Written by  Jake Newnham, Nuffield Scholar

Improving sweet cherry fruit quality—the cold chain (part 4)

Research has shown that decreasing the water drop height in shower-type hydrocoolers can significantly reduce the incidence of pitting damage in cherries.

Hydrocooling (cont from last issue)
Water drop height
Water drop heights of one metre could cause pitting damage to approximately 40% of all fruit directly impacted by water.
Conversely, water drop heights below 20cm reduced the amount of damaged cherries to under 10%. This figure lowers once again when a plastic mesh screen was added to diffuse the water and reduce water droplet size and velocity.
The same research showed that shower-type hydrocoolers were causing pitting damage to 18.6% of fruit and bruising damage to 9.6% of fruit in current systems at the time.
Pitting
Pitting is a major post-harvest defect characterised by irregular ‘pits’ on the cherry surface.
These marks often appear on the shoulders of the cherry and are caused by physical damage during packing.
Pitting generally becomes visible several days or even weeks after picking, meaning that it is next to impossible to grade out, even with modern electronic graders.
Apart from detracting from the visual appearance of fruit, pitting also increases the respiration rate which speeds up the rate of decay at the injury site and in turn reduces shelf life.
Whilst pitting can be caused at many different stages of the packing process, most notably by the cluster-cutter or cherries dropping onto dry belts, improper hydrocooler setup can cause damage to fruit and poor systems can and should be easily redesigned.
Packing
Tipping fruit
Packing begins with full bins or totes of cherries being transferred, usually into a water filled dip tank, before being sized and sorted. This tipping is done in a number of ways including:
Lifting the bin slowly and letting cherries roll down a ramp into water.
Rotating the bin upside down with a canvas lid that slowly retracts letting cherries fall into water.
Fully submerging the bin and completing the tip entirely underwater
Tipping bins underwater is now commonplace for large commercial sheds as it offers far less potential for damaging fruit, although perhaps it is only viable for packhouses with a high throughput.
Tipping a full bin instantly requires that fruit to be transferred from the water and through the system fairly swiftly as excessive submerged periods can lead to fruit cracking, particularly in some cultivars.
Tipping fruit from totes once again provides the greatest benefit to fruit quality but increases the cost of handling and reduces fruit throughput.
In the Pacific north-west of the USA, many large sheds use a rotary tote tipping machine to handle sensitive cultivars such as Rainier. The premium returns for these cultivars offset the extra care in packing and increased cost of labour.
In fact, most packing sheds handling these cultivars have designed their lines so that the electronic grader can be fed through the conventional bin dump, or through the rotary tote tipper.
Cluster cutter
Generally, the fruit from the totes will travel through an alternate cluster cutter. This cluster cutter belt will be running at a slower speed to the conventional line as it has been shown to reduce damage (Figure 1). (continued next issue)

See this article in Tree Fruit Oct 2021