Improved postharvest management of chestnuts (CH13005)

What was it all about?

Unlike other tree nuts, chestnuts are quite perishable and need to be kept refrigerated after harvest. Growers usually store harvested nuts in bins, with rooms set as low as ‐3^oC in order to optimise storage quality.

This study conducted a series of trials to ascertain the effects of storage methods on nut quality. Specifically, it sought to find out…

• If current practices, particularly room cooling in bins, cool chestnuts efficiently
• If storage temperatures can be reduced to reduce costs without jeopardising quality
• The critical temperature at which freezing injury can occur.

Researchers measured cooling rates inside chestnut fruits under different commercial systems.

Three grower packers (small, medium and large operations) took part in the trials, which compared cooling rates in polypropylene bags, mesh bags, wooden bins and plastic bins as well as room cooling in a lined bin or a bin with ventilation pipes through the load. Other technologies that can be used to speed up cooling rate were tested, including hydrocooling and forced air cooling. 

Researchers found the following results…

• Freshly harvested chestnuts packed into a lined bin and simply placed in a cold room failed to cool in reasonable time and resulted in rapid mould growth.
  
  
• While chestnuts placed in open wooden or plastic bins cooled more quickly, lack of air circulation through the centre of the bin meant cooling rates were still relatively slow. There was no obvious benefit from additional venting in the plastic bin.
  
  
• A system of home‐made ventilation pipes through the stack significantly increased cooling rates, providing a possible low cost solution for growers.
  
  
• Forced air cooling was by far the fastest way to cool chestnuts, reducing cooling times from around 24 hours to just 60 to 100 minutes. The system is inexpensive and can be used in an existing cold room. 
  
  
• Freezing of chestnuts caused kernel darkening, softening, weight loss and development of ‘off’ odours and flavours.
  
  
• Freezing appears to be a combination of time and temperature, with the critical temperature around ‐3^oC. As little as 2 hours below ‐3^oC caused significant damage. As time below ‐3^oC increased so did weight loss, darkening and rancidity. 
  
  
• Not only can very low temperature cause damage, but it may provide little improvement in storage quality. A comparison of storage at 0^oC and 5^oC found little difference between samples stored for up to eight weeks, with variation due to storage temperature only seen after longer storage times, and not for every variety. 

The researchers concluded that storage at close to 0^oC can extend chestnut storage life without risking freezing injury or adding extra costs for growers. Even colder temperatures should be used with caution, and only if fruit is to be stored for an extended period.