Fruits and vegetables
Fruits and vegetables
outline
• Plant cell composition
• Plant pigments
–
–
–
–
Chlorophyll
Carotenoids
Anthocyanins
Anthoxanthins
• Enzymatic browning
Fruits and vegetables
outline
• Sulfur compounds
• Changes during ripening
• Fruit and vegetable cookery
– Osmosis
– Diffusion
Plant cell
Vacuole
Middle
lamella
Amyloplast
Image courtesy of Beginner’s guide to Molecular Biology (www.
res.bbsrc.ac.uk/molbio/guide/cell.html)
Pectic substances
• Pectin (pectic substances)
• This is the “glue” that holds the cells of
the tissue together
• Very important to plant tissue texture
Stone walls and plant
tissue texture
Stones held together by mortar between stones. Strength (texture)
of the wall largely determined
by mortar.
Cells held together by pectic
substances in middle
lamella. Texture of plant tissue largely determined by
pectic substances What happens over time?
In both cases the texture changes
due to a change in what is
gluing the stones/cells
together.
Cellulose vs. starch
Glucose
Glucose
Glucose
Beta-1,4
0 calories/gram
Glucose
Glucose
Glucose
Amylose
Alpha-1,4
4 calories/gram
Fruit and vegetable
characteristics to preserve
• In preparation, processing, or
preservation of fruit and vegetables we
want to preserve
–
–
–
–
Color
Flavor
Texture
Nutrient value
Chlorophyll
Chlorophyllin
(bright green)
heat Chlorophyll
(green)
alkali
heat
acid
Pheophytin
(olive green)
Chlorophyllase
Chlorophyllide
(bright green)
heat
acid
Pheophorbide
(olive green)
Chlorophyll/pheophytin
acid
heat
Mg++
pheophytin
chlorophyll
Images courtesy of NYU/ACF Scientific Visualization Laboratory
(www.nyu.edu/pages/mathmol)
Fresh chlorophyll
Chlorophyll with base
Chlorophyll with acid
Chlorophyll in broccoli
Carotenoids
• These are the most stable class of
pigments in fruits and vegetables
– Carotene--orange
• Carrots
– Lycopene--red
• Tomato
– Xanthophylls--yellow
• Corn
Carotenoids
O
H
v it a m in A
beta-carotene
lyco p e n e
Carotene in carrots
Anthocyanins
• These are the purple-blues-reds of
grapes and red cabbage, for example
• They are acid stable but in alkali they
turn vivid blue, blue-green, turquoise,
etc
Anthocyanins in red cabbage
Anthoxanthin
• These are really non-pigments that
only appear if you mess up
• These compounds are in onions,
cauliflower, potatoes, etc.
• They are acid stable and colorless, but
in alkali, they turn a bright yellow
Anthoxanthins in onions
Lightning Quiz
A general rule
• Less cooking produces fewer
undesirable pigment and texture
changes in fruits and vegetables
Enzymatic browning
O
H
H
O
O
+
R
a phenolic
oxygen
O
polyphenoloxidase
(PPO)
R
an ortho-quinone
Enzymatic browning
Enzymatic browning
Phenolics
(cell walls)
Oxygen
(air)
Enzymes
(PPO from
cytoplasm)
Brown pigments
Enzymatic browning
• Enzymatic browning is usually
undesirable when it occurs in foods
(apples, pears, lettuce, potatoes, etc.)
• Exceptions
– Dried fruit, coffee, tea, and fruit butters
Control of enzymatic
browning
• Destroy the activity of the enzyme
– Heat
• Remove oxygen from the system
– Vacuum or inert gas packaging
– Antioxidants (Vitamin C in Fruit Fresh)
• Remove substrate (phenolics) from the
system
– Genetically breed out
Control of enzymatic
browning
• Alter the activity of the enzyme
– Acid
• Use a physical barrier
– Concentrated sugar syrup
O2
Reduced solubility
syrup
fruit
Sulfur compounds in
vegetables
• These may produce distinctive, strong,
and/or unpleasant tastes and odors
• They occur most importantly in two
distinct classes of vegetables and the
compounds in these vegetables behave
very differently on prolonged heating
(cooking, setting on a steam table, etc.)
