Improving Kiwifruit Quality 

• Dry matter (DM) and postharvest management will determine the final kiwifruit quality, so precisely estimating DM at harvest is essential.  
• The main aim of postharvest handling is to reduce fruit softening due to ripening, as the fruit is delicate. 
• Research in postharvest handling and processing suitable for local conditions is in varying stages of readiness around the globe, so the quality of fresh and processed fruits is similar.   

Kiwifruits are one of the newer temperate fruits whose widespread cultivation began around a century ago. The fruit is growing in popularity due to the novelty of the new cultivars’ appearance, taste, and nutritional value. Since kiwifruit is a relatively new crop, research into postharvest management is needed. However, decades of work have given us enough information to improve quality. Continue reading to explore this topic. 

What People Want in Kiwifruits 

Successful cultivation of kiwifruits started only at the turn of the last century. Kiwifruit is, therefore, a new food. Depending on consumer attitude, this novelty can be advantageous if consumers seek variety in their food. People are generally open to trying new flavors and fruits with different appearances. Hence, consumers are willing to pay more for red-fleshed cultivars like ‘Hongyang’ than for the standard green ‘Hayward.’ 

Consumers value taste, freshness, fruit size, affordability, and safety. A balance of sweet and acidic and only sweetness were both judged necessary. The place of origin was significant in some cases. In a Japanese study, New Zealand kiwifruits were rated as having the best quality compared to fruits from other countries.  

The nutritional value of the fruits was significant, and people were prepared to pay more for safe organic kiwifruits with less environmental impact. Kiwifruits are full of nutrition and rich in carotenoids, flavonoids, and phenolic compounds, lowering the risk of cardiovascular diseases, cancer, and degenerative diseases. 

One of the significant barriers to kiwifruit sales is the inability of consumers to judge when the fruit is ready to eat. The other reported barrier is the need for utensils to cut and scoop out the fruit. In addition, the thin skin and delicate flesh make handling difficult. For this reason, cut kiwifruits are gaining popularity as they combine taste, nutrition, and convenience. 

Kiwifruits are also consumed as juices (as it has 83% water), jams, marmalades, jelly, wine, puree, ice creams, canned, dried, and baked products. 

Innovative breeding, which has produced new, tastier, healthier varieties with a novel appearance, like red and gold kiwifruits, also attracts more consumers. However, the green “Haywards” remains popular and accounts for 90-95% of international trade.   

Global Kiwifruit Production 

A rise in urbanization and veganism are driving demand for kiwifruits.  

Regarding quantity, China is the largest producer and consumer of kiwifruits, with 2230 thousand metric tonnes in 2020, contributing around 50% of global production. Chinese consume 2.2 million tonnes which is 57% of the worldwide kiwifruit production.   

New Zealand, the second largest producer, which grows 624.94 thousand metric tonnes, is the largest exporter, with 22% going to China alone. 

Italy, Greece, and Iran, producing 521.53, 307.44, and 289.61 thousand metric tonnes, are the other major producers of kiwifruits. 

South African kiwifruit demand and production are also rising, though the country relies heavily on imports. South Africans hope to produce enough to export in seasons when New Zealand can’t.  

Figure 1: Expected growth in kiwifruit markets in different regions between 2022-2027, Mordor Intelligence. (Image credits: https://www.mordorintelligence.com/industry-reports/kiwi-fruit-market) 

 

Travel restrictions during the pandemic had made international trade of kiwifruits more difficult, but there was a Y-o-Y increase even in 2020. But since then, the demand for kiwifruit has been picking up globally. As a result, the estimates of growth vary for the kiwifruit market. 

Based on different sources, the global market is expected to grow at a rate of 4.3% or 5.8% CAGR between 2022 to 2027. The demand for the fruit is not expected to be even, as shown in Figure 1. The demand will be high in North America, Asia, Australia, and New Zealand, medium in Europe and South America, and low in Africa. The Asia-Pacific region will see the highest CAGR in demand.  

The growth is most noticeable in the East, with China emerging as the leading importer, even though it is the largest producer. China increased its imports, driven by the demand for healthy food, to 128.7 metric tons in 2019. 

Quality Parameters and Monitoring Methods 

Kiwis are climacteric fruits harvested when they are mature but not ripe. Fruit quality parameters, such as firmness, sweetness, color, and dry matter, are used singly or in combination.  

The traditional quality parameters for estimating harvest and maturity were firmness and sweetness. Dry matter Content is quickly becoming the most critical maturity index as it is correlated to soluble solids content (SSC) and can predict consumer satisfaction.  

According to the University of California (UC), the maturity indices for harvest for good quality fruits are as follows:  

• Dry matter content around 16% 
• A minimum sweetness/SSC of 6.5%  
• Flesh firmness of 14 pounds force 

Late-harvest kiwifruits have more firmness and SSC than early-harvested kiwifruits.   

According to UC, good quality ready-to-eat kiwifruits should have an SSC of 14% and a flesh firmness of 2-3 pounds. They should also be free of internal injury, decay, external scars, bruises, sunscald, and growth cracks.  

