Titanium to improve fruit quality

Gordon Brown reviews Chinese research on the use of titanium dioxide to improve the colour and quality of apples.  

Introduction 

The first thing consumers do to decide if they will buy fruit is to look at its appearance. Chinese consumers generally like full-coloured red apples that are clean, undamaged and glossy, although they don’t want their apples waxed to improve their appearance. 

As growers, it is possible to improve red colouration by exposing fruit to extra sunlight by pruning and using reflective cloths, or using ethephon close to harvest. While these practices improve colour, unfortunately they have disadvantages such as cost and potential issues with poorer quality due to sunburn and advanced maturity. 

In the search for other useful treatments Chinese researchers have explored the potential of titanium dioxide – as used in sunscreen products – to improve apple colour and other fruit quality characteristics. Nano particle titanium dioxide has been safely used as a colorant and whitening agent for more than five decades. It is commonly added to a variety of products, including coffee whiteners, condiments, confectionery, baked goods (to enhance whiteness), processed seafood and fruit products (for colour improvement), as well as toothpaste and beverages. It is typically present at around 5 per cent in sunscreen products. In Europe, titanium dioxide was banned in 2022 for use in food due to safety concerns. In the same year, Food Standards Australia New Zealand reviewed it and found no evidence of genotoxicity, toxicity, or carcinogenicity so it is still permitted here.  

In the research article being reviewed today, researchers compared the impact of titanium dioxide and ethephon treatments on fruit colour and quality. 

Method 

In the field trials on both Royal Gala and Red Chief apple trees, a nano particle titanium dioxide spray (1kg/1000l) was applied at five-day intervals for the month prior to harvest. Four applications were made to the Royal Gala trees and three to the Red Chief trees. This was repeated over two seasons. At harvest, fruit firmness, sugars and acidity were measured to assess impacts on fruit internal quality. In addition the red skin colour was characterised and the epidermal layers of the fruit studied with an electron microscope. 

Results 

For this article, the averages across both seasons for the Royal Gala trees will be studied. Identical trends were observed for the Red Chief trees. 

Fruit maturity 

It was observed that ethephon reduced fruit firmness by 17 per cent and acidity by 11 per cent, while fruit sugars were marginally increased (Table 1). These measures of fruit maturity all indicate that the preharvest ethephon treatment slightly advanced fruit maturity. This was not the case for the titanium treatment, where there were only minor reductions in all measurements, suggesting that the fruit maturity was not affected by this treatment.  

Table 1: The average level of fruit firmness, sugar content and acidity for Royal Gala averaged across two seasons after application of either titanium dioxide or ethephon to the trees. Fruit firmness was measured with a 2mm diameter probe rather than the 10mm probe used in Australia.

Fruit appearance 

In China, blush area is the most important character of red skinned apples that impacts consumer desire to purchase the fruit. Averaged across the two seasons, preharvest application of both products dramatically increased the red colouration surface area of the fruit (Figures 1 and 2). Titanium dioxide was not quite as effective as the ethephon spray, with increases of 32 and 53 per cent respectively. 

Figure 1: Surface area of fruit covered in red blush, with preharvest titanium dioxide or ethephon application averaged across two seasons.

Figure 2: Red colouration of Royal Gala apples after preharvest application of titanium dioxide or ethephon (year 1 of trial).

As well as area of red colouration, the depth of red colouration is also important for consumer acceptance of apples in China. This was instrumentally measured and it was found that both the titanium dioxide and ethephon increased the quantity of red pigmentation three-fold (a* from 6 to 18), resulting in an elevated and similar intensity of red colouration. 

As mentioned in the introduction, the glossiness of the fruit is also important for consumer satisfaction. In this trial, glossiness was measured with a gloss meter and, although this instrument is not designed for use on curved surfaces such as fruit, it showed that the titanium dioxide increased the fruit glossiness by 23 per cent to a gloss unit of 10.6. It should be noted that not only is a gloss meter designed to work on flat surfaces, its response is not very sensitive to gloss units below 10. However, this response for titanium dioxide is similar to results in other publications by the research article author, indicating this change is real. The gloss units for a carnauba waxed apple is not available; however, for indicative purposes, the shine on an eggplant fruit returns a gloss unit of around 15 units. 

The glossiness of apples is related to the natural wax on the skin surface. This research went on to study the cuticle with an electron microscope and this revealed several impacts of titanium dioxide application on the fruit skin (Figure 3). These results may be more important to cherry growers than apple growers as there is potential for this treatment to have an impact on rain splitting of the fruit. 

Significantly, the preharvest titanium dioxide application increased both the number of epidermal cell layers (by 50 per cent) and the quantity of cuticular wax on the surface of the fruit (33 per cent greater). Of interest is that ethephon also had these impacts but to a lesser degree (11 and 25 per cent greater, respectively). Hence these treatments both increased the strength of the skin as well as the waterproof protective coating on the skin of the fruit. This means that there will potentially be fewer stalk end punctures and subsequent decay, and less water loss from the fruit during storage. 

Figure 3: The number of epidermal layers and quantity of cuticular wax in response to preharvest.

Genetic analysis of the fruit identified that both the titanium dioxide and ethephon treatments upregulated the genes for wax biosynthesis to different extents. While both materials increased alkanes and fatty alcohols, titanium dioxide reduced ester production, while ethephon increased it. It is pertinent to note that liquid esters are related to the appearance of greasy fruit, although this characteristic was not measured in this trial. 

Conclusions 

A recent review of titanium dioxide in Australia found it to be a safe food ingredient, although it was banned in Europe over food safety concerns. As a result, titanium dioxide is still a common food ingredient in Australia. The application of titanium dioxide to apple trees in the month prior to harvest increases fruit red colouration, with minimal impact on fruit firmness, sugars or acidity. In addition, this treatment increases the quantity of cuticular wax on the skin, which imparts a slight improvement in the glossiness of the fruit as well as additional epidermal cell layers which may lead to tougher skin. In contrast, preharvest ethephon application has a similar impact on fruit colouration, but this is associated with a slight advancement of fruit maturity and a smaller effect on the fruit skin epidermis and cuticle. 

Take home messages 

• Titanium dioxide is a common additive to food products in Australia (E171). 
• Preharvest titanium dioxide application improves harvest fruit colour with minimal impact on fruit maturity. 
• Preharvest ethephon application improves fruit colour, with a slight advancement in fruit maturity. 
• Preharvest titanium dioxide application increases the number of epidermal cell layers and the quantity of cuticular waxes to a greater extent than ethephon sprays. 
• Increased epidermal cell layers may lead to tougher skin less prone to damage in postharvest activities. 
• Increased cuticular wax is associated with improved fruit glossiness. 

Further reading 

Wang et al. (February 2026) “Nano-titanium dioxide enhances apple appearance quality via wax biosynthesis and cuticular remodelling”, Plant Nano Biology, 15:100253, https://doi.org/10.1016/j.plana.2026.100253 

G Ward and A Nussinovitch, (September 1996) “Gloss properties and surface morphology relationships of fruits”, Journal of Food Science, 61:973–977, https://doi.org/10.1111/J.1365-2621.1996.tb10914.x 

Gloss measurement: www.byk-instruments.com/support/knowledge/white-papers/appearance/gloss-measurement

This article was first published in the Winter 2026 edition of AFG.