Transcript Slide 1

Waxing of Horticultural Produce
Dr. Ron Porat
Dept. of Postharvest Science of Fresh Produce
ARO, The Volcani Center, Bet Dagan, Israel
Waxing (coating) fruits and vegetables is used to:
1) Enhance shine
2) Reduce water loss and shrinkage
3) Retard ripening
4) Provide a carrier for application of chemicals
(fungicides, plant growth regulators, etc.).
The term “wax” once referred only to beewax, but now
is used for any substance with wax-like properties.
Waxes may consist solids derived from plant materials
(carnauba, candelilla, and wood rosin), insect
excretions (shellac, beewax), or petroleum products
(paraffin, polyethylene, mineral oil).
Plant resin and waxes
Resin from
Douglas trees,
Montana, US
Wax extracted from stems of Candelilla, Mexico
Another species of Candelilla, Mexico
Wax extracted from leaves of Carnauba, Brazil
Edible shiny coatings of carnauba
Resin from the “Elephant tree”, Guatemala
Scale insects- drain the sup from the bark of trees
About 300,000 insects can secrete 1 kg shellac!
Commercial products of bee wax
Waxes are usually classified according to their main
solid constituent. For example, shellac-based wax,
polyethylene-based wax, carnauba-based wax, etc.
In other cases, waxes may be classified by the solvent
in which they were dissolved. For example,
petroleum-based waxes or water-based waxes.
The petroleum-based materials (paraffin, polyethylene,
mineral oils) are generally restricted to use in
coatings for fruits and nuts where their peel or shell
is not normally ingested, including avocado, banana,
citrus, coconut, mango, melon, papaya, pineapple,
pumpkin, and different nuts.
Water soluble shellac waxes are commonly used in
coatings of apple fruit.
Natural edible coatings, mainly based on
polysaccharides, are developed for use in fruit and
vegetables that are digested with their peel,
History of wax development
The practice of waxing began in China already in the
12th and 13th centuries in order to ferment and
preserve food.
However, the development of modern days waxes
began in the US in the 1930’s as followed:
1930s – coating fruit with paraffin waxes
1940s – solvent waxes based on wood rosin or synthetic
resins dissolved in petroleum solvents replaced the
use of paraffin
1950s – the first water soluble wax emulsions were
introduced but had poor shine.
1960s – high-shine, water soluble waxes based on shellac
and alkali-soluble resin were developed.
1970s – high solids (concentrated waxes) were developed.
Nowadays – there is an increasing interest in the
development of “edible coatings” based on natural
polysaccharides, proteins and lipids, but they are not
yet good enough.
In addition to wax lipid components, coatings can
include other materials, such as resins, proteins,
carbohydrate compounds, bases, surfactants,
buffers, plasticizers and emulsifiers.
Resins – are a group of acidic substances, many of
which are wound-response products secreted by
specialized plant cells of trees and shrubs.
Wood rosin is manufactured from pine and other
trees.
Synthetic resins are petroleum-based products like
coumarone indene.
Shellac resin is secreted by the insect Laccifer lacca
(found in India).
These compounds provide shine, but have low
permeability to gases and provide moderate barriers
to water vapor.
Proteins – film-forming proteins my be derived from
plant or animal sources.
Plant proteins include zein from corn, wheat gluten,
soy protein, peanut protein and cottonseed protein.
Animal proteins include casein from milk, keratin,
collagen, and gelatin.
Almost all of these proteins are considered GRAS!
Proteins are moderately permeable to gases but offer
little resistance to water vapor.
Polysaccharides – provide an abundant resource of
hydrophilic film-forming agents with a wide range of
viscosities.
Polysaccharide materials include cellulose, pectins,
starches, chitosan and various gums.
Polysaccharides have low permeability to gases but
offer little resistance to water vapor.
Commercial coatings developed from carbohydrate
polymers include ‘Pro-long’ (England), ‘Semperfresh’
(US), ‘Nature Seal’ (US) and ‘Nutri-Save’ (Canada).
Use of ‘Nature Seal’ to preserve fresh-cut apples
Plasticizers – are low molecular weight compounds
that increase the strength and flexibility of coatings.
Common plasticizers include glycerol, sorbitol, and
propylene glycol.
Sucrose fatty-acid esters and acetylated
monoglycerides can also be used as plasticizers.
Addition of plasticizers to the coating, however, also
increases permeability to gases and water vapor
exchange.
Emulsifiers – are macromolecular stabilizers.
Common emulsifiers are gums and starch.
Bases – are used to promote the solubilization of solids.
Common used bases are morpholine and ammonia.
