What Is The Best Apple Harvest Maturity Index?

• Apple harvest maturity indices are mainly based on physical and chemical attributes.
• The most reliable method is the starch index.
• The harvest maturity index values differ based on cultivars and the storage time.

Each fruit is distinctive, and the methods used to fix harvest time must be customized for each cultivar. Determining the optimum harvest window is crucial as harvesting apples at the correct maturity defines the fruit quality at the retailers and the storage conditions. Fixing harvest time also helps growers schedule labor and reduces the number of fruits that get spoilt and rejected. Many methods, like the starch test, are unique to apples. Find out more about how apple harvest time is determined.

Harvest Maturity of Apples

Apples are harvested when unripe but fully mature as they are climacteric fruits. Immature fruits do not store well, have poor color and aroma development, and are starchy after storage. Immature fruits are also more prone to internal breakdown and loss of moisture content due to incomplete cuticle formation, scalding, and bitter pits. Apples that are over-mature at harvest are susceptible to diseases, cold temperatures, and mechanical injuries, even under optimum storage conditions.

Harvesting at correct maturity ensures the best quality fruits that meet consumer demands. Moreover, the supply chain needs precise data on fruit maturity at harvest, which will affect labor requirements and postharvest operations like sorting, grading, packing, and storage conditions. Therefore, the harvest maturity index determining the correct picking window is essential for apples.

Apple undergoes many physicochemical changes as they ripen. These are an increase in fruit respiration and ethylene production rate, which leads to lowering chlorophyll levels and alterations in peel, pulp, and seed color. The flesh softens as starch is converted to sugars, and acidity decreases. As apples mature, most changes can be tracked on the field to fix harvest time.

Harvesting at a pre-climacteric minimum is ideal, as the fruits’ respiration rates are minimal, providing long storage and quality development potential.

The harvest maturity index involves tracking apple quality a few weeks prior to picking. For a harvest maturity index to work, sampling correctly is critical. Each variety and orchard block should be sampled separately as maturity varies based on genotype and growing conditions. Sample collection and testing of harvest maturity indices can begin 4-5 weeks before the approximate harvest date or days after full bloom (DAFB).

The harvest maturity index can involve destructive and non-destructive methods. For apples, chronological, physical, and chemical parameters are tracked as the harvest maturity index.

Chronological Apple Harvest Maturity Index

The chronological harvest maturity indices used for apples are days after bloom (DAFB). It is used only as a general reference as the average harvest date can vary 5-20 days from the optimum harvest time. Growers should record bloom and harvest time by block and cultivars to estimate specific DAFB for their fields and crops over the years.

Depending on the variety, DAFB can be from 110 to 145 days in India. It is 110-120 days for red and royal and 135-145 days for golden apples.

In the USA, ‘McIntosh’ Apples have a DAFB of 133 days. It is correlated in some states with the day the accelerated ethylene production (AREP) rate begins.

Physical Harvest Maturity Index for Apples

The physical characteristics of harvest maturity are color, size, sound, fruit abscission, and firmness.

Color

The peel background or ground color, surface color, DA index, and internal flesh can be measured destructively and non-destructively. Apple peel color doesn’t change during storage, so the color at harvest is crucial. The apple can be too firm and starchy without the correct background color. 25-25% of starch conversion occurs at optimum color, and firmness is above 15 lbs.

Peel color is checked non-destructively by comparing fruits’ skin background and surface color to traditional color charts. Near-infrared (NIR) spectroscopy can be used for external and internal color checks for objective and quantifiable color and hue data.

Background color: Peels lose their chlorophyll and change from green to yellow to red. Ground color is used for many varieties, where a light green or yellow indicates harvest time for fruits meant for extended storage. Background color changes to yellow or cream are the correct shade for apples stored briefly. The background color is a crucial index for Gala and Fuji. The exact color depends on cultivars, as Cortland and McIntosh are harvested at a deeper green shade, while Honeycrisp is picked when slightly yellow in the USA.

Surface color: Stripes of green, yellow, or red characteristic of each cultivar can also work as a harvest maturity index. In India, apples are harvested when they have developed 50-75% color.

DA meter: The Index of Absorbance Difference (IAD) indicates the difference of absorbance (DA) that measures the amount of chlorophyll-a in peel and ethylene evolution. It is an objective and precise method that is also useful in fields. As apples mature, the DA value declines, and each cultivar has a DA index. However, general recommendations are a DA index of 0.60-0.70 for long-term storage and a 0.35 index for short-term storage apples.

