Engineering and thermal processing
Engineers are different from scientists
• What do scientists do?• What do engineers do?• The importance of simplifying assumptions
– examples
Engineers deal with • Heat transfer, always runs from hot to cold so
refrigerators or freezers need pumps (energy input)
• Mass transfer• Mass balance
– Consider a simple drier
The 2nd Law of Thermodynamics:everything goes to maximum
randomness (entropy)• Unless you put in energy
– Electric, microwave, pressure, thermal, ionizing
– Types of Heat Transfer (demo)
• Conduction• Convection• Radiation
Most food processing involves the addition or removal of energy.
Most types of food spoilage have engineering solutions.
Heating and Cooling:• Heat transfer (heating or cooling) is driven by ΔT (the
temperature difference)
ΔT (°F)
Boiling H2O 152
Oven 350°F 292
Broiler @ 2000°F 1940
Refrigerator 40°F 20
Household Freezer 10°F 50
Commercial Freezer -40°F 100
Liquid Nitrogen -380°F 440
Assume a Room Temperature of 60°F.
Pasteurization: • 7 log kill for Salmonella in Milk = 99.99999%
• Pasteurization of eggs:3 log reduction = 99.9% 145°F for 3 to 4 mins.
• Fruit juices are pasteurized to reduce microbial count and inactivate enzymes; they were thought not to carry pathogens.
Temperature:145°F (63C)161°F (72C)212°F (100C)280°F (131C)
Time:30 min15 sec0.01sec6 sec
Log NumbersTime (min) # survivors Log # survivors
1 1,000,000 62 100,000 53 10,000 44 1,000 35 100 26 10 17 1 08 0.1 -19 0.01 -2
10 0.001 -3
Log vs linear
Pasteurizers (Ohio State University)
60 C for thirty minutes80 C for thirty seconds
Canning:• Invented by Appert in 1790s.• Commercial sterility.• Destroys pathogens and spoilage organisms
that can grow.• Requirements for high acid foods (pH < 4.6)
are not very stringent. Why?• Can be done in a boiling water bath.
Home canned tomatoes, OK
Home canned beans, No
Requirements for low acid (pH >4.6) foods are very strict
• Low acid foods pH > 4.6• Target is C. botulinum
12 log reduction = 99.9999999999%
• 2 to 4 min @ 250°F• Requires certified retort
operators, approved process
Flow Chart: Canning of Corn (FAO)
Blanching!
Flow Chart for the pea canning process
Flow Chart: Canning of Peas (FAO)
Blanching!
Mild vs. Severe Heat Treatment:Mild Heat Treatment):
•Aims:•Kill pathogens•Reduces bacterial load (Food is not sterile)•Inactivate enzymes
•Advantages:•Minimal damage to flavor, texture, and nutritional quality.
•Disadvantages:•Short shelf life•Another preservation method must be used, such as refrigeration or freezing
•Examples:•Pasteurization•Blanching
Severe Heat Treatment:•Aims:
•Kills all bacteria•Food will be commercially sterile
•Advantages:•Long shelf life•No other preservation method is necessary
•Disadvantages:•Food is over-cooked•Major changes in texture, flavor, and quality
•Examples:•Canning
Refrigeration and Freezing:(85 % of food is frozen at some point)
Refrigeration: Freezing:
Micro growth Slow Stops
Chemical reactions Slow Even Slower
Quality “Fresh” “Frozen”
Shelf life Short Long
Margin of error Small (spoilage, chill injury, cross contamination)
Large
Freezing:
Slowly:•Water migrates•Freeze “pure”•Solutes concentrate•Large crystals
Rapidly:•Freezes in place•No solute concentrate•Small crystals•Better structures
How refrigeration works
Expand
Energy Input
Condense
Simplified
Freezing:• 3 Types:
– Plate Freeze• Contact with very cold surface
– Blast Freeze• Very cold air• Very high velocity
– Immersion• N2
• Flash Frozen = “IQF”
Processing can create multiple products– Apple Juice
Bottle – “hot pack” Clear glassBrown plastic
ConcentrateLiquidFrozen
Metal canAseptic – Brix pack
(“Sipps”)
“Nonthermal” processingnext lecture…
• High pressure• Irradiation• UV light• Pulse light• High intensity light• Pulsed electric field
How pumpkin pies are made…
Happy Thanksgiving!