Postharvest Handling of Tree
Nuts and Dried Products
Jim Thompson
UC Davis
Handle Chestnuts like Fresh Fruit
Rather than Tree Nuts
• Store at -1 to 0°C (30 to 32°F).
• Cooling promptly to stop decay development
and preserve quality.
• Storage humidity = 90 – 95%.
• Package in microperforated plastic film to
minimize water loss.
Maturity Stages
Almond
Walnut
Tree Shaker
almonds & walnuts
Windrow & Pickup
almonds & walnuts
Concealed Damage of Almonds
Storage at high temperature
and relative humidity
Pick-up Machine for Walnuts
Walnut Hulling & Drying
Kernel Darkening from Sun
Exposure
Walnut Kernel
Color Quality
Price is inversely related
to kernel color
Pistachio
Maturity
- Pistachio shell split
is desirable.
- Early hull split is not
desirable because it
increases potential
for fungal infection.
Pistachio Nut Maturity Indices
• Ease of hull separation from shell.
• Shell dehiscence (splitting).
• Change in shell color (green to ivory).
• Decrease in fruit removal force.
• Kernel dry weight and fat content.
Shake-Catch System for
Pistachio Harvest
Sorting Pistachio Nuts to Remove
Defects
Pistachio Hull Removal
Shell Staining
Shell Staining Scale for
Pistachio Nuts
Pistachio Drying
• Two-stage
– Heated-air drying to about 12% moisture
– Ambient-air drying to 5-6% moisture
• Heated-air
• Sun
Pistachio Dryers
Cross-Flow
Continuous Belt
Drying Temperature
Drying
Drying Time
Open shells –
Temperature to 5 – 6% MC kernels released
(°F)
140
(°C)
60
(h)
14
(%)
0
160
180
71
82
10
7
1
3
200
93
5
6
Should not exceed 71°C (160°F)
Storage Potential
Storing Nuts &
Dried Fruits and Vegetables
• Water activity – maintain the dry chain
• Temperature
• Oxygen concentration
• Effective insect control
• Time
• Cultivar
Water Activity of Selected Nuts and
Dried Fruits & Vegetables
Equilibrium Moisture Content
Taste
Hedonic Scale
9
Low quality
Odor
8
Flavor
7
6
5
4
High quality
3
0
20
40
60
80
Storage Humidity (%)
Placentia Perfection walnuts stored for 7 months at 72°F
Rockland, 1957
Kernel Darkening
Dark Kernels (%)
20
16
Serr
12
8
Pedro
4
0
0
3
6
9
12
Storage Time (mo)
Lopez et al., 1995
Insects and Decay
Insects cause Quality and
Weight Loss
• Navel orangeworm
• Indian meal moth
• Dried fruit beetles
• Saw tooth grain beetle
• Merchant grain beetle
• Raisin moth
• Fruit fly
Aeration to Control Storage
Temperature and Humidity
• Regularly ventilate storage with outside air
to maintain uniform nut moisture in
storage.
Average Air Temperature
Temperature
°F
90
80
70
Red Bluff
°C
30
Sacramento 20
60
50
10
Maintain temperature as
long as possible
Cooling
40
30
S
O
N
D
J
F
M o n th
M
A
M
0
J
J
A
Grain Aeration
Components
Perforated floor
Metal grain bins
Ventilation fan
Insect Control Procedures
• Fumigation - methyl bromide, phosphine, propylene
oxide.
• Freezing at -18 ºC for more than 2 days.
• Heat treatment (50-55 ºC).
• Exposure to 100% carbon dioxide for more than 2 days.
• Storage temperature <10 ºC reduces insect activity.
• Storage in 0.5% oxygen (balance nitrogen) atmosphere
reduces insect activity.
• Irradiation at 750 Gy.
Sensory Quality of Irradiated Almonds
Quality is reduced at 1.5 kGy or higher
Experimental Insect Control
Treatments
–Fumigation with carbonyl sulfide, sulfuryl
fluoride, or ethyl formate.
–Controlled atmospheres (<0.5% O2 and/or 4060% CO2).
–Heat treatments (radiofrequency).
–UV & IR radiation.
Reduce Losses of Dried Products
• Dry below 0.65 water activity.
• Store below 10°C (50°F), storage time increases
with lower temperatures.
• Clean storage before filling.
• Protect from birds and rodents.
• Insect disinfestation as needed.
• Protect against reinfestation using insect-proof
packaging.