Earlier, firmness was measured by penetrometers, SSC by refractometers, and dry matter by the oven method. Unfortunately, all these methods are destructive and time-consuming.   

Since fruit quality has to be monitored and measured several times before harvest to establish maturity and later in postharvest storage and transport, newer tools like non-destructive Near-Infrared (NIR) spectroscopy devices are becoming popular. The devices scan the fruit with NIR, and state-of-art Chemometrics analyzes the generated spectra to give results that can be read from the devices.  

Tools such F-751 Kiwifruit Quality Meter from Felix Instruments- Applied Food Science allow accurate field estimation within seconds. In addition, the device comes equipped with GPS so that farmers can use it to create fruit harvesting maps that can advise future harvest management.  

Figure 2: “Changes of color in three kiwifruit cultivars. (A–C) Bisected fruits of ‘Hongyang’ (HY), ‘Jinnong-2’ (JN), and ‘Hayward’ (HWD) at nine developmental stages. The red star indicates a fruit harvested at 145 DAP (days after pollination),” Liu et al., 2017. (Image credits: DOI: 10.3389/fpls.2017.01507) 

 

The NIR spectroscopy allows non-destructive measurement of internal and external color to track maturity and ripeness for green, red, and gold kiwifruits, as shown in Figure 2.  

The F-751 can measure SSC, acidity, external and internal color, and dry matter content simultaneously. Since the sweetness and acidity balance are also critical criteria for consumers, the same NIR spectroscopy tools are also helpful for monitoring ripening during storage, transport, and retailing. 

Storage & Transport Conditions 

Harvest is guided by estimating maturity through internal parameters as described above. The goal in the postharvest stage is to minimize the flesh softening of kiwifruits, which is associated with ripening and an increase in SSC.    

Packing House: Sorting and Grading 

The entire crop is harvested, and defective fruits are sorted and sent for processing. Good quality fruits are stored for fresh consumption, before which they are sorted according to size. Dry brushing is enough to clean kiwifruits before grading.  

Grading is essential as larger fruits ripen and soften more slowly than smaller fruits under controlled atmospheres (CA). So it is best to sort fruits to maintain consistent quality in a batch. 

Currently, kiwifruit production in most parts of the globe suffers from proper quality inspection during grading and storage.  

New Zealand, the first country to start growing kiwifruits, is ahead of other countries in mechanizing operations. However, even they lack automated grading and rely on manual sorting. The country, however, gets higher profits per unit than other countries by grading kiwifruits according to size and quality. In these circumstances, tools like F-751 can make monitoring quality easy. 

Storage and Transport 

Kiwifruits lose one-third to half their firmness within a month, even when stored at 0°C. Therefore, the storage temperature should be close to zero but not below to avoid freezing kiwifruits. Freshly harvested fruits with lower SSC, around 6.5%, will freeze at 0.5°C, especially at the stem end, where the SSC content is the least. Over time as starch hydrolyzes and SSC content rises, the freezing temperature drops to -1.5°C.  

Several disorders impacting quality can affect kiwifruits below zero temperatures, like chilling injury, internal breakdown, translucency, and pericarp granulation.  

Ethylene levels should be below ten ppb during storage and transport, as the fruit is very sensitive to ethylene and will soften. Regular scrubbing of ethylene is necessary. Avoid storing or transporting kiwifruits with ethylene-producing products or machines. Fruits can suffer from disorders like hardcore due to high ethylene levels. 

The best storage conditions for kiwifruits are controlled atmospheres (CA), where temperature and ethylene levels are monitored and controlled. Oxygen (O2) levels of 1-2% and carbon dioxide (CO2) levels of 3-5% are used. CO2 levels higher than 7% can induce internal breakdown. In CA conditions, kiwifruit storage length depends on size; small, medium, and large fruits can be stored for 20, 30, and 49 weeks, respectively. 

During transport, mechanical damage to fruits due to vibration should be prevented by proper packing, travel speed, and maintenance of transport vehicles.  

The storage and transport conditions can vary depending on the production region since the requirements are designed not only to prevent softening, but also the incidence of postharvest diseases like Botrytis, rot, and mold.  

Fresh-cut kiwis have a shelf life of 9-12 days. They are stored at 0-2°C, 90-95% relative humidity, O2 levels of 2-4%, and CO2 levels of 5-10%. Ethylene levels as low as 2-20 ppm can soften slices. 

Artificial ripening 

Ethylene is used for the artificial ripening of kiwifruits at the concentration of 100 μl l–1 and applied for 6-12 hours at temperatures of 0°C. After treatment, the fruits can be kept for up to a month in cool storage. To reach the ready-to-eat stage, the ethylene treatment will reduce the ripening time required at 20°C. 

During retailing, the fruits should be kept cool.  

Best Practices – Preharvest and Postharvest 

With increasing research, we better understand factors affecting quality—these insights guide recommendations for best practices pre- and postharvest for kiwifruits. 

