Temperature and humidity

Environmental conditions after harvest can make or break stone-fruit quality. By Anna Mouton.

“Low temperatures are important for maintaining fruit quality,” says Handré Viljoen, stone-fruit research manager at ExperiCo. “At the correct temperatures the fruit will develop fewer internal defects and fungal growth will be reduced.”

Most growers are very aware of the need to remove field heat after harvest. Bins must be kept out of the sun in a cool place. “In the old days the fruit would be covered with a wet blanket,” says Viljoen, “but nowadays they say that it could be a source of spores that cause decay.”

Getting fruit into a pre-cooling cold store quickly is critical. From there, the sooner stone fruit is packed and put into forced-air cooling the better. But Viljoen cautions that there is a danger in cooling fruit too fast. He recently concluded a three-year trial on heat damage in plums which showed more internal damage in fruit cooled within 24 hours than in fruit cooled within 48 hours.

“Our research was based on Laetitia, so I don’t want to generalise to other cultivars,” says Viljoen. “Commercial operators cool within 36 hours without problems. But it can be easy to cool too fast.”

Cooling too slowly holds other dangers. “We tested extended cooling for 72 hours, but then we saw problems with moisture loss, resulting in shrivel and, in some cases, loss of flesh firmness,” says Viljoen. He adds that use of 1-MCP – 1-methylcyclopropene – enabled maintenance of firmness.

Why moisture loss occurs

Humidity is the amount of water vapour in the air. When air contains the maximum amount of water it can hold, it is said to be saturated. Warm air can hold more water vapour than cold air before being saturated.

Specific or absolute humidity refers to the amount of water in saturated air at a given temperature. Relative humidity is the degree of saturation expressed as a percentage. For example, at 25C, every cubic metre of saturated air will contain 23 grams of water vapour. If the same air is 95% saturated, it will contain 22 grams of water.

In contrast, at 00C, every cubic metre of saturated air contains less than 5 grams of water vapour. Attempting to add more water vapour to this air will lead to condensation of excess water.

Water tends to move from regions of high relative humidity to regions of low relative humidity. This is what drives moisture loss from fruit. The relative humidity inside fruit is 100% because the intercellular air spaces inside the fruit are saturated with water vapour. When the fruit is placed in air which is not saturated, the moisture inside the fruit moves out, potentially leading to shrivel.

“From the moment a fruit is picked, it starts losing moisture,” says Viljoen. Moisture loss is much greater at high than at low temperatures, so the warmer the fruit, the faster it dehydrates. Viljoen stresses that any delay in cooling stone fruit significantly increases the risk of shrivel.

The packaging predicament

Some stone fruit types and cultivars are packed in perforated plastic wrappers or bags to limit moisture loss. The bag creates a region of elevated relative humidity around the fruit by trapping water vapour. Plastic is an attractive option for this packaging because it is effective, inexpensive, lightweight, and easy to use.

Unfortunately, plastic is also polluting, and producers are under pressure to reduce its use. Viljoen has been experimenting with alternatives to single-use plastics as part of a Hortgro-funded project on shrivel control in nectarines and plums.

“We initially looked at two different weights of a paper liner. A 19-gsm – grams per square metre – liner was too thin, and it tore. A 55-gsm liner was hopelessly too thick. So we tried an in-between weight and had very good results, although not as good as plastic,” says Viljoen.

Viljoen also applied different edible coatings, two of which gave similar results to the paper liner. “Our next step is to look at a combination of paper and a coating, because hopefully the combination may give you the same results as plastic.”

Another aspect of his trials was a comparison of fruit quality after storage in cold rooms with and without humidity control. He found that maintaining relative humidity above 90% reduced the levels of shrivel and internal defects after shelf life.

Surveying the storage scene

Controlling humidity during pre-cooling and packing is impossible, explains Viljoen, because these activities take place in open or high-traffic areas. However, humidity control during forced-air cooling is essential because dry air moving over the fruit surface continuously takes away moisture.

Viljoen recently conducted a survey of commercial stone-fruit cold rooms in the three main stone-fruit production areas. He monitored temperature and relative humidity during pre-pack bin holding, accumulation, and forced-air cooling. The relative humidity levels in the pre-pack cold rooms were mostly 85%-90%. Data collected during forced-air cooling showed that relative humidity was usually below 90%.

“The aim should be at least 95%,” says Viljoen, “but the best commercial rooms in our survey were in the low nineties.” He believes that part of the problem is lack of awareness. Very few people measure the temperature and humidity in their cold rooms. The cold rooms are also not equipped with humidity-control systems, even though this technology is available.

Lack of humidity control during shipping can also exacerbate shrivel. Container vents are usually kept open when transporting stone fruit to prevent carbon dioxide building up. Unfortunately, this increases moisture loss. Therefore, it was decided to test controlled-atmosphere storage on four different plum cultivars to determine their tolerance to elevated carbon dioxide levels.

“Based on one year’s data, Laetitia didn’t do well with any modification,” reports Viljoen. “The other three had better firmness and fewer internal quality problems at 6% carbon dioxide than under regular atmosphere.” He is currently planning work with Maersk to measure carbon dioxide levels in containers, and to trial containers which have vents that open automatically when carbon dioxide levels rise above a certain level.

Whereas temperature has always been viewed as important, the damage caused by low humidity is being increasingly recognised. In the case of shrivel in stone fruit, prevention comes down to humidity control. “If you keep your humidity high, you can’t lose moisture,” says Viljoen. “That’s just the law of nature.”

For more about the relationship between humidity and temperature, and how relative humidity affects moisture loss in fruit, read our article on page 6 of the 2018 Hortgro Technical Symposium report. For more on shrivel in stone fruit, consult the September 2019 issue of Fresh Quarterly.