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.

Thermal Processing and Refrigeration

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!