Determining Apple Maturity and Harvest Timing
12 Sep 2022
|
|
Ethylene – All apples are sensitive to ethylene, a natural plant growth hormone that is produced in response to stress – natural or not – and which induces ripening in many different types of fruit as well as apples. Ethylene production begins a few weeks prior to the ripening phase of apples and can be measured using air samples from the apple’s core and analyzed using a gas chromatograph (GC). However, very few growers have access to a GC and yet there are different ways to determine the ripening of apples that doesn’t involve measuring this elusive gas. Nonetheless, it is important to understand the role of ethylene in the fruit ripening process as a trigger to “tell” the fruit to get on with it. This includes red color development, increase in sugars and polyphenolic compounds, fruit softening, development of abscission layer, changes in seed colors, and more.
Date – just because you harvested a particular variety on a specific date last year very little to when it’ll be ready this year. It is useful however for knowing that you’re getting close. But with so many variants, especially of dessert apples, the calendar is nothing more than a reminder to get prepared. For this reason, using a calendar to determine when fruit is ready to pick is extremely imprecise. Don’t do it!
Firmness – Since ethylene triggers fruit softening (via the breakdown of cell walls) it follows that fruit will get softer as ethylene levels rise and fruit gets riper. It is not, however, a good indicator of harvest readiness, merely of how soft the fruit is. Like other indices (like skin color and brix) fruit firmness is important to dessert fruit growers and can be used to guide harvest. Fruit that is too soft will not store well and then reach the consumer in good shape. Fruit that is too hard is likely underripe and devoid of other qualities that riper fruit contain. You will need a penetrometer, a sharp knife or special peeler, and a hard surface. Cut a small section of skin away from the fruit, place the apple on a hard surface (cut section facing you), place the penetrometer tip on the cut area, and push firmly (but not too hard) until it bursts the cell walls its in contact with. A firmness of 14-16 psi is very good, more than that unripe, lower than that still good; but once the firmness drops below 10-11 psi, the fruit is no good for long term storage.
Skin Color – skin color of apples is a very poor and imprecise way to determine apple readiness for harvest. Why? Some apple skins have dark red color, others very light, some none at all. Many apples are bicolored or orange. However, while the foreground skin color is not a good indicator of ripeness, the background color can be a good indicator of when to look to other more precise methods dialing in your assessment. What is the background color? It’s the part of the apple facing away from the sun and usually towards the interior of the tree. A fruit that is in full shade may only have “background” color as the red color – or anthocyanins – in fruit is developed in response to exposure to sunlight and high diurnal fluctuation of sunlight and temperatures. Warm sunny days and cool clear nights with low humidity are generally considered the best conditions for red color development and has nothing to do with the production of ethylene. However, once the background color starts to turn, then you can begin looking to other methods for determining the best time to begin harvest. The beauty of judging an apple’s skin color is that it is a non-destructive method – all other methods requiring damaging the fruit in some manner.
Seed Color – once the background color starts to turn from a green to off-green to creamy white, I’ll start to look at the seeds. Seeds, like skin color, are not the most precise method for determining fruit ripeness. But it is better and less subjective. Cut the fruit longitudinally to expose the star-shaped cavity. If the seeds are white or off-white, then the fruit is still not ready to pick. If they are light brown or beige, we’re getting closer. And finally, when they are dark brown or black, the time is nigh! However, don’t take seed color as the final determinant – some apple seeds never turn totally dark and are still ready to pick. How do I know? Having done this for many years, you can also test brix and starch at the same time as seed color.
Brix testing – When testing for brix you’re actually testing for total soluble solids (SS%), not just sugar. Nonetheless it gives a good idea of when the fruit’s ready to pick as many varieties have a certain brix level they’ll achieve at full maturity and ripeness. Some, like cider apples, can gain a very high brix that is important to fermentation. That said, many dessert apples are considered ready when the SS% is greater than 13% (though in some years its hard to get higher that 11-12% especially for early apples or in rainy seasons. Many cider apples can get as high as 20% in good years. And geographically the more sunnier growing days and more intense the sunlight the fruit gets can result in higher SS% due to greater net photosynthesis. But is brix testing the yardstick we’re all searching for? Not really, since SS% doesn’t necessarily reveal the overall potential of fruit, its just a yardstick – and sometimes used to declare “good enough.” You’ll need a good temperature compensated refractometer that measures the brix range you’re interested in (0-20% is generally good enough) and a garlic press. Cut a small wedge from the fruit, place it in the garlic press and express the juice onto the prism of the refractometer. Flip the lid firmly down and look through the viewer with adequate sunlight to illuminate the prism and give a clear result. There is a scale inside the viewer you can use to read the results. Focus as needed and record the results.
Starch-Iodine Tests – the real Holy Grail (even more than ethylene) are starch-iodine tests. When you cut an apple open you expose the interior portion that is made up of (for all intents and purposes): starches and sugars. As ethylene increases, it triggers ripening processes that begin the conversion of starches to sugars within the fruit. The use of potassium iodine brushed on to the cut surface of the fruit reacts with the starch to reveal a dark purple stain. It doesn’t react with the sugar (converted starches) and those portions stay white. The whiter the area, the more starch has been converted and the riper the fruit is. You don’t necessarily want a complete white pattern unless you plan on processing or using the fruit right away. About 33% (and this varies by variety and grower) of the interior should remain white at harvest if you plan on storing the fruit for a few months or longer. The pattern itself is unique to the variety, the year, and the individual fruit, but for the most part you can assess the percent starch vs percent sugar quite easily for harvest determination purposes. Bottom line: the more purple, the less ripe; the whiter, the riper the fruit. You’ll need potassium iodine and a small paint brush to brush it on. Cut the apple as described, paint on the K-I, and wait for it to react with the starch. Use the chart below to determine your starch level and level of ripeness.
Abiotic Stress – 2022 has been a very dry year. And like 2021, a wet year, each year brings its own environmental conditions, stressors, and influences on fruit maturity, ripeness, and quality. Each of the methods described above will vary and give different results from year to year. In a droughty year we can expect fruit to not only ripen sooner than it will in a wet year, but we can also expect increased pre-harvest drop due to high stress and lack of inputs (primarily water) to keep the fruit healthy, happy, and hanging on to its fruit. In this case, the drought increased the amount of ethylene produced by the tree and in turn enhanced the ripening process. Ethylene can be increased in very wet years as well. But in general a good growing season with adequate moisture, moderate temperatures, well-thinned trees, and few other stressors (such as increased pest damage) means the fruits will ripen more or less “normally.”
Summary – there is no perfect way to determine the precise best time to harvest fruit. Dessert fruit growers will use skin color since consumers prefer highly-colored fruit – so any fruits that are under-colored may be rejected. However, they can’t do this at the expense of good tasting dessert fruit since consumers also require sweeter more highly-flavored apples than ever before. Conversely, cider apple or processing growers care little about skin color and more interested in the highest quality fruit with the best internal and organoleptic qualities for their purposes. This could mean maximizing brix levels – even to the point of letting the fruit to drop from the tree or sweating it once it is off the tree. It is beyond the ability of most apple growers to test for polyphenolic compounds (though total acidity is a fairly easy test), so in the absence of fancy equipment and uncertainty about the methods above – the best way to determine when fruit it ready it to eat it: If It Tastes Good It Is Good!
**For the complete PDF with footnotes and citations, please email mike@knowyouroots.com
|
|
|
|
|
