Food Preservation Techniques - Dehydration PDF

NOTA FST564: PRESERVATION – DR. NAFISAH BINTI MUSA 7. Maintains nutritional value (if properly dehydrated). TOPIC 5: DEHYDRATION 8. Long shelf life without preservatives. DEHYDRATED...

NOTA FST564: PRESERVATION – DR. NAFISAH BINTI MUSA 7. Maintains nutritional value (if properly dehydrated). TOPIC 5: DEHYDRATION 8. Long shelf life without preservatives. DEHYDRATED DRIED Physical and Biochemical Changes Food: no more than 2.5% water Food: more than 2.5% water (dry 1. Size/Shape: Shrinkage during drying. (dry basis) basis). 2. Colour: Browning due to high temperature or oxidation. o Prevent with blanching or ascorbic acid. Definition and Purpose 3. Flavour: Loss of volatile compounds.  Definition: Application of heat under controlled conditions to o Prevent with sulphur dioxide or blanching. remove water from food by evaporation. 4. Nutritional Value:  Purpose: Extends shelf life by reducing water activity. o Loss of water-soluble nutrients (e.g., Vitamin C). Theory  Combines heat application with moisture removal. Effects of Drying on Food Examples  Shrinkage and scorching.  Commercial dried foods: Coffee, milk, raisins, pasta, beans,  Case hardening from fast drying. spices.  Colour changes (e.g., browning).  Dried ingredients: Egg powder, enzymes, yeasts.  Reduced nutritional value, lipid oxidation.  Increased porosity due to fluid loss. Dehydration Process  Oldest method: Solar/sun drying. Factors Affecting Drying Process  Pre-treatment (e.g., blanching, sulphide dipping) preserves colour 1. Drying Temperature: Ideal drying temperature is 65–70°C, then and flavour. 60°C. High heat may burn stuff.  Foods are dried whole or transformed (e.g., powdered milk, dried 2. Relative Humidity of Air: Lower humidity and higher temperature fish). speed up drying. Moisture Content 3. Velocity of Air: Faster airflow eliminates the air food surface  Safe storage: Moisture content below: moisture. o Fruits: 20% 4. Drying Time: Dependent on food type, size, and tray load (e.g., vegetables: 6–16 hours, fruits: up to 48 hours). o Vegetables: 10% 5. Capacity of Dryer and Tray Loading: Affects drying performance. o Grains: 10-15% 6. Moisture Content of Food: Foods with high moisture (e.g., lettuce)  Dehydration reduces water activity (Aw) below 0.5. are unsuitable for drying. Drying Principles 7. Moisture Absorption: Dependent on the air’s capacity to absorb 1. All organisms need water to survive. moisture. 2. Dehydration reduces available water, stopping spoilage. 8. Size of Food Particles: Smaller pieces dry faster than larger ones. 3. Decreases enzyme activity and undesirable reactions. 9. Surface Area: Larger surface areas facilitate faster drying. 10. Atmospheric Pressure and Vacuum: Lower pressure accelerates Mechanism of Dehydration evaporation at lower temperatures. 1. Removes water, deactivating enzymes and microorganisms. 2. Reduces microbial spoilage. Factors affecting Drying Rate/Process 3. Concentrates sugar in fruits, deterring bacterial growth. 1. Speed of Drying/Airflow Drying Process o Affects product quality.  Water loss rapid initially (surface water). o Pre-treatment may prevent discoloration.  Outer dried layer insulates, slowing further drying. o Drying must be fast but controlled to avoid "case  Requires constant temperature and airflow to avoid hardened hardening" (outer surface hardens while inside remains surfaces. moist). 2. Temperature: Objectives of Drying o Low temperature encourages microbial growth. 1. To reduce water content (12 inches for thicker foods). o Delaying ripening. o Inactivating or controlling parasites.  Uniform dose delivery. o Insect disinfestation.  Decays into non-radioactive nickel and barium. o Inhibiting sprouting of vegetables.  Low environmental risk.  Also referred to as: o Disadvantages: o Cold pasteurization.  Requires frequent replenishment (Co60 pencils). o Ionizing radiation. o Electric pasteurization (achieves the same objective as  Cannot be turned off. thermal pasteurization without a substantial temperature 2. Accelerated Electrons (E-Beams): increase). o Produced by electron guns converting energy to high-  Used in over 40 countries for more than 100 food items, including energy electrons. potatoes, onions, cereals, flour, fresh fruits, and poultry. o Characteristics: Advantages of Food Irradiation  Maximum energy: 10 MeV. 1. Reduces or eliminates disease-causing microorganisms.  Adjustable energy levels. 2. Nutritional value remains essentially unchanged. o Process: 3. Food does not become radioactive.  Electron beam generator directs electrons at o Considered a safe and effective technology for food to a shallow depth (3 cm or 1 inch). preventing foodborne diseases.  Two opposing beams can treat slightly thicker food. Irradiation Process o Advantages:  Food (packaged or in bulk) is exposed to carefully controlled  Can be turned on/off as needed. amounts of ionizing radiation for a specific time to achieve desired  No need for replenishment (unlike Co60). objectives.  No radioactive waste. o Disadvantages: Mechanism of Irradiation 1. Effect on Microbes:  Shallow penetration (suitable for foods