Properties of major wax components
Shine
Gas
Water vapor
permeability resistance
Lipids (wax) Moderate
Moderate
High
Resins
High
Low
Moderate
Proteins
Low
Moderate
Low
Polysacc.
Low
Moderate
Low
Summary
Water loss
Lipid materials (wax and oils) provide the best barrier to
prevent water vapor exchange, followed by shellac.
Carbohydrates and proteins are much less effective.
Gas permeability
Shellac, followed by wax, proteins and carbohydrates
restrict gas exchange, and can modify the internal
atmosphere inside the fruit leading to anaerobic
respiration.
Shine
Addition of shellac to the wax provides high-shine.
Physical characteristic requirements of waxes
• The wax should cover the entire fruit surface
• The wax should dry up quick enough
• The wax should be resistant to rewetting resulting
from water condensation.
• The wax should remain stable for long storage
periods.
Commercial application of waxes
wax
dryer
dryer
Imazalil
tank
Waxing defects
Powdering
(resolubilization)
Friction
between fruit
Wax particles
The exact formulations of commercial waxes that
were designed to address these requirements are
‘trade secrets’ and were not released to the public
domain!!!
Moreover, the companies often perform all kind of
changes in their own products without the
knowledge of the consumer.
It is necessary to fit a specific wax formulation for
each commodity.
For example, with citrus, we use different waxes for
oranges and grapefruits, mandarins and pummelo
fruit.
Effects of waxes on gas exchange and
water vapor loss
Gas and water vapor movement through the peel may
occur:
1) Through holes, such as stomata pores, the stem
scar and wounds.
2) Through the cuticle layer.
Gas exchange through holes is rapid, but is very slow
through the cuticle.
For example, O2 diffusion coefficient are 1.8 10-5 m2 s-1
through air but just 2.5 10-14 m2 s-1 through tomato
cuticle.
Most gas and water vapor movement occurs through
pores in the cuticle layer!!!
Primary routes of water loss in fresh produce
Air injected in pear travels through gas pathways
to the fruit surface
Gas and water vapor movement through polymers
depend on:
1) The difference in the concentration of the gas or
water vapor inside and outside the peel.
2) The diffusivity of the barrier.
3) The thickness of the barrier.
For most polymer barriers, the permeability of CO2 is
several times higher than that of O2!
Waxes may influence internal gas levels in several
ways:
1) Waxing reduces peel permeability by adding a
barrier to gas exchange.
2) Waxing reduces perforations (holes) on the peel
surface by blacking them.
3) Waxing may indirectly affect fruit respiration rates
(waxes usually reduce internal O2 levels and,
therefore, reduce respiration by 20-45%).
Storage temperatures greatly affect respiration and gas
exchange rates.
However, since respiration (O2 uptake and CO2
production) are influenced much more than the
exchange of these gases through the peel, the O2
levels decrease and CO2 levels increase with
increasing storage temperature.
Effects of storage temperature and waxing on
internal O2 and CO2 levels in grapefruit
Anaerobic respiration
Application of waxes may block gas exchange and
cause O2 concentrations to fall to a value below the
so-called extinction point, where aerobic respiration
is replaced at least by part by anaerobic
fermentation, resulting in the production of ethanol
and off-flavors.
A simplified description of plant aerobic and
anaerobic respiration pathways
Glucose
Glucose 6-phosphate
Fructose 6-phosphate
Fructose 1,6-bisphosphate
Glyceraldehyde 3-bisphosphate
NAD
+
NADH
1,3-Bisphosphoglycerate
ADP
ATP
3-phosphoglycerate
Cytosol
2-phosphoglycerate
Phosphoenolpyruvate
ADP
Pyruvate
ATP
kinase
Lactate
Pyruvate
Lactate
Pyruvate
dehydrogenase
decarboxylas
e
Anoxia
Pyruvate
AT
P
AD
P+
Pi
CO2
Citric
Acid
Cycle
Pyruvate
dehydrogenase
AT
ADP+P P H
H
+
O + H2O i
NAD
+
H
NAD
NAD
H
Mitochondria
2
NAD+
Ripening
Wax
CA, MA
Storage conditions
Ethylene
Stress
Acetaldehy
de
NADH
Alcohol
dehydrogenase
NAD
+
Ethanol
Effects of polyethylene and shellac-based waxes
on internal O2, CO2 and ethanol levels in grapefruit
Effects of storage temperature on respiration rates
2.0
A
1.8
Grapefruit - Wax
Grapefruit +Wax
1.6
1.4
1.2
Grapefruit
0.8
0.6
0.4
0.2
0.0
2.0
B
1.8
Mandarin - Wax
Mandarin +Wax
1.6
1.4
1.2
1.0
Mandarin
.
