SECTION 2. POSTHARVEST TECHNOLOGY OF CHESTNUTS
2.1 The three Cs
CLEANLINESS, CARE and COOLING are the 3 keywords for each postharvest
process of fresh horticultural crops. For chestnuts, these 3 actions are very
important from the harvest to the consumer table. Cleanliness means the adoption
of all procedures to avoid, as much as possible, external contamination (microorganisms,
toxic compounds, metals, pesticide residues, and more) of chestnuts; care means
to deal with chestnuts as with a very delicate commodity, protecting them from
each potential injury; cooling means to keep down the temperature as much as
possible from the harvest to the consumer table.
2.2 Maturity index and Harvest
There are no commercial indices of maturity to apply for chestnuts
harvest. Depending on the country, harvest usually is affected by the size of
chestnuts tree and the labour cost:
1. When the trees are huge, even 20 m high, the only way to harvest
is waiting for the drop of the spiny fruits which start to senesce and open.
At this stage, chestnuts are perfectly ripe and the taste is very good. The
problem with this kind of harvest is that frequently chestnuts remain on the
ground, very humid, for more than one day, thus contamination of fungi is favoured.
Low cost polypropylene net tarps are suggested, but often chestnut trees growth
environment is rough because of mountain or hill areas. In this case, several
pickings per day must be organised.
Clean baskets (plastic is better because is easy to clean and lasts longer)
are required and care must be taken: in order to not break the chestnut tuft,
to place chestnut leaves on the bottom for cushioning the basket.
If chestnuts must wait in the field, inexpensive canvas tents are very useful
to shade chestnuts and to permit ventilation.
2. Another harvest technique is hitting branches when chestnut
trees are not too tall. The advantage of this technique is that chestnuts drop
in the burr and so fungi contamination is reduced; the disadvantages are:
- chestnuts are often unripe and so without flavour;
- spiny fruits must be opened and this is not so easy to handle;
- insect worms cannot escape in the ground, remaining inside and coming out
later.
The spiny fruit is a protection for the chestnut but in the harvest
basket the volume occupied is too high and less chestnuts are carried per time
unit, wasting time to make several trips from the tree to the unloading point.
This means that fruits can warm up waiting to be carried to the packinghouse.
Inexpensive canvas tents are recommendable for shading jute sacks or baskets
with spiny fruits, permitting ventilation. In any case, in this harvest system
is always better to harvest early in the morning to reduce the field heat.
In conclusion the best procedure where useable is:
- waiting the chestnuts drop because of right maturity and the optimum flavour
(sweet and less starchy)
- placing polypropylene net tarp
- picking up chestnuts quickly and placing them in the basket with care
- providing shading canvas tent for waiting chestnut or leaving under tree
shadow but not waiting for more than 3 hours
- providing clean harvest baskets or sacks and clean shipping container (jute
sacks, box)
In the case of hitting harvest technique:
- hitting only the yellow-brown, slight opened, fruits; this means making
several hitting operations for each tree;
- picking up the fruits and placed them under shading canvas or under tree
shadow
Mechanical harvest is becoming common by using vacuum system especially
where chestnut orchards are located in flat area and ground under the tree is
clean. The disadvantage of this technique is the tuft break (Fig. 7) which increases
the potential for fungi contamination during water curing or heat treatment
with hot water.
Fig. 7 On the left, chestnut without tuft after mechanical harvest
(Source: ARSIAL, 1999)
Transport from the field requires care and speed. Care because all
kind of impacts due to uneven road or too fast driving can break the chestnut
tuft; in the case of spiny fruits this precaution is less important. Speed is
important to ensure the rapid processing of chestnut or rapid cooling. The shorter
is the time from harvest to the farm house or packinghouse the less is the chestnut
perishability with time.
2.3 Quality parameters and procedure
Quality standard for chestnut is usually the size, but quality
must be something more. Quality is also appearance, taste, and free from internal
defects. So, it is very important that growers or packinghouse managers guarantee
high quality chestnuts keeping in mind these quality parameters and providing
information on what has been done on chestnuts in the orchard. The packinghouse
manager should follow these steps:
- select the grower which ensures yearly high quality chestnuts or,
in the case of grower, selecting the orchard in which produces high quality
chestnuts (size, taste, appearance, good response to storage);
- mark all containers coming from different growers sampling the chestnuts
and scoring for the quality (Total Quality Score = TQS, see Annex 1); this
is important for traceability during storage and distribution; give a code
;
- keep clean the equipment and all the facilities: yearly sterilise
walls, floor, ceiling, equipment with compounds permitted for food; keep out
mice and other animals;
- check accurately the equipment, few weeks before starting of chestnuts
season;
- monitoring temperature and RH in the cold room (shut the door and check
the temperature, RH, and ventilation inside the room in all the corners,
below the cooler, and close to the door);
- testing the efficiency of each machine by sampling chestnuts before and
after the operation;
- testing washing or water curing tanks; water quality is important, no
rust, no other potential contaminants; check the pump for water recycling;
- checking the performance of horizontal and elevator belts: their speed
must be controlled and constant;
- test job quality of sorters by sampling chestnuts and
measuring how many are the rotted ones after the sorting table (pay the sorter
on the basis of quality job); do not keep more than 2 h the sorter on the
table sorting; guarantee good quality environment of work (safe, clean, less
noisy it is possible); oblige them to use gloves and masks and, if the environment
is too noisy, the headphones;
- pack chestnuts with a label where it is written beyond the weight,
all the information on what you have done: code of the sample, water curing,
sorting, brushing, storing, and suggestions for keeping in the shop and at
home.
