By Erica Faber – Orchard Productivity Manager
The following is an overview of factors that adversely impact fruit quality and shelf life with tips to ensure you minimise risk to your fruit and ultimately your pack out.
Avocados are one of the few fruits that do not actually ripen on the tree and have to be picked for the ripening process to begin. This is known as climacteric fruit. Studies indicate that C7 sugars and other metabolites are responsible for inhibiting the ripening process of the fruit while on the tree.
Also, unlike other fruit, avocados accumulate oil instead of sugars and the oil content increases as the fruit matures on the tree. Oil content was previously used as an indicator of fruit maturity but testing has shifted to dry matter content for determining maturity as there is a very close correlation between dry matter and oil content and dry matter testing yields quicker results and is more economic to do.
In New Zealand, fruit needs to test at an average of 23% dry matter for local and 24% for export markets before harvest can commence. These values ensure minimum oil content and fruit that the consumer will find both tasty as well as buttery and smooth. It is the content of oil and the profile of fatty acids that determine fruit quality for avocados unlike other fruit where the balance of sugars and organic acids are used to determine quality. Harvesting too soon when the oil content is not yet high enough will result in watery, tasteless avocados.
Once harvested, the ripening process commences. Even though the fruit has been picked, it is still “alive” so to speak and continues to respire; absorbing oxygen and giving off Ethylene, carbon dioxide, water and heat. The increase in Ethylene production is accompanied by numerous complex changes resulting in fruit softening, flesh colour change as well as the synthesis of aroma and flavor chemicals. Cooling of the fruit soon after harvest is therefore important to slow down the ripening process as well as moisture loss.
CORRECT HANDLING PROCEDURES FIELD HANDLING
In many countries Hass fruit is snap picked. However in New Zealand, Hass is snip picked to a pedicel length of 5mm. This not only reduces moisture loss and fungal disease entry but also differentiates the fruit from windfall or stolen fruit which either have no stem / button or a long stem. Cutting the pedicel too long though will cause scratching and abrasions on neighboring fruit, affecting pack out.
After picking, the fruit must be carefully and gently handled. When transferring the fruit from the picking bag to crates or bins, a drop height higher than 10cm will cause bruising leading to localized softening. Bruise severity increases with increasing fruit drop heights and will often only manifest after a few days. Any mechanical damage caused during field handling and transportation will affect the fruit’s cosmetic appearance impacting your pack out and also acting as entry points for postharvest pathogens that cause decay and rots. It is important therefore to ensure pickers have short nails, and bags, bins and crates have no sharp edges or protrusions and that there are no stray twigs in the bags or bins as all of this can result in scratches, cuts and abrasions on the fruit.
Also ensure that your access roads are well maintained and tractor speeds are at a minimum so as to avoid the fruit jostling around in the bins.
Bins that are overfilled will also cause compression damage and bruising.
Keep bins out of direct sunlight.
Keep bins in the shade and out of direct sunlight and use bin covers if possible. This will not only prevent sunburn and keep any nasties from landing up in the bins (food safety) but will also reduce heat buildup and moisture and weight loss of the fruit.
The harvested fruit should reach the pack house as soon as possible so that the fruit can be cooled down as an increase in pulp temperature will accelerate ripening and shorten the shelf life of your fruit. If harvesting late in the season, avoid picking when temperatures are high.
With regard to phytosanitary requirements – ensure that clippers are sterilized daily, bags and bins are checked and cleaned, fruit does not come into contact with soil or mud and that when moving bins, the forklift is not carrying soil or debris that is knocked off into the bins. Hand sanitizer or washing facilities should also be available to ensure good hygiene when handling fruit.
FRUIT DAMAGE AND DEFORMITIES AFFECTING FRUIT QUALITY AND PACK OUT
While you are admiring your fruit that has been harvested, you may notice some odd deformities or perhaps wonder what is impacting your pack out. Below are some of the issues that I have come across to date this season, first and foremost being lenticel damage.
In New Zealand, harvesting in the rainy season makes our harvest not only more challenging logistically but also can attribute to poorer pack outs due to the fruit being more susceptible to lenticel damage, compression and external bruising, all of which can result in a significant increase in fungal diseases, rots and internal bruising. It is important therefore to follow industry harvest protocols and not harvest if there has been 5mm or more of rain in the preceding 24 hours.
