Kiwifruit

Infobox on Kiwifruit
Example of Kiwifruit
Freshness facts
Optimum carrying temperature	0°C
Highest freezing point	-1,5°C
Acceptable product temp. at loading into containers	Max. 2°C above carrying temperature
Optimum humidity	90%-95%
Ventilation setting for containers	60 m³/hr
Storage life	2-3 months
Climacteric / non-climacteric	Climacteric
Ethylene production	Low
Ethylene sensitivity	High
Modified / controlled atmosphere	3%-5% CO2; 1%-2% O2
Potential benefits	Excellent Low O2 delays ripening Elevated CO2 can aid in retaining firmness
Availability
New Zealand South Africa South America	June - January May - June March - August

Kiwifruit

Harvesting and handling

Kiwifruit are harvested at the pre-climacteric stage (picking ripeness), to ensure the longest possible storage and transport life. One way of determining the stage of ripeness is the colour and size of the fruit. These days, however, the ripeness criterion commonly applied is the Brix value, which is a measure of the sugar/acid ratio of the juice. New Zealand law states, for instance, that a minimum value of 6,25% SSC (Soluble Solids Content) must be reached before harvest can begin. Late harvested kiwifruits retain their firmness better than early harvested fruit and have higher SSC at harvest and when ripe.

For the trade, the firmness of the kiwi fruit is of utmost importance, as it is on this that the storage/shelf life for the wholesale, as well as the retail trade mainly depends. Roughly, the following stages can generally be distinguished:

Harvest time : firmness 15 - 20 lbs. Optimally firm : >10 lbs (upon delivery) Hard fruit : firmness more than 5 lbs. Sensitive fruit : firmness between 2 and 5 lbs. Soft fruit : firmness lower than 2 lbs.

The fruit is considered to be eating ripe when the firmness drops below 2-3 lbs. and having a minimum of 14% SSC.

The following graduations can be used:

• Optimally firm: The fruit flesh is hard and does not deform even when subjected to considerable pressure. This is the ideal condition upon arrival in Europe.
• Firm: The fruit flesh is hard, but yields slightly to moderate pressure. At this stage the kiwi fruit is in excellent condition for transport to the retailer.
• Hard ripe: The fruit flesh clearly yields to moderate pressure. This fruit is still fit for retailing, but cannot be kept for a long period of time.
• Ripe: The fruit flesh is kneadable, but not soft or pulpy. This is the ideal stage for consumption.
• Overripe: The fruit is soft, pulpy, watery or shriveled and loses its commercial value.
  
  

From observation of kiwi fruit arriving on European markets it would appear that the most serious defect is excessively soft fruit. Cargo parties (mostly large importers) claim that in the event of soft merchandise, the storage, shelf-life and flavour are impaired. As wholesalers (and retailers) are mostly very reluctant to buy this fruit in non-optimally firm condition, allowances are often required. High quality kiwifruit should not be shriveled and should be free from sunscald, scars, growth cracks, insect injury, bruises, internal breakdown, and decay.

Cooling and storage

With regard to preserving kiwi fruit, research specialists indicate that at a temperature of -1°/+1°C the product can be stored for at least 3 months. For a longer period, up to 6 months, the ripeness grade at the moment of harvest is of utmost importance. The relative humidity in storage should range between 90% and 95%. Moreover, the product should be cooled to a pulp temperature of approx. 0°C within 12 hours. Hereby there must be no ethylene gas present, e.g. produced by apples, bananas, etc., as this gas accelerates the ripening process even at low temperatures.

Also of importance is the total storage period prior to shipment, storage condition, season influences (late or early harvest), timely picking, etc.

Kiwi fruit continues to ripen after harvest, changing from a hard and acid flavoured fruit to one that is soft and sweet; the rate at which this ripening process occurs being largely dependent on fruit temperatures and post harvest treatments. Even at 0°C the rate of softening is generally quite rapid during the first two or three months of storage; softening rates of 1 kg per week have been recorded.

As the fruit flesh temperature rises, so does the rate of softening, thus to ensure safe carriage of kiwi fruit and good storage life, the fruit flesh temperature should not be allowed to rise above 1°C.

Although fruit temperature is important in kiwi fruit storage, a more important factor is the presence of the ripening hormone ethylene. Ethylene is a hydrocarbon gas produced by most ripening fruits, including kiwi fruit. At 0°C, 0.1 ppm ethylene had been found to ripen freshly harvested fruit to eating ripeness in less than three weeks. The same treatment at ambient temperatures ripens kiwi fruit in less than 10 days, thus the presence of a few overripe fruit in a box could also accelerate the softening of the remaining fruit.

Several pathogens can cause postharvest deterioration of kiwifruit. Botrytis grey mould rot caused by Botrytis cinerea is the most important and can directly invade the fruit or enter through wounds. Kiwifruit become much more susceptible to Botrytis (and other fungi) as they soften. Thus, maintaining fruit firmness (by rapid cooling, cold storage, and use of controlled atmospheres) can significantly reduce pathological breakdown. Sunburned fruit and physically damaged fruit are also more susceptible to postharvest diseases.

Freezing damage is manifested by flesh translucency starting at the stem end of the fruit and progressing toward the blossom end as the severity increases. Susceptible fruit become somewhat yellow fleshed with prolonged storage. There was no "graininess" observed in the fruit that showed these symptoms. Freezing damage can occur on early picked kiwifruit when stored at temperatures below 0°C or when subjected to an early frost in the vineyard. Fruit frosted late in the season are usually affected on the shoulder where the cells collapse to cause a pinching of the fruit at the stem end.

Mixed loads

Kiwifruit should never be shipped with ethylene-emitting produce. The cargo is highly odour-sensitive and must therefore not be stored together with odour-emitting products either.

Cautions

O₂ below 1% can cause off-flavours. CO₂ greater than 7% for longer than four weeks may cause internal flesh breakdown.

Special considerations

One of the most troublesome concerns for storage of kiwifruit is management of ethylene in the atmosphere. Enormous losses are incurred when fruit soften prematurely in storage without ripening. The effectiveness of various ethylene removal techniques, e.g. catalytic converters, potassium permanganate filters, ozone generators, simple ventilation systems, etc., must be evaluated carefully for specific situation. For example, the frequency of opening the storeroom doors, proximity of the storeroom to sources of ethylene, such as the packinghouse itself or nearby roadways, presence of decaying fruit in the storeroom etc., all influence the efficacy of ethylene removal.

Kiwifruit respond well to CA and remain harder for longer. Commercial experience has shown response to be variable with obscure physiological disorders developing.

Storage disorders

Blue mould, Chilling injury, Grey mould rot, Phomopsis rot, Ripe rot.