • Adjust moisture content for further processing.

  • Remove moisture added in previous operations.

  • Provide convenience and save time for users.

Dehydration vs. Drying

  • Dehydration: Controlled removal of moisture using mechanical dryers.

  • Drying: Uncontrolled removal (e.g., sun drying).

Methods of Drying

  1. Thermal drying process.

  2. Osmotic dehydration.

  3. Mechanical dewatering.

  4. Freezing (freeze concentration).

Differences in Drying Methods

Dehydration vs. Sun Drying

  • Controlled vs. uncontrolled conditions.

  • Product quality comparison.

  • Land and resource requirements.

  • Speed and sanitation conditions.

The Dehydration Process

  • Steps involve:

    1. Raw materials.

    2. Preliminary preparations.

    3. Pre-drying treatments (blanching, sulphuring/sulphiting, dipping treatments).

    4. Drying (single-stage or multi-stage).

    5. Post-dehydration treatments (packaging, casing, storage).

Pre-Drying Treatments Explained

Sulphuring

  • Exposing prepared material to fumes of burning sulphur in a closed chamber for 30-60 minutes.

Sulphiting

  • Treatment with sulphite solutions (e.g., 0.2–0.5% sulphurdioxide).

  • Acts as an anti-microbial, anti-enzymatic, and anti-oxidant agent.

Dipping Treatments

  • Utilizing various chemical solutions (e.g., magnesium oxide for color preservation, citric acid for rehydration properties).

  • Specific examples include:

    1. 2% starch solution for carrots.

    2. Blanching protocols for cauliflower.

Post-Dehydration Operations

  • Sweating: Holding dehydrated fruits for equalization of moisture.

  • Screening: Removing unwanted sizes from dried products.

  • Instantization: Improving rehydration rate through processes like flaking.

Water Movement During Drying

  • Mechanisms include liquid movement by capillary forces, diffusion caused by solute concentration differences, and water vapor diffusion in air spaces.

Drying Curve Analysis

  • Describes different drying phases: a) Initial heating. b) Constant rate period. c) Falling rate period - involves critical moisture content.

  • Factors determining drying rate include temperature, moisture content, surface area, shape and size of the product, relative humidity, and air flow rate.

Drying Equipment Overview

Types of Driers

  1. Hot Air Driers: Kiln driers for high capacity.

  2. Cabinet/Tray Drier: For various food types with flexible applications.

  3. Tunnel Drier: Insulated construction with specific airflow characteristics.

  4. Belt/Conveyor Drier: Wet materials conveyed through the system.

  5. Fluidized Bed Drier (FBD): Heated air suspends and dries food materials.

  6. Pneumatic Drier: Uses heated air streams for rapid drying.

  7. Spray Drier: Sprays food into heated air for fast drying.

  8. Rotary Drier: Rotated materials agitated in heated air.

  9. Vacuum Band Drier: Continuous process using vacuum conditions.

  10. Freeze Drying: Sublimation drying method for heat-sensitive materials.

Freeze Drying Process

  • Involves:

    • Pre-freezing.

    • Sublimation drying.

    • Secondary drying.

  • Advantages include minimal product shrinkage and better rehydration properties.

Quality Evaluation of Dehydrated Products

Evaluative Parameters

  • After dehydration:

    • Bulk density.

    • Moisture content.

    • Sensory attributes.

  • After rehydration:

    • Rehydration ratio (RW/DW).

    • Coefficient of rehydration.

    • Percent water in rehydrated sample.

Quality of Dehydrated Powders

  • Evaluated based on:

    • Wettability.

    • Sinkability.

    • Dispersibility.

    • Solubility.