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Home » Table grape post-harvest is not a dark science

Table grape post-harvest is not a dark science

By Oscar Salgado (during his latest visit to South Africa)

When visiting Agrostar in Egypt, I ran into Jan Lievens who was installing his applied post-harvest technologies in their two receiving pre-coolers. It was actually just by accident that we ran into each other!

I read an article in 2020 which Jan wrote for Cold Link Africa: “Houston we have a problem…” and that sparked my interest. I then found his postharvest philosophy and incorporated it in my world-wide-use presentation as it makes complete sense.

Jan asked me if I could review another article which he wrote for this publication, and I decided to add a contribution from my side – in writing a preface to the article which you’ll find in this month’s publication as well.

We are staying in touch with each other as we both have the same drive and conviction to change the approach of the industry to table grape post-harvest. Enjoy both readings!

Table grape post-harvest is not a dark science, it is a balance between common sense, a bit of theory and the mother of the battles… experience, as well as an open mind – which must be as open as a parachute. If it is not open, the parachute doesn’t work, and the same is valid for the human brain.

However, it is a continuation of good field agriculture and production practices, and so post-harvest is not a science of miracles, we are “thanatologists”, and we try to keep a dead body “edible and commercially attractive” for as long as possible for our customers, so it is literally a race against time (senescence of the tissues).

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Pre- and post-harvest is a sort of culture of the details (the devil sticks his tail in the details) as we, ever since we cut the bunch out of the tree, are counting down the commercial life of our bunches.

Every variety has its own maximal post-harvest life (and the happy days of 3 to 4 seedless varieties maximum and many nice strong seeded varieties commercially are gone). That maximum is not well-known and is based on the agronomical potential of each one, ruled by the genetic pool inherited from the parents’ line. So, we as technicians and growers, are targeting to maximise that expression, based in a bunch of major technical decisions, investments and details, in every step and we are aiming to delay the inevitable end. So, every action will reduce the anticlockwise reduction of days, hours, or even minutes.

Big things are the cooling culture, reducing the field and metabolic temperature as fast as possible of the final product, however 24, 36 or even the 72 hours of South African “fast precooling” are out of the question.

Since 1997/98 I have been repeating over and over that the cooling techniques are not up to a world standard in South Africa, we need real fast single precooling tunnels to take our pulp temperature (and please not the average), to a -1,2 °C to -0,5°C, so just before the stem freezing point, as it is well reported that Botrytis can grow nicely even at 0,0°C, allowing a ΔT of maximum 0,5 to 1,0 unit of degree between the hottest pulp point and the coldest pulp point. Otherwise, one of our hidden enemies will strike somehow, along the cold chain – “condensation”.

Condensation is a physical phenomenon which is very easy to understand, it will be always present, but we can minimise it, and is irreversible, triggering a number of issues within our box and in transit to our consumers. Design of packing materials, boxes, cross ventilation -horizontal and vertical, liners and placement of the boxes on the pallet, as well as a good understanding of how your SO2 pad is working in reality, which can be a gun in a hand of a kid or at the same time a gun with no bullets, are all important.

Then the details strike one by one, keeping your fruit commercially alive, but one rather very important and easy to solve, is the relative humidity from the harvest till the consumer.

Now a bit of theory: understanding of the vapor pressure deficit (VPD), which is simple, it is the difference between the inner tissue, stem, or rachis, which is almost at 100% RH and the environment surrounding your grape. That delta (Δ) is what we should reduce, reducing the vapor pressure deficit (VPD) between the tissue and the environment, is what will keep your stem greener and in most of the case the berries turgid and crispy.

Remember we should focus more on the stem rather than the berry; a table grape bunch is a mixture of many sorts of “Plums”. By the way, each berry is a fruit by themselves and connected by a lettuce. Easy to understand now, but by the way the respiratory rate of the stem or rachis is approximately 15 times higher than that of the berry.

From all the complaints and claims that we received from different origins and in different markets with grapes, they are mainly on condition issues. Remember we split quality and condition. Condition issues are leading the list and alternating with the pole-position are decay (mainly Botrytis) and dry stem. In general, we are not facing that many quality problems, we are confronting condition challenges, moreover the new genetics are not showing a particular strong stem, and this can too be a generalisation, but I firmly stand for my remark.

To take it too simply perhaps, I will summarise that decay is managed in post-harvest, and both production and pre-harvest is a different story – by cooling, ie quality cooling, SO2, and dry stem by managing the reduction of vapor pressure deficit (VPD) from the picking moment till the table of our consumer.

A big chunk of our commercial Agronomic expression of the life span or shelf life of our table grapes was swallowed by the logistic, not the sailing time only, the logistic from the pack shade till the deck of the vessel, and the unloading till the dining table of our consumer, adding up to 70 days till the berry was heated.

Whether we like it or not, we need in most markets from 2 (if we are lucky) or up to 4 weeks to sell the fruit, then 5 to 7 days of shelf life at the retail store, and our grapes were not prepared to endure such conditions, end of the story.

How can we prepare our table grapes? Change our pre- and post-harvest cultures and open our minds to new ideas. “Hope for the best, but be ready for the worst.”

“Do not relax, everybody has the right to be in business, but nobody is obliged to remain in business, remember our business is changing very fast and what is changing even faster and faster than our business, is the speed of the change…….” (Oscar Salgado 2014)

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