Postharvest Handling of Tree Nuts and Dried Products

Jim ThompsonUC 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.

AlmondAlmond

WalnutWalnut

Maturity Stages

Tree Shaker almonds & walnuts

Windrow & Pickup almonds & walnuts

Storage at high temperature and relative humidity

Concealed Damage of Almonds

Pick-up Machine for Walnuts

Walnut Hulling & Drying

Kernel Darkening from Sun Exposure

Price is inversely related to kernel color

Walnut Kernel Color Quality

- Pistachio shell split is desirable.

- Early hull split is not desirable because it increases potential for fungal infection.

Pistachio Maturity

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 Time to 5 – 6% MC

Open shells – kernels released

(°F) (°C) (h) (%)

140 60 14 0

160 71 10 1

180 82 7 3

200 93 5 6

Drying Temperature

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

Rockland, 1957

Placentia Perfection walnuts stored for 7 months at 72°F

3

4

5

6

7

8

9

0 20 40 60 80

Storage Humidity (%)

Hed

on

ic S

caleOdor

Flavor

Taste

Low quality

High quality

Kernel Darkening

0

4

8

12

16

20

0 3 6 9 12

Storage Time (mo)

Dark

Kern

els

(%

)

Pedro

Serr

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

Month

30

40

50

60

70

80

90

S O N D J F M A M J J A

Sacramento

Red Bluff

Cooling

Tem

pera

ture

30

20

10

0

°F °C

Maintain temperature as long as possible

Grain Aeration Components

Perforated floor

Ventilation fan

Metal grain bins

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 40-

60% 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.