Preharvest:   

1. Adequate nutrient addition, especially nitrogen, for the crop is necessary to ensure fruit quality and maturity. 
2. Kiwifruits should not be overfertilized. Fruits from trees with leaf nitrogen of less than 2.0% nitrogen retain their firmness longer in storage. 
3. Training and pruning can ensure good exposure to light to maximize fruit quality. 
4. Fruit thinning of defective fruits allows carbon allocation to other fruits to improve quality and taste. Leaf thinning to restrict leaves number to 2-3 per fruit is recommended. 
5. Lateral fruits’ removal allows better development of the larger king fruit in the center.
6. Girdling twice in a season removes the phloem that conducts photosynthates to the roots. As a result, there is more carbon available for shoots and fruits, increasing fruit dry matter, size, and weight. 
7. Controlling crop pests and diseases can reduce postharvest fruit spoilage. 

Post-Harvest:  

1. Fruit should be air-cooled within 6 hours of harvest and prepared for storage because higher temperatures speed up ripening. 
2. ‘Haywards’ kiwifruits storage time can extend to six months if kept at 0°C and relative humidity of 90-95%. The gold and red cultivars will last only for 1-2 months in similar conditions. 
3. Fruits in normal conditions can be stored for 3-4 months. 
4. Kiwifruits should be brought to CA storage no later than a week after harvest, as it delays ripening. 
5. Use polyliners while transporting kiwifruits to prevent water loss and wilting. 
6. Keep ethylene levels below ten ppb during storage and transport to delay ripening.
7. An ultrasound treatment followed by edible coating preserves the appearance, freshness, taste, and quality of fresh-cut kiwifruit pieces- coatings such as chitosan act by reducing microbial and non-microbial spoilage.  
8. Postharvest treatments with aloe vera, calcium, ozone, polyamines, salicylic acid, potassium permanganate, and hot water enhance quality, maintain firmness and ascorbic acid levels, and extend storage time. 
9. Active packaging or 1-methylcyclopropene (1-MCP) delays softening and ethylene production in kiwifruits in the supply chain.  

Regulatory and Industry Support Organizations 

Regulatory organizations can play a vital role in any country’s effort to increase trade. New Zealand decided to regulate its producers since competition among its seven primary exporters drove down prices and profits and affected supply. Other countries were able to exploit this situation to New Zealand’s detriment.  

Therefore, the “Kiwifruit New Zealand” (KNZ), which regulates New Zealand’s kiwifruit trade, was set up under the Kiwifruit Export Regulations 1999 (‘Regulations’) and Kiwifruit Industry Restructuring Act 1999 (‘Act’), to provide collaborative marketing for growers. Zespri is recognized as the “single desk” exporter of New Zealand’s kiwifruits to all countries except Australia. Other exporters who want to export must collaborate with Zespri.  

Zespri International is now the world’s largest marketer of kiwifruits, sourcing produce from 2800 New Zealand and 1500 international growers and selling to 50 countries.   

The KZN also sets quality export standards and invests in research and market development.  

New Zealand Kiwifruit Growers Incorporated (NZKGI) supports the grower’s economic and political interests and the single desk exporter. 

Maturity Requirements Thresholds 

Exporters must meet the varying quality standards in different regions. For example, the United Nations Economic Commission for Europe (UNECE)’s Working Party on Agricultural Quality Standards defines the quality of kiwis of the three classes- Extra, Class I, and Class II in terms of defects, freshness, size, and weight. 

The UNECE requires that all fruits at packing be mature enough to continue the ripening process. For this, the fruits at packing must have a dry matter of 15% and 6.2° Brix. Fruits entering the distribution chain must have 9.5° Brix.   

Processing Kiwifruits 

Most kiwifruits are eaten fresh, but they are processed to make several products, such as juices, drinks, purees, leathers, distilled spirits, and frozen, dried, and lyophilized products.   

In most cases, minimal processing is used, where heat is not applied. For example, to make juices. However, the high levels of polyphenols can cause browning and bitterness while making juices. People try to get around this problem by using resins to absorb the chemicals. Ultrafiltration through osmotic distillation is another possibility to get high-quality juice.  

Vacuum drying and hot air are used to dehydrate kiwifruit slices. The former is better as there is less loss of ascorbic acid or vitamin C. Osmotic hydration and solar drying are some other drying techniques possible.    

Green kiwifruits are not processed much as chlorophyll is destroyed during processing, making products less attractive, except in a few cases. The golden cultivars are sought after for processing and are found in jams and jellies. However, since these cultivars are more expensive, the processing is limited.  

Green, red, and orange kiwifruits are preferred due to their color for products like icecreams.  

Cut fruits are popular for their convenience and are Packed in MAP to extend their shelf life. 

One of the primary considerations during the choice of processing method is to ensure that the quality of the food remains good in terms of freshness, color, and aroma. The other is to limit the loss of nutrients since people consume kiwifruits for their nutritional value. 

More Research and Trials Needed 

The newness of kiwifruits requires more research to improve and maintain their quality in the entire supply chain. More work is also needed in processing the fruit, where developments must catch up to other aspects of kiwifruit production. Stakeholders will also have to allow consumer preferences for safe food and environmentally friendly production methods to guide their management practices while improving kiwifruit quality.  

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