CONTRIBUTORS
Table grape post-harvest is not a dark science
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 ).
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 postharvest life ( and the happy days of 3 to 4
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 !
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 ) CLA
32 www . coldlinkafrica . co . za COLD LINK AFRICA • July / August 2022