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Lecture 3: Processing of fruits and vegetables (Post-harvest handling…
Lecture 3: Processing of fruits and vegetables
Introduction to vegetables
Excellent sources of certain minerals and vitamins and are often the main source of dietary fiber
Structure
Stem
(asparagus and celery)
Leaf
(lettuce and spinach),
immature flower bud
(broccoli and brussels sprouts)
Roots
(potatoes and carrots)
Fruits
(tomatoes and cucumbers)
Post-harvest handling operations
3. Sorting
4. Packing
2. Washing
5. Cooling
1. Harvesting
6. Storage
7. Transportation to retail stores
Fresh and minimally processed vegetables
Roots and legumes
- harvesting by machines; sorting and grading are performed at field or collection stations
Bulk handling
Bundled as whole plants
- cutting will injure the cells and liberate ethylene (promote senescence and shortens shelf life)
Vegetables for long-term storage
-
treatments
to avoid microbial spoilage, insects, and small-animal invasion may be necessary
Leafy vegetables
- do not require harvesting by mechanical device; cooled immediately (remove field heat), sorted (remove debris), washed (remove dirt), bundled or packed (shipping and retail)
Washing and waxing step
- improve the shelf life and attractiveness of the produce (eg. cucumbers)
Packaging
Veges go through preparation steps (washing, sorting, grading, cutting, packaging into retail-size containers)
Examples
Vacuum packaging
Modified atmosphere (MA) packaging
Properties
Air is replaced by an atmosphere
high in carbon dioxide
and
low in oxygen
Modified atmosphere can
slow the respiration rate
and the
senescene of cut vegetables
Modern packaging -
"clean room
concepts
Shipment -
refrigerated containers
Comparisons
Advantages
do not contain any preservatives
have not gone through any heat or chemical treatment
Disadvantages
refrigeration storage is essential; limiting the practice to developed countries
Processing of vegetables
Goal
: to deter microbial spoilage and natural physiological deterioration of the plant cells
Techniques
2. Dehydration
(removal of water)
applying heat - through radiant energy of the
sun
or through air heated by
electrical energy
Major advantage:
reduction in volume and weight
- aids in storage and transportation of the dried products
Modern drying techniques: tunnel drying, vacuum drying, drum drying, spray drying, freeze drying
Example:
Instant potatoes
. Dehydrated potato granules - high bulk density and easy to handle in large quantity,
air-lift drying
; dehydrated flakes -
steam-heated drum drier
Sulfite
- an antimicrobial agent, aids in heat transfer and acts as a blanching agent (potatoes)
3. Canning
usage of metal cans or glass jars
Advantage: useful in developing countries where refrigeration is limited or nonexistent
Canning process
: cutting into pieces, packed in cans, put through severe heat treatment (to ensure destruction of bacteria spores); containers are sealed while hot to create a vacuum inside when they are cooled to room temperature
Disadvantage:
1. Minor defects - result in bulged cans
; Causes - formation of gas from chemical reactions between the metal cans and their acidic contents, and possibility that inadequate heat processing did not destroy all bacteria spores. 2. Severe heat treatment can have
inferior quality and less nutritive value
than fresh and frozen products.
Aseptic canning:
High-temperature-short-time (HTST) processing
- pre-sterilized containers are filled with a sterilized and cooled product and sealed in a sterile atmosphere with a sterile cover; avoids the slow heat penetration - creates products with superior quality
4. Freezing
Individually-quick frozen (IQF) method
- does not allow large ice crystals to form in vegetable cells; individual freezing cause particles to not cohere, final product is not frozen into a solid block
Techniques: blast freezing, plate freezing, belt-tunnel freezing, fluidized-bed freezing, cryogenic freezing, dehydro-freezing
Choice of method
type of vegetables to be frozen
capital limitations
quality of end product desired
products to be stored as bulk or as individual retail packages
Advances in packaging materials and techniques - led to
bulk frozen products
being stored in
large pouches
5. Fermentation and pickling
Acid
is used to preserve the products
Requirement: less heat treatment
1. Blanching
Blanching (heating) in
hot water
at 88 C (190 F) for 2-5 minutes or with
steam
in a conveyor at 100 C (212 F) for one-half to one minute; then undergo
rapid cooling
in either cold water or cold air (better quality retention)
Purposes
inactivates natural enzymes that would cause discoloration and off-flavors and aromas
reduce the number of microorganisms
render vegetables limp for easy packing into containers
remove harsh flavors
6. Irradiating
Ionization radiation -
gamma ray
Preventing potatoes from sprouting during long-term storage