Lesson 8-

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Oct 30, 2023

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Dehydration as a Food Preservation Method LESSON 8
Lesson Objectives illustrate the underlying concepts of various methods of food dehydration outline the basis for extension of storage life of foods by dehydration compare and contrast methods for dehydrating different foods, and the consequences in terms of food quality explain factors affecting the rate of dehydration describe the packaging requirements for foods dehydrated by various dehydration methods
During dehydration: 1. Heat /energy - transferred to the food promoting water removal from the food 2. Water - transferred from the food into the dehydrating environment The process of dehydration
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• Microbial growth need specific a w • Staphylococcus aureus (a w 0.85)- pathogen! • Mould (need a w > 0.6) • Below 0.6 no microbial growth • Enzymatic reactions • Chemical reactions (eg. Maillard browning) at a w > 0.3 In dehydration the water is physically removed to lowe r the water activity ( a w ) Dehydrated foods have low water activity ( a w 0.2-0.6 ) But Microorganisms are not killed and resume growth after the food is rehydrated Preservation principle Water activity
Water activity (a w ) for some dehydrated and concentrated foods Product a w Whole milk powder dehydrated soups 0.03 Noodles 0.50 Dried Fruits: berries, apples, apricots, raisins (grapes) 0.60 Fruit juice concentrates, condensed milk 0.80 E.coli, Salmonella (a w 0.95) Staphylococcus aureus (a w 0.85) Mould (a w > 0.6) Chemical reactions (eg. Maillard browning) at a w > 0.3
Note differences between Concentration and Dehydration ⚫ Both use physical removal of free water from foods: ❖ Dehydration : removal of as much water from the food as possible ❖ imparts a long storage life .
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❖ Concentration: some of the water is removed from the food ❖ “ concentrate ” the food constituents ❖ Concentrated foods are not fully shelf-stable ❖ other forms of food preservation : refrigeration, freezing, dehydration, thermal processing… ❖ Fruit juices concentrates, jams, evaporated milk, condensed milk, maple syrup Note differences between Concentration and Dehydration ( cont…)
Changes during Dehydration ⚫ Shrinkage ⚫ Case hardening ⚫ Chemical changes Not all dehydration methods result in these changes Newer techniques try to protect the product against these changes
⚫ Shrinkage • Water migrates from the interior of the food to the surface where it evaporates & is carried away by the dehydrating medium… • As water migrates to the surface of the food it carries with it the water soluble substances dissolved in it. Changes during Dehydration
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Changes during Dehydration ⚫ Case hardening ◦ Rapid drying causes compounds (such as sugars) to form a hard, fairly impermeable case around the food piece ◦ Can cause the rate of dehydration to decrease ◦ Common in high-sugar products ⚫ tropical & temperate fruit products
Case hardening… 50x 100x
Changes during Dehydration Chemical changes • Browning and flavour changes due to reactions • Maillard browning ↑ as the conc. of solutes ↑ • Denaturation of proteins, aggregation of polysaccharides • loss of water-binding capacity • Loss of water-soluble components or • Conc. on the surface of the food (leads to case hardening) • Loss of volatiles (esp flavour compounds)
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Grapes vs. Raisins… ⚫ Think about differences in: ◦ Appearance ◦ Texture ◦ Flavour (aroma + taste) Note: 4.5 lb of grapes = 1 lb raisins! ⚫ Which one is sweeter? ⚫ Which one will be shelf stable at room temperature ? ⚫ Why?
