The link between pre- and postharvest quality

What happens in the orchard doesn’t stay in the orchard. By Anna Mouton.

Apples’ storage potential is largely determined by their condition at harvest. Most growers are well aware of the crucial role of optimum maturity, but factors such as fruit size and structure also affect how fruit interact with different atmospheres and temperatures during storage.

“During the past two or three seasons, I have come to the conclusion that the interaction between orchards and fruit quality is very complex,” observed Willie Kotze, technical adviser at Dutoit Agri. “So how can we make it a little less complicated? One way is to reduce the variation of what we put inside the cold rooms.”

Speaking at the 2023 Hortgro Science Postharvest Symposium, Kotze acknowledged the influence of seasonal shifts in heat and cold units on maturity, but he challenged growers to relook at their orchards.

“We can explain a lot by evaluating the tree and canopy on which we produce the fruit,” he said. This will help us make better decisions about which orchards to put into short- or long-term storage.”

Lipstick on a pig

“If we want to reduce the variation of what we produce, we have to start with the canopy structure,” said Kotze. He explained that most of the carbohydrates in an apple are made by the leaves closest to it. High-quality fruit draw on actively photosynthesising leaves powered by abundant sunlight.

Kotze advocates smaller canopies with fewer branches spaced well apart so that each potential bearing position has sufficient light exposure. This will facilitate even flowering of strong buds throughout the tree. Uniform flower quality and condensed bud break contribute to fruit of similar size, quality, and maturity.

He showed a photo of a Rosy Glow orchard where the trees are too vigorous and the canopies too dense. The fruit on the top and outside of the tree had already been picked, and the grower was attempting to promote red colour development in the remaining fruit by stripping the remaining leaves.

The problem with this strategy is that the inner canopy fruit will probably reach post-optimum maturity before adequately reddened. “It’s like putting lipstick on a pig,” said Kotze, quoting New Zealand horticulturist Dr Ken Breen.

Post-optimum maturity of Cripps Pink and its relatives increases the risk of internal browning. More advanced maturity is also associated with postharvest problems such as softer fruit, lenticel disorders, and internal breakdown.

In addition, multiple picks require more labour and increase the opportunities for handling errors such as bruising and fruit drop.

“If you have good light penetration inside the canopy, the potential for the inside fruit to develop colour is the same as for the outside fruit,” said Kotze. “We need to look at the inner and outer canopy, and the top and bottom of the tree, and produce the same quality fruit in all those positions.”

The golden mean

According to Kotze, both over- and undercropping are detrimental to postharvest quality. Overcropping tends to delay red colour development, increasing the chances that fruit will be harvested at post-optimum maturity, with all the attendant ills already discussed. Overcropping usually delivers smaller fruit.

Undercropping is equally bad because it promotes tree vigour and too-large fruit. Bigger fruit are prone to the same postharvest issues as over-mature fruit: lower firmness, lenticel disorders, internal breakdown, and poor storability.

Over- and undercropping can influence the next season’s bloom and may initiate a vicious cycle of alternate bearing.

Kotze presented data from two Golden Delicious orchards to illustrate how unbalanced canopies result in unbalanced crop loads. Both orchards yielded more than 90 tonnes per hectare, but one had 32% bitter pit while the other had only 5%.

“We calculated that we required more or less 400 fruit per tree from these orchards,” recalled Kotze. “But the actual amount from the problem orchard was only 300 fruit per tree. The yield was made up by the larger fruit size.”

On examining the trees, he found that the canopy balance was disturbed so that some branches carried very few but very large fruit. Analysis showed that the large fruit were the ones suffering from bitter pit. Large fruit also led to more push-offs and bruising.

In addition, the fruit size distribution demonstrated a mixed-maturity nightmare of as many as four generations of apples on the trees.

Kotze’s message is that balancing the crop load on each branch and better managing the canopy as a whole would have netted this grower many more apples in an export carton and fewer postharvest defects.

Keeping score

Although he strives for uniformity and quality in the orchard, Kotze also has a system for dealing with the variation in storage potential that inevitably remains. “We visit each orchard,” he said. “The horticulturist, soil scientist, and production team score it in terms of short-, medium-, and long-term storage potential.”

Their scorecard considers soil characteristics, leaf and fruit mineral analyses, tree vigour, crop load, and fruit-size and orchard variation. For example, fruit from a vigorous tree with a low crop load and high fruit-size variation would be allocated to short-term storage.

The picking windows for different cultivars are defined in days after the release date. For fruit to be stored long-term, the orchard must have the necessary storage potential and be picked in the first window. If picked in the second or third windows, the fruit would be stored medium- or short-term, respectively.

Once fruit are harvested, storage potential is re-assessed based on maturity indexing and the fruit’s behaviour in cold stores.

“Maximum storage potential is not only determined by the maturity at the beginning of storage,” emphasised Kotze. “Even if the fruit was picked at the right maturity, there were already orchard factors that determined its potential.”

Kotze has seen how reducing variability in the orchard goes a long way to reducing losses due to postharvest disorders. “We have very little control over the climate, but we need to focus on what we can manage,” he concluded. “The weakest link determines the strength of the chain.”