Seed color: The seed color becomes darker and changes to brown in apples. This change in seed color indicates full maturity for harvest in India. However, seed color is not used extensively in the USA, as the seed color change indicates ripeness and is used only for local sales. Seed color change is determined only through destructive means.

Firmness

Apples become less firm as they mature. However, several factors affect firmness, such as fruit size or watercore, making it unreliable as a stand-alone harvest maturity index. Larger fruits are softer than smaller fruits, so sampling should include representative and homogenous fruits from an orchard block. Apple firmness is measured by a penetrometer with an 11 mm tip.

In the USA, flesh firmness is used as a harvest index. Apples for long-term storage are harvested at firmness of 15 pounds (lbs) and above, and those for short-term storage with less firmness of 13-15 lbs.

Size

Each cultivar’s fruit size and weight characteristic can be used as a harvest index, though the latter will involve destructive sampling. In India, the size range, depending on cultivars for harvest maturity, is 55-85mm.

Sound

Mature apples ready for harvest sound like wood, so this property is used as a harvest maturity index in India.

Abscission

The ease of fruit separation from spur indicates harvest maturity and is used in India.

Chemical Harvest Maturity Index for Apples

Starch, acidity, and sugar content are the chemical parameters used as harvest maturity indices.

Starch

A maturity index unique to apples is the starch staining pattern.

As apples ripen, starch turns to sugars, first in the core and later in the cortex, see Figure 1. All the starch is broken down when the apple is ripe and ready for eating. The rise in ethylene is associated with a certain level of ethylene generation. However, the starch levels in a fruit depend on the crop load.

Figure 1: “Starch staining patterns in McIntosh apples,” University of Maine. (Credits: https://extension.umaine.edu/fruit/harvest-and-storage-of-tree-fruits/maturity-indicators/)

Iodine is used on an apple cross-section to stain the starch blue, and areas without starch remain white to give patterns characteristic of each cultivar. The starch index numbers 1 to 8 are arbitrary, where 1 indicates no starch breakdown, and 8 indicates no starch presence. Newer varieties may not have a well-established starch index.

In the USA, various starch levels are used for long and short-term storage:

• Long-storage apples are harvested at a starch index of 3-5.
• Apples for fresh consumption with short storage times must have a starch index of 6-7, while some regions (Washington) use a starch index of 4.5-6 for the same purpose.

For more details on the starch index of a specific variety, see Table 1.

The starch method is a traditional and also the most widely used method of establishing harvest maturity. It is more reliable than firmness, soluble solids content, and acidity.

Table 1: Starch index for short- and long-term storage of apple varieties, University of Maine. (Credits: https://extension.umaine.edu/fruit/harvest-and-storage-of-tree-fruits/maturity-indicators/)

Soluble solids content

The soluble solids content (SSC) increases as apples mature and ripen. However, SSC is influenced by many factors such as seasons, years, temperature, sunlight levels, shade, irrigation, and crop load, making it less reliable as a harvest maturity index, so it must be used with other indices.

General recommendations for the USA suggest that apples must have 12-14% SSC at the time of harvest. However, variations exist based on cultivar and target markets.

In India, the SSC of mature apples must be 12%.

Acidity

Malic acid is the primary acid in apples, and along with sugars and volatiles, it contributes to the fruit flavor. As apples mature and ripen, their titrable acidity reduces. Titrable acidity is not used as a maturity index, as the reduction rate of acidity and the optimum acidity values vary widely between cultivars. Acidity is only good when frequent measurements are made on fruits from the same tree. So, a grower can estimate the optimum acidity for each block through values recorded over several harvests.

Dry matter

The average apple dry matter content at harvest is 16.2%. However, dry matter content is not used as a harvest maturity index for various reasons, even though it is correlated to postharvest consumer preferences after prolonged storage. One of the reasons is that starch and firmness change more than dry matter during maturity, and these parameters are better to establish the correct maturity for harvest. For example, dry matter content can’t estimate Gala apple harvest time accurately.

Special Indices

Besides the quality attributes, some indices have been developed to fix the harvest window time for apples. These are the Streif index and biospeckle activity.