• Surface area (surface to volume ratio) • Temperature of the drying air • Air velocity • Humidity of the drying air • Atmospheric pressure and vacuum Factors affecting Dehydration
Factors affecting Dehydration ⚫ Surface area (surface to volume ratio) ◦ Smaller food piece, more rapid the rate of moisture loss-more surface to volume ratio- faster the moisture loss ⚫ Temperature ◦ ↑ temp. will ↑ Dehydration (DH) rate ⚫ Air velocity ◦ Maximize velocity of heated air moving around the food particles ⚫ Humidity of drying air ◦ The drier the air, the more moisture it can absorb ◦ % RH of drying air determines the final moisture content of food
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Factors affecting Dehydration Atmospheric pressure and vacuum ◦ Water boils at 100 ° C (at a pressure of 1 atm =760 mm Hg) At lower pressure , the boiling temperature will decrease ◦ eg. under vacuum, water will boil at 32 ° C ◦ important for heat-sensitive food products
Methods for Dehydration ( “ Drying Methods ” ) ⚫ Sun Drying ⚫ Spray Drying ⚫ Tray and Tunnel Air Drying ⚫ Drum Drying ⚫ Freeze Drying ⚫ Vacuum Microwave Drying ⚫ Deep Fat Frying ⚫ Vacuum Extrusion Drying
Compare and Contrast Questions Tray drying vs Sun drying Spray drying vs Drum drying Freeze drying vs Vacuum Microwave drying Feature to compare ◦ Principle ◦ Process time (slow or fast) ◦ Quality and sensory attributes
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Drying Methods- Sun drying • Dry, warm climates • Slow drying method (several days) • Fruits, vegetables, fish • Inexpensive • Invasion by insects, birds, rodents, microorganisms • Shrinkage • Loss of volatiles • Poor rehydration
Saffron Case
• Trays or racks • Heated air at a set velocity and with low %RH • Quick & inexpensive • Pasta, vegetables, fruit, spices • Poor/moderate rehydration properties Food shrinkage, dense product • Case hardening may happen Drying Methods- Tray (Air) Drying
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Note the counter- current flow of food and hot air Tray drying
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Air (tray) dried RH of air: 40% Aw: 0.25
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Tray (Air) dried strawberries 7-10 hrs
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Drying Methods- Spray drying • Liquid foods and purees • Tiny droplets of food are sprayed into a stream of heated air (fine powder) • Rapid method • Good rehydration • Powder skim milk, instant coffee, tea, eggs
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Spray drying
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• Semi-solid foods: food paste, purees • Rotated heated drum • Rapid drying (thin layer) • Dried food is scraped off (flakes) • Re-hydration is fairly good • Mashed potatoes, baby cereal , Drying Methods- Drum Drying
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Drum drying
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Drying Methods-Freeze-drying (lyophilization) ◦ Food must be in the frozen state first ◦ Placed in a vacuum chamber (not a rapid method, depending on the size, it may take hours/days to complete dehydration ) ◦ Low heat provided from radiant heaters – energy ◦ Water will sublime : Water (solid) → Water (vapour) ⚫ no transition through the liquid state
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Freeze-drying (lyophilization) (cont …..) • Voids from ice crystals when water sublimes (act as channels)- → Excellent rehydration • No translocation of water-soluble constituents to the surface • Shape of food is retained (no shrinkage) • Flavour is retained • food closely resemble starting material • Very expensive($$) • Used for high-value expensive food • Instant soups, high-quality coffee, vegetables, military rations, space food
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Source: adapted from Food Science and Nutritional health by T.P. Labuza and J.W. Erdman. West Publishing Co., St. Paul MN. 1984. Freeze drying
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Freeze dried dough
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– Very expensive ( $$ ) – ” high value foods ” Instant soups, coffee, vegetables, military food, space food Freeze dried products
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Drying Methods- Vacuum Microwave Drying • Vacuum (keep temp. low = lower boiling point) • Microwave (rapid energy transfer) • High-quality products • Less nutrient loss • Flavour retention, less colour change • Complete rehydration • Quick drying method (minutes vs hours compared to freeze- drying) • Very expensive ( $$$ )
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◦ Hot oil → evaporation of water ◦ Food will “ pick up ” oil ◦ Snack foods, bakery products e.g. instant noodles! • frying: 140-150 o C , 1-2 min compared to • hot-air drying: 80 o C , 30 min Drying Methods-Deep-fat frying
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Drying Methods- Extrusion drying ◦ A slurry of food – steam heating , under pressure → release pressure → steam ( puffing ) ◦ The moist heat causes starch gelatinization and cooking of the product. ◦ eg. breakfast cereals, snack foods
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Source:Understanding Food Science and Technology by P. S. Murano. Wadsworth/Thomson Learning Inc. 2003 Extrusion Drying Food expands (puffs) as it is released Die cuts food into the desired shape
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Packaging requirements for dehydrated foods ⚫ Protect against moisture absorption – DH foods – hygroscopic – prevent transmission of water vapour ⚫ Physical protection – Prevent crushing – Freeze dried foods – porous structure with spaces; easily crushed ⚫ Protection from O 2 and light – Photo-oxidation
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Terms to remember Water activity Dehydration vs. Concentration Case hardening Water soluble components Sublimation Hygroscopic Atmospheric pressure & vacuum Different dehydration methods (sun drying, tray (air) drying, freeze drying, etc.)
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