If only it were as simple as counting days to maturity! But apple growers know that the optimal development of apple flavor can’t be properly determined by counting days after full bloom to harvest alone. Beyond the obvious – you don’t want your customers to eat hard, small, undeveloped apples, nor do you want to sell too-soft, starchy, oily-feeling apples that waited too long to be picked – getting harvest timing correct means apples with the texture, taste, aroma and color to make your farm stand out in the crowd.
But picking at the optimal time isn’t always easy. The pressures of customers seeking a certain cultivar before its prime can lead to a premature harvest. Labor issues can lead to over-maturation, with fruit being picked past its peak. Either way, the result is a less than optimal eating experience, and the fruit may not be suitable for your marketplace.
Knowing how to assess ripeness and recognize the factors which influence the development of flavors can prevent post-harvest yield losses. Bitter pit and storage scald can happen when fruit is picked too soon; development of water core and shortened storage life occurs if apples are over-mature at harvest. To prevent crop loss, it’s important to pick at the correct degree of ripeness.
Yet the nuances of when to pick to capture and retain the best flavor depend upon many factors.
Ready or Not?
The intended end use is a primary factor in deciding at what stage apples should be harvested. If they are going to be shipped or held in storage for later sale, picking occurs at an earlier stage of maturity. For immediate use, apples are harvested at peak.
Ethylene causes the ripening of fruit. The level of ethylene can be most accurately measured using gas chromatography, which isn’t practical on the field level – but knowing the ethylene level is important. Apples will quickly pass their peak of ripeness once a final surge of ethylene is produced, and their quality will rapidly decline.
During the fruit’s maturation process, respiration increases. At the peak of maturity, the respiratory climacteric (a pulse of ethylene release) happens. In order to prevent loss of quality, harvesting needs to occur during a short window of about seven to 11 days just prior to this respiratory event.
Since measuring ethylene directly is not practical, assessing other characteristics which are impacted by fruit ripening is the primary method of detecting harvest maturity level. Ripeness tests, using various indicators, are industry standards. These include assessing sugar content, firmness, seed and flesh color and starch levels. These indicators correlate to some degree with other characteristics such as acidity levels, flavor, aroma and flesh texture.
Using more than one indicator enhances harvest timing. Each block of trees should be assessed separately, as growing conditions can change throughout the orchard and impact maturation. Growing conditions also change throughout each tree’s canopy.
Apple color changes can be indicative of starch content, but can also be influenced by nitrogen levels. In general, peel color changes from green to yellow when the sugars have developed fully. On red apples, the side protected from the sun can be best assessed for peel color.
Combining color with refractometer readings (or Brix) paints a more accurate picture of the degree of ripeness. Brix, measured in degrees or percent, varies along with the amount of soluble solids, indicating the sugar content. Each cultivar has differing levels of these solids when at peak of ripeness.
Proper calibration of the instrument is needed, as is holding all apples to be tested at the same temperature, as changes will affect the results. Brix levels in apples are impacted by the degree of sunshine. Fruits grown with more sunlight will have higher Brix readings than those in shaded locations. Seasons with low moisture levels result in overall higher Brix readings, as do those with higher temperatures and sunnier days.
Iodine staining also helps to indicate the degree of harvest maturity and is used to measure starch content. As in Brix levels, the pattern of starch disappearance varies from cultivar to cultivar. Iodine staining is a visual assessment of core versus flesh uptake of the iodine solution.
Penetrometer readings are taken to measure the firmness of apple flesh. Done properly, they measure ripeness based on the principle that fruit softens as it matures. Each cultivar will have its own firmness levels for storage or fresh eating. The instrument must be applied to the flesh, not the skin, and the speed of penetration impacts results. Large apples tend to be softer than smaller ones from the same batch.
Measuring the chlorophyll content is newer means of determining maturity, using an instrument called a DA meter. Each cultivar will have its own standard DA readings. Chlorophyll content decreases as the apple matures.
Some Factors Affecting Ripening
Fruit on the periphery of the tree matures the most quickly. Trees with light loads will mature their apples more rapidly, having higher Brix readings than neighboring trees with a heavy load. Seasons with higher temperatures will mature fruit faster than cool seasons, but each cultivar will respond differently to environmental factors.
Excessive nitrogen not only alters changes in peel color, it also causes softening of the fruit and premature drop. Fruits growing on weak spurs will mature differently than better-situated neighbors. If there has been an extended bloom period, maturity dates can vary significantly.
Orchard pruning and training systems affect maturity dates too. A high-density orchard block should mature at a different rate than a nearby standard orchard block will.
If apples are put into storage, ongoing assessment of maturity levels is needed to keep fruit quality high and prevent yield losses.
Picking at the correct harvest time for your markets is the first step in ensuring a quality crop at the point of sale, and is the end goal of all you’ve worked for all season. Using all the tools of the trade, including maturity reports put out by many universities in apple growing regions, can help you “pick” the best time to harvest your fruit.