An adequate drying out period should be allowed for not only fruit to dry off but also for the cells to respire, lose moisture and “shrink” back to normal. This will depend on your orchard drying conditions and whether you have pruned open canopies and inter rows and orchards that facilitate sunlight and air movement. As mentioned, lenticels are susceptible to handling damage after rain when the cells around and in the lenticel, expand and fill with water.
Lenticels are the macroscopic openings occurring on the surface of avocado fruit and are responsible for gaseous exchange and transpiration. These lenticels originate from preexisting stomata during fruit enlargement and growth. The fragile cell membranes, stressed under increased turgor pressure may rupture with added impact or compression during harvest handling causing visible browning. Not only does this impact on pack out but we also know that damaged cells are more susceptible to fungal diseases impacting on both fruit quality and shelf life. Imagine if you will, two balloons – one hardly blown up at all and one blown up almost to bursting point. It doesn’t take much handling or impact for the balloon under massive internal pressure to burst or as in the case of avocados, for the cells expanded with water to rupture with harvest handling.
Instead of ending up in an export crate, many of the fruit with lenticel damage (also known as peel handling damage) end up in domestic classes or processing. The tell-tale brown speckling with lenticel damage gets progressively worse over time and what may appear at first as only 2% or even less at harvest can easily end
up being over 20% at end of the pack line or on the super market shelves.
Incidences of vascular browning are also reported to be higher in fruit harvested when they are wet.
Not only does wind contribute to windfall fruit affecting total production but it also causes wind rub on fruit affecting pack out. This will be exacerbated when:
- There are insufficient windbreaks or shelterbelts
- Shelterbelts are too dense and instead of breaking the wind velocity by filtering it, it forms a barrier, creating turbulence over the shelterbelts and into the orchard
- Trees have been allowed to grow too tall or
- Row direction funnels the wind
If you had any exposed fruit, you will have fruit that will show sunburn damage in varying degrees from pale yellow to black lesions.
Spraying sunburn protectant products e.g. Surround or Bud Mate will help prevent sunburn and improve pack outs. Sunburn can also occur after the fruit has been harvested and exposed to direct sunlight.
Sunburn damage that occurs on exposed fruit growing on the tree.
Sunburn damage that occurs after harvest when fruit is exposed to direct sunlight in the picking bins or while being transported.
The two main culprits responsible for damage to fruit and affecting pack out if not properly controlled are Greenhouse Thrips and Leaf Roller.
Regular pest monitoring from fruit set until harvest is important to ensure your best possible pack out.
Greenhouse Thrip damage.
Physical deformities can develop due to many reasons. Abiotic or environmental stresses such as cold, heat, wind, or water stress at the early stages of fruit development can cause ridging and bumps, neckiness (also known as goosenecks) and cricks (also known as crick-side). Water stress will also reduce fruit size, increase fruit drop and the incidence of sunburn as well as lead to irregular maturity.
Excess water on the other hand can cause fruit cracking or splitting; all of which will affect pack out.
Netting or Ridging.
Chimeras are genetic mutations and can manifest as stripes of different colour or peel texture or can also manifest as fruit deformities e.g. Crooks (not the kind wanted for stealing avos!). Quite often all the fruit on an entire branch or even the entire tree will exhibit these symptoms.
Nutrient deficiencies can not only manifest as internal fruit disorders, which we will cover in another newsletter, but also as physical deformities.
Sunken corky lesions.
Symptoms of boron deficiency.
As the fruit matures and we advance further into the harvest season, many of these reasons that adversely affect pack out, fruit quality and shelf life will become exacerbated. More fruit drop, more sunburn on exposed fruit, more handling bruising as well as colouring and decay will manifest. One should weigh up both the economic losses and the impact on the upcoming fruit set and next year’s production when hanging your entire crop late for tempting, late market prices.
When growers put so much effort and expense into an entire season to try and ensure premium quality fruit that will pass export grading, it seems that sometimes at the last hurdle we falter and those decisions and mistakes impact our returns. Plan your harvest strategy well and take into account current tree health, environmental impacts on your crop, your pruning window and the upcoming fruit set. Ensure fruit is handled well in the field when harvesting and study your pack out reports to see where improvements and adjustments can be made for next season’s crop.