GUIDELINES FOR HANDLING & STORAGE OF CHESTNUTS

Summary
  • the cleaner the chestnuts the better
  • the drier the chestnuts the better
  • the colder the chestnuts the better
  • And in most instances the detail of chestnut handling and storage will be at the direction and primary concern of the packhouse and buyer with whom you will need to consult

Background

Most NZ chestnut orchards are harvested by hand, chestnuts are then dipped in a weak solution of sodium hypochlorite and packed into plastic bags ready for coolstorage at 0-2ºC. Recent research has identified an improved chestnut sterilant to hypochlorite and storage treatments which results in substantially improved nut quality for our customers.

Sodium hypochlorite is a good, safe and commonly used surface sterilant but it breaks down quickly in contact with organic matter and, even when working well only eliminates some surface fungi; not internal rots, not latent infections under the shell, and not all resting-spores. There are now better surface sterilants and better washing procedures we all need to be using.

Similarly with plastic bags. Getting fresh chestnuts into plastic bags as quickly as possible after harvest has the great virtue of minimising water loss and maintaining weight an important consideration when you're being paid by weight but chestnuts that are “too fresh”, with too high a moisture content and surface wetness, are a target for fungi rots. Wet chestnuts go rotten much faster than drier chestnuts.

The definitive French chestnut handbook “Chestnuts and Marrons” authored by Dr Henri Breisch (extracts have been reproduced in “Chestnutz News” by permission) is quite clear on this point. A typical, freshly harvested chestnut, at 58-67% moisture content is simply “too wet for successful storage”. The “ideal water content” is considered to be 50-52%. To reach this point may require some deliberate drying. At the very least there should be no surface water or dampness. Putting fresh nuts into plastic bags is sometimes just asking for trouble (especially when sweating or condensation occurs inside the bags).

Coolstores, even when running at the correct temperatures can also cause problems. To avoid sweating and condensation adequate air movement and ventilation is required. This is necessary to prevent fungal growth. Large volumes of tightly packed bags of chestnuts can work against this. Lots of in-and-out movement, frequent restacking, removal of bags of chestnuts and their later return to coolstorage, and the presence of rotten nuts in some bags doesn't help. It commonly takes up to seven days from time of admission for a consignment of chestnuts to drop to stable coolstore temperatures which, even then, have averaged 2-3ºC, (allowing rots to continue to develop) even though the coolstore itself has been set to run at 0-1ºC. Conversely, chestnuts removed from coolstorage, even for a short time, can warm up very, very fast (and be much, much slower to cool down again).

Ironically, it can sometimes be easier for small producers to store chestnuts for long periods, using only modest resources, far more successfully than larger commercial coolstore operators. Using the correct cool storage conditions it's possible to store chestnuts for over 18 months. Sure they loose some weight and some chestnuts germinate but the incidence of surface mould is negligible, and the incidence of internal rots is no higher than when they first entered the coolstore. They are still perfectly OK to eat. So it can be done.

General guidelines:

  1. Whenever possible, put only good quality chestnuts into coolstorage. (Even the best handling and storage conditions in the world won't stop a chestnut that is already rotten, going rotten). Even a small percentage of “bad” chestnuts can jeopardise the storage life of an otherwise good batch : respiration rates will be greatly increased, water loss (and therefore condensation and sweating) will then be increased, and fungal (or bacterial) infections can then spread. There may even be localised heat build-up. To prevent this means washing and dipping chestnuts, throwing away all “suspect” chestnuts and if possible use flotation grading (see below). (Ideally this should be done twice, once on the orchard and again, in the packhouse, just before export or processing).
  2. In general, the drier the chestnut the better it will store. This doesn't mean drying the nut so as the shell and kernel are soft to touch (many chestnuts at this stage are “dead”, and this can make storage problems much worse) but certainly a chestnut that is bone-dry to the touch, with no sign of sweating or condensation during storage is preferable. If you want your chestnuts to store well, be prepared to accept some weight loss.
  3. In general, the colder the temperature the better chestnuts will store. Just a few degrees can make all the difference to fungal growth, especially Phomopsis. 2-3ºC is way too high. Below ~ -2ºC is too low due to the risk of freezing damage. (If you want to freeze them, then freezing as fast as possible, which usually means using very low temperatures, minimises damage to the actual nut and helps maintain quality). “Just a little bit of freezing” can be the worst of both worlds giving only partial protection and causing marked deterioration in subsequent taste, texture, colour, etc.

