Respiration is characterised by consume of oxygen and production of carbon dioxide, heat, and water. Because all perishable are living organisms we must care about all these parameters during the postharvest life, handling, storage, and transportation.
Usually seeds are characterised by low and constant respiration rate without any peak (non climacteric curve); in contrast, chestnut shows, high respiration rate, but anyway without variation. As reported in the Annex 3, respiration rate, varies between 5 and 20 mg/kg-h, depending on the temperature, which is similar to potatoes and onions (see footnote ).
Ethylene is known as ripening or senescence hormone, and also
as stress hormone. It is produced by all plant tissues, from seed through leaves
to flowers and fruits. Seeds produce very little ethylene and chestnuts behave
similarly (Annex 3). Presence of decay (insects, fungi) increases ethylene production
which can promote sprouting. However ethylene is not a problem for chestnuts
storage.
Plant cells use transpiration to cool down their temperature. Transpiration is triggered by vapour pressure deficit (VPD= tissueVP - environmentVP) and temperature: the higher is the VPD the higher is the transpiration and so the weight loss of tissue. VPD depends on the water content of tissue and air, thus it is related to the temperature because the water-air diffusion coefficient increases with the temperature.
At constant relative humidity (RH), decreasing the temperature, VPD decreases; increasing RH in the environment, VPD decreases when the tissue and the surrounding environment are at the same temperature.
If the temperature is decreased in the environment, even keeping the same RH, VPD increases until the fruit reaches the same temperature of the environment; when tissue and environment are at the same lower temperature, VPD is much lower than at higher temperature. This is the reason to remove quickly heat from fruits and vegetables (precooling).
Chestnuts have high transpiration rate. In one day at 20°C and 70% relative humidity (RH) chestnuts can lose even 1% of weight. If temperature is decreased rapidly and kept down, VPD is reduced and so the weight loss:
| 20°C | 70%RH | VPD = 7 mbar |
| 0°C | 70%RH | VPD = 2 mbar |
| 0°C | 90%RH | VPD = 0.6 mbar |
It means, just reducing the temperature (with the same RH) the weight loss is
cut down 3.5 times (if chestnut loses 1% at 20°C and 70% RH, at 0°C
and 70% RH will lose 0.3%); if the RH is increased to 90% we will have further
decrease of weight loss (only 0.08%).
1% = 10 kg of chestnut/1 ton in one day
0.08% = around 1 kg/1 ton in one day
Transpiration rate is also increased by mechanical injuries which
slightly break or scratch the peel (Fig. 4) or break the tuft. So attention
must be paid to all handling practices.
Fig. 4 Mechanical injuries on chestnuts after mechanical harvest and rough handling (ARSIAL, 1999)
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(ARSIAL, 1999)
In conclusion transpiration rate of chestnuts must be controlled absolutely by keeping high RH and low temperature in the storage environment.
With the same easiness of water loss, chestnuts uptake water when are dipped in water. So, during water curing they take up a lot of water which must be removed to avoid a strong fungi diffusion during the storage.
Chestnut kept in relatively high RH and at room temperature (20°C), sprouts easily. If kept at low temperature sprouting occurs at the end of winter, sometime before. Sprouting can occur even soon at the harvest time if chestnuts absorb water from the ground. High CO2 and low O2 have shown strong effect to delay sprouting while water curing promotes. Chlorine at certain concentration reduces sprouting. Anyway, once sprouting starts it is difficult to stop it.
Insects and fungi are the causes of losses during the storage.
All of them comes from the field and show up during storage and distribution.
When chestnuts drop, worms (Cydia and Balaninus)
escape from the seed and penetrate in the ground; chestnuts show a visible hole
(Fig. 5) so they can be removed. When chestnuts drop with the spiny fruit, worms
are able to escape later during distribution or storage and so immediate sorting
is difficult. Worms of insects will come out from chestnuts during the first
months of storage.
Fig. 5 Holes left by worm escape. These chestnuts need to be sorted out.
Different fungi attach the chestnuts: during the storage the most diffuse is the Cyboria which turns black and spongy the flesh (Fig. 6), but other fungi are known such as Rhizopus, Fusarium, Collectotrichum. Several are the ways of spores entrance but the break of the tuft is one of the most common. Water is vehicle of spores diffusion, from one chestnut to other during traditional storage practices such as sand storage or water curing.
Fig. 6 Chestnut black decay. At the initial stage the water sorting is inefficient.
When decay is diffuse inside the peel becomes dull and superficial mould
can develop in high humidity condition.
Chestnut sorting is easy when seed flesh is completely altered because the brown peel becomes black and dull, but, at the beginning of decay, sorting is difficult and even the floating is not succeeding.
Chestnut is not susceptible to any specific disorder. It is not chilling sensitive but it can freeze when temperature go below -5°C for few days.
Chestnut is not CO2 sensitive but it ferment in absence of oxygen for several days. Thus it is sensitive but only in the extreme environment conditions.
Internal browning can appear after cutting the seed flesh at the end of storage period which is increased after cooking. This might be due to the soluble sugars accumulation during senescence and beginning of sprouting.
Sprouting can be considered as disorder which reduces chestnut quality.
If we multiply these respiration rates by 2.6 (ratio between
1 mg CO2 and calories) we obtain the heat release by respiration:
5 x 2.6 = 13 cal/kg-h at 0°C; 20 x 26 = 52 cal/kg-h at 20°C
This means that: 100 ton of chestnuts kept at 20°C in 1 hour produces 5200 kcal (about 6 kW) .
In the same time: 20 mg CO2 /kg-h x 100 ton = 2 kg CO2 /h which corresponds approximately to 1 L /h and so 24 L/24h.
If we consider that:
free volume of 440 m3cold room of chestnuts, 70% loaded, is around 130 m3 (130000 L),
it means that each day CO2 rises of 0.02% (it is approximately the same amount contained in the atmosphere).
This at 20°C. At 0°C this amount is much lower.
In the same time, the same amount of O2 is consumed.