2.4 Handling procedures
At farm
As soon the spiny fruit arrives to the farm house the following
procedure must be applied:
- remove soon the chestnut from the burr paying attention not to injury or
break the tuft; hand removal can be used but even a simple handy machine made
by two counterrotating plastic rollers (even car tires) which squeeze out
the fruit can fasten the operation;
- if chestnuts are sold immediately, do not dip in the water; dipping in
the water is used frequently for removing the chestnuts which are floating
(rotted) but the sorting efficiency is very low; if this system is required,
chlorinate the water;
if chestnuts need to be stored, dipping chestnuts in the water with some chlorine
(approximately 500 ppm = 0.5 ml/l) for 3 to 7 days (7 days is more effective
in killing the worms; longer time gives the risk of high amount of ethanol
inside the flesh), in the shadow; removing the floating chestnuts;
- placing a plastic film over the floor and spreading chestnuts on the floor
in a very thin layer and open the windows or turn on the fans to let them
well drying superficially;
- fill the jute sacks (better if plastic or wooden box).
Precautions:
- clean very well the tank
- add clean water possibly at 10-15°C
- renew the water when new chestnuts are dipped
- dry very well the chestnuts before shipping but, above all, before
storage.
At packinghouse
The area for the handling equipment must be in a closed room with
a door for the entrance of the chestnuts and a door for the exit of packaged
chestnuts. During working time, the doors can be kept opened but thick, flexible,
plastic strips curtains must be installed which provide some kind of protection.
Room temperature must be between 10 and 15°C and 3-4 meters
around the equipment must be left free. This area must be nearby to the cold
storage facility.
If chestnuts are in the burrs, they must be removed. This can
be done by hand or by machine. The former takes longer time but it is more reliable
and less injurious. The latter is faster and the capacity of the machine must
fit with the capacity of the other machines.
Chestnuts can come directly from the farmer or from the storage
facilities:
Case 1 - From the farmer and sold immediately
- sort by appearance (Fig. 8) rejecting chestnuts with superficial defects;
if hot water treatment must be done (49-50°C/40') to kill the worms, be
sure the tank is clean and the temperature is correct. After this treatment
chestnuts must be cooled down;
Fig. 8 Sorting table for chestnuts
- brush chestnuts by using gentle rotating brushes, blowing air
to reduce the dust and removing residues;
- size chestnuts (Fig. 9): in China, in XinXian area, the chestnuts diameter
ranges between 15 and 30 mm (approximately 42 chestnus /kg), so 3 sizes is
enough; chestnuts are collected in 3 tanks from where they can be hand loaded
on sorting table or by using a cup elevator (Fig. 10) which move from one
tank to the other;
Fig. 9 Perforated sizer for chestnuts .
Fig. 9 Perforated sizer for chestnuts (close view).
Fig. 10 Chestnuts collector and cup elevator belt
- pack and label: ·
- choose the pack (type and size) in function of the market destination; vertical
filling of nets or sacks is fast but should be used for lower quality chestnuts;
- top quality chestnuts (0.5 kg weight) must be packed in plastic tray wrapped
with plastic film with an extensive information label.
Case 2 - From the farmer and then to storage
- unload chestnuts in tank for water curing (Fig. 11) or in the case high
CO2 shock treatment just brush chestnuts; drying well the water cured chestnuts
before starting the storage (in the cold room or other ventilated place)
Fig. 11 Water tanks for curing chestnuts
- load chestnuts in plastic (better) or in wooden box or special
wooden container of bigger size (Fig.12) and place them in storage.
Fig. 12 Special wood container for handling chestnuts
Case 3 - From the storage
- for the sorting, dip chestnuts in chlorinated water, wipe chestnuts by air
stream then take to the sorting table; the reason of dipping in the water
at this step is because the internal decay is very spread and so chestnuts
can float and be discarded;
- sort very accurately by appearance rejecting the one which looks dull and
black or with superficial defects;
The further steps are the same of the Case 1.
Testing equipment performance
Sizer
- sampling: take 100 chestnuts per exit and measure with a round sizer; if
more than 10% are smaller for the big sorted chestnuts, the performance is
not good; for the other sizes the percentage must be over 20%
- solution:
· check sizer loading
· check the slope and the rotation speed
· keep clean the perforated roller
Brusher
- sampling:- mark with a pen 100 chestnuts with the tuft, before and after
the brushing; if more than 20% of chestnuts loses the tuft performance is
not too good
- solution:
· check brushes hardness by rubbing one chestnut against the brush
and see what happens to the tuft: if the tuft is broken or removed,
the brush is too hard;
· check the rotation speed of the brushes
Check the brightness of chestnuts, exposing them against the sun
or white light: they should shine.
Hand sorting for appearance and external defects
- check the job of the workers by sampling 100 chestnuts and see accurately
how many are contaminated; if more than 10% for the big size chestnuts, the
sorter efficiency is low and the worker needs to be moved to another operation
in the packinghouse, momentarily;
- keep good work environment compatible with chestnut requirements (T=10-15°C;
RH= 80%; oblige the sorters to use gloves and masks)
- provide enough light over table (1 m long lamp/m table at an height of 70-100
cm; the light cone must cover all the table surface and not spotted; chestnuts
must bright under the light)
- replace the sorter every two hours;
- not overload the sorting table; the sorter must have in front only one layer
of chestnuts, rolling and spread (his/her eyes must look 0.5 m2);
- not speed up the sorting belt (5-8 cm/sec).
Worker conditions
- try to reduce the noise as much as possible; for workers working close to
a noisy machine, oblige them to use headphones;
- when it is possible oblige the use of gloves and masks;
- forbid the entrance to the plant of foreign persons
- forbid the operators to throw away waste in the working area.