Respiration Rate (CO2: ml /Kg.h)
1.0
0.8
0.6
0.4
0.2
0.0
Weeks
Temp. (oC)
1
2
2
4
1
2
6
4
1
2
11
4
1
2
20
4
Effects of storage temperature on internal CO2 levels
20
18
A
Grapefruit - Wax
Grapefruit +Wax
16
14
12
Grapefruit
10
8
Internal CO2 (%)
6
4
2
0
20
18
B
Mandarin - Wax
Mandarin +Wax
16
14
12
Mandarin
10
8
6
4
2
0
Weeks
Temp. (oC)
1
2
2
4
1
2
6
4
1
2
11
4
1
2
20
4
Effects of storage temperature on internal O2 levels
24
A
Grapefruit - Wax
Grapefruit +Wax
21
18
Grapefruit
15
Internal O 2 (%)
12
9
6
24
B
Mandarin - Wax
Mandarin +Wax
21
18
Mandarin
15
12
9
6
Weeks
Temp. ( o C)
1
2
2
4
1
2
6
4
1
2
11
4
1
2
20
4
Effects of storage temperature on ethanol levels
1600
A
Grapefruit - Wax
Grapefruit +Wax
1200
Grapefruit
Ethanol (nl/ml)
800
400
0
1600
B
Mandarin - Wax
Mandarin +Wax
1200
Mandarin
800
400
0
Weeks
Temp. ( o C )
1 2
3
2
4
1 2
3
6
4
1 2 3
11
4
1 2
3
20
4
Effects of storage temperature on fruit taste
10
9
Grapefruit - Wax
Grapefruit +Wax
A
8
7
6
Grapefruit
5
4
3
Taste Score
2
1
0
10
9
8
7
6
Mandarin - Wax
Mandarin +Wax
B
Mandarin
5
4
3
2
1
0
Weeks
Temp. ( o C)
1 2
3 4
2
1 2
3 4
6
1 2 3 4
11
1 2 3 4
20
Conclusions
1) Waxing greatly affects internal gas levels.
2) Shellac-based waxes restricts gas exchange much
more then polyethylene-based waxes, and may lead
to anaerobic respiration and development of offflavors.
Effects of waxes on fruit ripening
Ripening processes, especially in climacteric fruit,
involve increases in respiration and ethylene
production rates, and requires oxygen.
Modification of the fruits internal atmosphere by
application of waxes, decreases oxygen levels and
thus retards ripening.
Low gas-permeable coatings, such as resins (shellac)
greatly modify the internal atmosphere and
effectively retard ripening but may create anaerobic
conditions.
High gas-permeable coatings, such as polyethylenebased waxes, have less effect on the modification
of the internal atmosphere, and create less risk of
development of anaerobic conditions.
Tommy after 3 wks at 12C + 10d at 20C
Organic wax Z538
Uncoated
Tommy after 3 wks at 12C + 6 days at 20C
Com.
Z639
Z637
Z640
Banana stored for 2 weeks at 12C + 2d
ethylene + Wax + 1 wk at 17C
Wax
Z 538
WAX
PA
Control
Biological Ettinger after 3.5 wks at 5C
+ ethylene + 1 wk at 20C
Non-waxed
Organic wax
Effect of organic wax on chlorophyll
breakdown in biological Ettinger peel
Hue
Waxed
129.3 Ho
Control
117.2 Ho
Chlorophyll
188.8 µg/g
128.5 µg/g
Oroblanco after 6 weeks at 12C
Carnauba wax
Non-waxed
Effects of waxes on appearance (gloss)
Resins and various wax emulsions can impart high
gloss to coated products.
Non-waxed
PE wax
Effects of waxes on nutrition value
Waxing may decrease internal oxygen levels and,
thus, generally decrease metabolism.
For example, in carrots and peppers, it was reported
that coatings resulted in higher carotenoids
(vitamin A precursors) and ascorbic acid levels.
Effects of waxes on decay
Waxes on fruits and vegetables can also act as
lubricants to reduce surface injury and seal
wounds. Therefore, with less wounding of the fruit,
decay caused by wound pathogens usually
decreases on waxed fruit.
In addition, in commercial practice, waxes also
provide a carrier for the application of fungicides,
food preservatives, biocontrol agents, etc., which
reduce decay development.
Conclusions
1) Waxing is used to:
Enhance shine
Reduce water loss and shrinkage
Retard ripening
Provide a carrier for application of chemicals
2) It is important to chose the correct wax formulation
for each type of fruit or vegetable.
3) Application of waxes may decrease internal 02
levels, and lead to anaerobic respiration and
accumulation of off-flavor volatiles.
4) Research is now aiming to develop natural edible
coatings for use in fruits and vegetables.
Thank you for your attention!