The following three principles have been successfully put into practice by individual growers such as Ray Knowles and commercial companies such as Kiwi Chestnut Co-operative Company Ltd (KCCC).

KCCC now uses a new washing and dipping procedure involving “Vortexx”, a hydrogen peroxide-based chemical which HortResearch has shown to be much more effective than the previously used sodium hypochlorite. Vortexx is much longer-lasting and more acceptable for organic production. (Like any such chemical it also works best on chestnuts that have first been washed and rinsed to remove excess dirt, grass and other debris). More detailed information about this chemical and how best to use it is available from David Klinac, HortResearch or Judy Fitness, KCCC, Ph. 07-823 6692, or the manufacturer Craig Scoun, Ecolab Ltd, Ph. 025-444 994.

Growers who supply packhouses are now required as part of their supply agreements to use this product in association with on-orchard flotation grading of their chestnuts to remove as many rotten and suspect chestnuts as possible prior to coolstorage. This technique is both simple and very effective, but needs to be done properly. The water solution must be adjusted regularly for different batches of chestnuts. It won't work if using non-standard chemicals or procedures (as some growers have attempted). Some of the “finer points” of flotation grading remain proprietary to HortResearch. (Contact David Klinac if you require further information). Whatever approach is adopted, the fewer “inferior”-grade chestnuts that are put into storage to begin with, the better they will keep.

KCCC has also developed use of open-weave onion bags to store chestnuts, rather than plastic bags. Coupled with a special coolstore layout involving careful stacking to allow ease of air movement, and low temperature storage (0 to -2ºC). Partial drying of the chestnuts is carried out under controlled conditions and the development of rots and surface mould is minimised. Although being used primarily for the storage of processing-grade chestnuts, fresh nuts remain in saleable, even exportable condition for a surprisingly long time before drying out irreversibly.

Plastic bags and onion bags are however only the two opposite extremes of the spectrum. There are other ways to package chestnuts for coolstorage. Where longer-term storage is required, but without the water and weight loss of onion bags at sub-zero temperatures, then something as simple as “kleensacks” or potato-style 10-20kg multi-walled paper bags can be a useful compromise. Using these at conventional 0-2ºC coolstorage temperatures can work very well. Rots and moulds are kept to a minimum, but water loss is not excessive. (Care must be taken with nut quality going in, however, especially nut wetness). Leave bags open for up to a week, in coolstorage, to assist drying. A combination of a paper bag and a plastic bag together can also work well, and allows a degree of “fine-tuning”. Contact Ray Knowles (07-825 2744) for more details.

Looking further ahead:

There are some new dip and wash chemicals currently under test that promise to be much more effective, again, than “Vortexx”. Some revised washing and dipping procedures may also make it easier to get at deep-seated latent fungal infections sitting beneath the shell, on the surface of the pellicle : currently still proving troublesome.

Controlled drying of chestnuts is another key area. Even a few percentage points difference in moisture content can make a major difference to susceptibility to fungal rots and greatly increase storage life. The problems are:-

  1. How to measure and monitor these differences easily and non-destructively.
  2. How to carry out controlled drying of a large batch of chestnuts quickly and accurately : so that all chestnuts within that batch end up with the same moisture content. (Be warned : incorrect drying of chestnuts can produce bad taints and off-flavours).

Controlled atmosphere (CA) and modified atmosphere (MA) storage of chestnuts has been attempted with some degree of success. We now think we've got a much better, cheaper technique that will work. It's still experimental, and it would require some radical changes to current chestnut handling and storage procedures, but it could mean effective year-round storage of fresh chestnuts with negligible quality loss. Fingers crossed.

Until then :-

  • the cleaner the chestnuts the better
  • the drier the chestnuts the better
  • the colder the chestnuts the better
  • And in most instances the detail of chestnut handling and storage will be at the direction and primary concern of the packhouse and buyer with whom you will need to consult

CONTACT THE NZCC

For further information about the New Zealand Chestnut Council, contact:

David Klinac - 10 June Place, Hamilton. Ph 07 8569321
Email: