What Are Kiwifruit Harvest Maturity Indices and Why Are They Important?

• Dry matter content and soluble sugars are two quality parameters widely used as harvest maturity indices for kiwifruits.
• Dry matter is increasingly becoming the standard harvest maturity index because it is correlated to postharvest taste, consumer acceptance, and storability of kiwifruits.
• The two harvest maturity indices are used as they can be estimated non-destructively in real time.

Harvest maturity indices are used to ascertain that the kiwifruit yield is mature enough for picking. Objective and quantifiable data providing information on the internal fruit composition are increasingly becoming the norm in place of subjective visual and texture evaluations. Learn more about the parameters used as harvest maturity indices and their benefits for the kiwifruit supply chain.

What are Harvest Maturity Indices?

Harvest maturity index (HMI) is a parameter used to indicate the maturity stage of fresh produce to fix harvest time, ensure optimum storage and postharvest quality to meet consumer preference, and extend shelf life.

Harvest maturity is when fresh produce reaches physiological maturity when the fruit or vegetable fully matures.  The harvest index indicates the stage of maturity that will allow further postharvest development and maintenance of acceptable appearance and flavor, as well as adequate shelf life, see Figure 1.

Harvest indices can be fresh produce quality characteristics like visual, chemical, physical, and physiological parameters.

The methods that involve nondestructive sampling are the best to prevent the wastage of fruits due to frequent checks, especially since each check in an orchard can include extensive sampling to estimate maturity accurately.

Figure 1: Dry matter and sugar content changes in kiwifruits, Henry et al. 2023. (Image credits: https://link.springer.com/article/10.1007/s10460-022-10396-1)

Nondestructive Kiwifruit Harvest Maturity Indices

Two standard nondestructive kiwifruit harvest maturity indices are soluble solid contents and dry matter content. These harvest maturity indices can ensure that the correct harvest date is set when kiwifruits are fully mature but not ripe. Since kiwifruits are climacteric fruits, ripening changes like flavor and appearance occur postharvest. A minimum of 90 kiwifruits are tested to establish the harvest maturity index.

Firmness can also be used as a harvest maturity index but requires destructive sampling.

Soluble Solid Contents

A minimum percentage of soluble solid contents (SSC) is used as the harvest maturity index. SSC indicates sugars accumulating in fruits and is measured as °Brix.  After harvest, the climacteric kiwifruits continue to ripen, and the SSC continues to rise due to the breakdown of starch into sugars. The Brix is measured equatorially for all cultivars.

SSC estimation is still used, but it is losing importance to the more recent use of dry matter content as a harvest maturity index.

Dry matter content

The dry matter (DM) in fruits is the solids minus the water content. The solid consists of non-structural carbohydrates (sugars and starch), structural carbohydrates, organic titrable acids, oils, vitamins, pigments, volatiles, minerals, and ash.

Dry matter percentage is an ideal harvest maturity index for many reasons.

1. It is correlated with sugars at consumption due to the conversion of starch to sugars.
2. DM is associated with titrable acids at harvest.
3. Tests have shown that DM at harvest is also a good indicator of taste (the sugar-to-organic acid ratio) at consumption and consumer acceptance.
4. Dry matter is also correlated with structural carbohydrates and firmness and is an excellent indicator of kiwifruit’s storability.

While higher dry matter indicates preferred taste, the dry matter matter also has an optimum value. Very high dry matter results in very sweet fruits that do not have the correct sugar-to-titrable acid ratio. Therefore, the dry matter content chosen as the harvest index can depend on the required SSC and titrable acidity at consumption.

Dry matter in fruits can vary based on cultivar, region, year, rainfall, temperature, and orchard management. So, the same dry matter content may not indicate the same physiological stage of maturity for all cultivars or even fruits from different areas for a single cultivar. However, despite these concerns, dry matter content at harvest remains crucial as a quality criterion.

Optimum Harvest Indices

Harvest maturity indices can vary worldwide, considering the factors influencing dry matter and sugar accumulation. Below are some examples of optimum indices for kiwifruits in different parts of the world.

European Union

A significant portion of kiwifruits are exported, and the United Nations Economic Commission for Europe (UNECE) ‘s Working Party on Agricultural Quality Standards has set standards for kiwifruits sold to the European Union (EU). Fruits at packing, which usually takes place soon after harvest, must have a minimum SSC of 6.2° Brix and dry matter content of 15%. Therefore, these levels can be considered as the harvest indices for fruits grown for the EU.

New Zealand

New Zealand accounts for 40% of kiwifruit exports, mainly of’ Zespri’ cultivars, followed by ‘Hayward’ cultivars.

Zespri

For ‘Zespri GOLD,’ the harvest index is ~17% of dry matter to achieve 14% SSC at consumption. In New Zealand, Zespri kiwifruits can get premium prices if they meet specific dry matter and SSC content standards.

Hayward

General recommendations for ‘Hayward‘ are as follows:

• The ‘Hayward’ harvest index is 6.2% SSC.
• The ‘Hayward’ harvest index based on dry matter is 15-20%. This band of dry matter content gives an SSC of 16% at consumption.

Recommendations are also given for Kickstart fruits, picked in March at the start of the season, and Mainpack fruits, harvested in May.

Harvest maturity indices for ‘Hayward Kickstart’ are as  follows:

• Dry matter content of ≥15.9% for conventional fruits and ≥15.5% for organic fruits.
• Average SSC content of ≥4.7°Brix.

Harvest maturity indices for ‘Hayward Mainpack’  are listed below:

• Dry matter of >15.5% for conventional fruits and ≥15.5% for organic fruits.
• The SSC is ≥6.2°Brix.

Sweet Green

‘Sweet Green’ has a harvest index of 16.8% dry matter or ≥5.4°Brix.

Sungold

The ‘Sungold‘ harvest maturity indices are as follows:

• Kiwistart and Mainpack fruits must have a dry matter of ≥16.1 for size 16-36 and ≥16.6 for size 39.
• The SSC for Kiwistart must be ≥5.0°Brix and ≥8.0°Brix for Mainpack fruits.

Rubyred

‘Rubyred‘ harvest maturity indices are a dry matter of ≥17.2% and SSC of ≥ 9.0°Brix.

California

Californian ‘Hayward’ kiwifruit requires a harvest maturity index that will give  >11.6% SSC at consumption.

South Africa

The South African golden kiwifruit cultivars ‘Y368’ and ‘Soreli’ need a harvest maturity index of 6-7°Brix SSC.

South Korea

The optimum harvest maturity index for hardy kiwifruit is 8.1-9.1% SSC.

India

In India, the harvest maturity index for cultivars Abbott, Allison, Bruno, Hayward, Monty, and Tomuri is 6.5°Brix.

In the absence of specific recommendations, a harvest maturity index of ~16% dry matter should be used as a minimum to produce quality kiwifruits. If the titrable acidity at consumption is 1.2%, then a dry matter of ~15% is also suitable.

Nondestructive Estimation of Harvest Indices

The standard tools used for nondestructive estimation of harvest indices are based on Near Infrared (NIR) Spectroscopy. Felix Instruments Applied Food Science offers a NIR spectroscopy-based device, the F-751 Kiwifruit Quality Meter. The spectrometer uses the NIR and visual light bands to quantify quality parameters such as internal and external color, dry matter content, SSC, and titrable acidity. The device can be operated on the farm and the rest of the supply chain, as it is portable, easy to use, and gives results in real-time. Kiwifruit growers can make the most of their harvest by using such tools to harvest at the correct time to optimize their harvest’s quality and help the rest of the supply chain.

Sources

Burdon, J., McLeod, D., Lallu, N., Gamble, J., Petley, M., & Gunson, A. (2004). Consumer evaluation of “Hayward” kiwifruit of different at-harvest dry matter contents. Postharvest biology and technology, 34(3), 245-255.

 

Burdon, J., Pidakala, P., Martin, P., Billing, D., & Boldingh, H. (2016). Fruit maturation and the soluble solids harvest index for ‘Hayward’ kiwifruit. Scientia Horticulturae, 213, 193-198.

 

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

 

Famiani, F., Baldicchi, A., Farinelli, D., Cruz-Castillo, J. G., Marocchi, F., Mastroleo, M., … & Battistelli, A. (2012). Yield affects qualitative kiwifruit characteristics and dry matter content may be an indicator of both quality and storability. Scientia Horticulturae, 146, 124-130.

 

Henry, M., Rosin, C. & Edwards, S. (2023). Governing taste: data, temporality and everyday kiwifruit dry matter performances. Agric Hum Values 40, 519–531. https://doi.org/10.1007/s10460-022-10396-1

 

Mahlaba, N., Tesfay, S.Z., Dodd, M., Magwaza, L.S., Mditshwa, A. and Ngobese, N.Z. (2022). Determination of optimal maturity indices for South African golden kiwifruit cultivars.

Acta Hortic. 1332, 335-342. DOI: 10.17660/ActaHortic.2022.1332.44

Nie, S., Al Riza, D. F., Ogawa, Y., Suzuki, T., Kuramoto, M., Miyata, N., & Kondo, N. (2020). Potential of a double lighting imaging system for characterization of Hayward kiwifruit harvest indices. Postharvest Biology and Technology, 162, 111113.

 

Park, H., Kim, M. J., Kim, C. W., Han, N., & Lee, U. (2022). Harvest maturity highly affects fruit quality attributes of ethylene-treated ‘autumn sense’ hardy kiwifruit. Forest Science and Technology, 18(1), 1–6. https://doi.org/10.1080/21580103.2022.2027275

 

Plant and Food Research. (2007). KiwiTech Bulletin No. N45 Taste, Dry Matter & Brix. Retrieved from https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://www.agribusiness.school.nz/pluginfile.php/2247/mod_folder/content/0/Consumer%2520Preferences/Taste%252C%2520Dry%2520Matter%2520and%2520Brix.pdf%3Fforcedownload%3D1&ved=2ahUKEwjMofakp92IAxUHwjgGHftrM-MQFnoECCgQAQ&usg=AOvVaw3Yrp2iR02WTYO7c5DcNodr