Sulfur compounds
– Family 1
•
•
•
•
Cabbage
Brussels sprouts
Broccoli
Cauliflower
Prolonged cooking results in
an intensification of the sulfur
aroma of these vegetables
– Family 2
•
•
•
•
Onion
Scallion
Leeks
Garlic
Prolonged cooking of these
results in them becoming more
and more mild and less
offensive
Ripening
• Changes on ripening include
– Starch-sugar interconversions
• Banana: Starch --------------> sugar
• Sweet corn: Sugar ------------> starch
–
–
–
–
–
Acids decrease
Sugars increase (generally)
Tannins become less soluble
Pigment changes occur
Texture changes occur
Pectic substances
Maturity
Material
Texture
Underripe
Protopectin
Hard
Ripe
Pectin
Tender-crisp
Overripe
Pectic acid
Mushy
Only pectin will form a gel (jelly). Protopectin and pectic
acid are no good at forming gels.
Control of ripening
• To delay
– Hydrocooling
• Put in ~32oF water immediately after harvest
– Controlled atmosphere
• To accelerate
– Ethylene treatment
• Lightning Quiz
Controlled (modified)
atmosphere storage
Nitrogen 78%
Oxygen 21%
Carbon dioxide <0.04%
95% Carbon dioxide
5% Oxygen
Controlled (modified)
atmosphere storage
Nitrogen 78%
Oxygen 21%
Carbon dioxide <0.04%
95% Carbon dioxide
5% Oxygen
Controlled (modified)
atmosphere storage
Nitrogen 78%
Oxygen 21%
Carbon dioxide <0.04%
95% Carbon dioxide
5% Oxygen
Controlled (modified)
atmosphere storage
Nitrogen 78%
Oxygen 21%
Carbon dioxide <0.04%
95% Carbon dioxide
5% Oxygen
Controlled (modified)
atmosphere storage
Nitrogen 78%
Oxygen 21%
Carbon dioxide <0.04%
95% Carbon dioxide
5% Oxygen
Ethylene treatment
Air only
Air + low concentration
of ethylene
Ethylene treatment
Air only
Air + low concentration
of ethylene
Ethylene treatment
Air only
Air + low concentration
of ethylene
Fruit and vegetable
cookery
• In fresh cooked fruits and vegetables you
want to retain texture. To do so, cook a
short time in a minimum amount of water.
• If too much water is used
– It takes longer (more energy) to cook
– There is increased water absorption, leading to a
mushy texture
– There are increased vitamin and mineral losses
Rules of thumb
• Amount of water
– Should be 1/2 (or less) the weight of the vegetable
• Initial temperature
– Place material to be cooked in boiling water, not
cold water
• Final texture
– Cook only to desired final texture, e.g., tendercrisp
Rules of thumb
• Use a lid on cooking utensils
– Exceptions: where green pigments may
be altered, e.g., broccoli, green beans, etc.
Other factors affecting
texture
• Initial moisture level
– Dried beans - very hard
– Watermelon - very soft
• Cooking with or without added solutes
(usually sugar)
– Without sugar - mushy texture (applesauce)
– With sugar - Firm texture (apple garnishes)
• These effects are due to diffusion and
osmosis
Osmosis
• Osmosis is the movement of water only
across intact cell walls
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x
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Cell wall
Osmosis
• Osmosis is the movement of water only
across intact cell walls
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x
x
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Cell wall
Osmosis
Osmosis
Diffusion
• Movement of water, sugar, and other
molecules through a tear in a ruptured
cell wall
y
x
y
y
x
x
y
x
y
Cell wall
Diffusion
• Entry of sugar into and exit of water
from the fruit tissue causes it to
become translucent and shrink slightly
which causes the firming action, e.g., in
apple and other fruit garnishes
• Lightning Quiz