Streif Index

This index is based on quality parameters and is calculated as follows:

[firmness/ (percentage soluble solids concentration x starch index)]

The Strief index declines with advancing maturity from 5-6 to 0.30-0.80. It is estimated through destructive sampling of apples starting a dozen days before approximate harvest time. The Strief index values to fix harvest maturity of some varieties in the USA for long-term controlled atmosphere storage are as follows:

• Redmax has Strief index values of 4.18 to 5.34
• Marshall needs 4.12 to 5.46
• Summerland ‘McIntosh’ must be 4.51 to 5.68
• Redcort ‘Cortland’ requires 5.23 to 5.99
• Wilmuta ‘Jonagold’ index is 1.38 to 2.34

Biospeckle activity

Biospeckle activity (BA) is a non-destructive method that reflects only the current status of the apple during measurement and cannot be correlated to the future. A characteristic drop in the BA occurs just before peak pre-harvest maturity and is used as the indicator. The BA method is new and has yet to be fully developed. However, it cannot be used alone as a harvest maturity index.

For detailed information on all apple harvest maturity indices for different varieties in the USA, check this resource.

Tracking Harvest Maturity Indices

Most harvest maturity indices for apples do not meet all the necessary criteria for a reliable index. The starch index is the best. Hence, a combination of indices must be used.

Several non-destructive techniques exist for physical and chemical parameter measurements, like VIS/NIR spectroscopy, dielectric spectroscopy, time-resolved reflectance spectroscopy, hyperspectral backscattering imaging, chlorophyll fluorescence, or laser-induced backscattering.

Felix Instruments Applied Food Science offers the F-750 Produce Quality Meter, which uses NIR spectroscopy to measure external and internal color, SSC, and titrable acidity. Besides being non-destructive, the readings are rapid, accurate, and easy to understand. The F-750 is a standard commercial tool that is easy to operate onsite on orchards and the supply chain.

Find out more about the F-750 Produce Quality Meter.

Sources

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CII. (n.d.). Postharvest Management Protocols. Retrieved from https://www.face-cii.in/cclrc/fruits-vegetables/29/Fruits/Apple.pdf

Douglas, J. B. (1983). An evaluation of harvest indices for ‘McIntosh’apples in two orchards. HortScience, 18 (2), 216-218 ref. 11. Retrieved from https://journals.ashs.org/hortsci/downloadpdf/view/journals/hortsci/18/2/article-p216.pdf

e Long, J.M.; Prange, R.K.; Harrison, P.A.; Shofield, R.A.; de Ell, J.R. (1999). Using the Streif index as a final harvest window for controlled-atmosphere storage of apples. Hortscience, 34, 1251–1257

Farcuh, M. (2023, July 2). How Can Growers Determine Apple Fruit Maturity and Optimal Harvest Dates? (FS-1180). Retrieved from https://extension.umd.edu/resource/how-can-growers-determine-apple-fruit-maturity-and-optimal-harvest-dates-fs-1180/

PSU. (2023, July 2). Fruit Harvest – Determining Apple Fruit Maturity and Optimal Harvest Date. Retrieved from https://extension.psu.edu/fruit-harvest-determining-apple-fruit-maturity-and-optimal-harvest-date

Reid, M.S. (n.d.). Maturation and Maturity Indices. Retrieved from https://irrec.ifas.ufl.edu/postharvest/hos_5085c/reading%20assignments/kader-6-maturation%20and%20maturity%20indices.pdf

Skic, A., Szymańska-Chargot, M., Kruk, B., Chylińska, M., Pieczywek, P. M., Kurenda, A., … & Rutkowski, K. P. (2016). Determination of the optimum harvest window for apples using the non-destructive biospeckle method. Sensors, 16(5), 661.

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van Hooijdonk, B.M., Tustin, D.S., Dayatilake, G.A., Oliver, M.J. and Richardson, A. (2018). Harvest index of apple and kiwifruit – how high can we go by optimising plant architecture and canopy management? Acta Hortic. 1228, 307-314. DOI: 10.17660/ActaHortic.2018.1228.46

Vieira, M. J., Argenta, L. C., Brancher, T. L., Freitas, S. T. D., & Mattheis, J. P. (2022). Relationship among dry matter content and maturity indexes at harvest and quality of ‘Gala’apples after storage. Revista Brasileira de Fruticultura, 44(2), e-841.

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