The problem sunburn actually causes
Fruit sunburn is a heat and radiation injury, not simply a cosmetic blemish. When fruit surface temperature climbs — and in exposed positions it runs well above ambient air temperature under direct sun — the tissue passes through progressive stages of damage: a loss of pigment and photobleaching at moderate stress, then browning, then necrotic tissue death at the extreme.
For Australian growers this sits directly on the bottom line. Sunburned fruit is downgraded from premium fresh-market packs to juice or processing, or discarded entirely. In a hot season the loss can be a substantial share of the crop on the exposed side of the canopy, and it lands on precisely the well-exposed, well-coloured fruit that would otherwise have been the highest-value pack-out.
Conventional mitigations — overhead netting, evaporative cooling, canopy management — all work, but netting is capital-intensive and cooling consumes water, which is not a comfortable trade in much of the Australian growing regions. A sprayable reflective film is attractive because it uses existing spray equipment and scales with the block.
How a kaolin particle film works
The principle is straightforward radiative physics. A suspension of fine kaolin particles is sprayed onto the canopy, and as the water evaporates it leaves a continuous white mineral film on leaf and fruit surfaces. That film changes the surface's radiative behaviour in a deliberately selective way:
- Infrared radiation is reflected, which is what carries the heat load. Less IR absorbed means a lower fruit surface temperature during the hottest hours.
- Ultraviolet radiation is reflected, reducing the photo-oxidative component of sunburn damage.
- Photosynthetically active radiation is substantially transmitted, so the leaf underneath continues to work.
Kaolin suits this job better than most white minerals because it combines high natural brightness, a platy particle habit that forms a coherent continuous film rather than discrete specks, chemical inertness, low abrasiveness on spray equipment, and — importantly for the fresh market — it washes off at the packhouse.
What the trial evidence shows
Kaolin particle films have been studied since the late 1990s and the literature is now reasonably deep. The headline findings:
- Kaolin films reduce leaf and fruit sunburn damage across a range of species including apple, pomegranate and grapevine, with reported reductions in sunburn incidence of up to around 60%.
- USDA Agricultural Research Service work reported sunburn losses reduced by up to 50% in high-temperature trial years.
- Measurable reductions in fruit and leaf surface temperature during peak radiation hours — the mechanism doing what it is supposed to do.
- Trials on Braeburn, Granny Smith and Fuji apples showed significantly reduced sunburn with reflective kaolin films.
- Secondary effects on insect pests, including repellency and oviposition deterrence, which is why the category features in organic orchard programmes.
The variance between trials is worth taking seriously. Efficacy depends on season severity, cultivar susceptibility, canopy architecture, spray coverage and reapplication discipline. Treat published figures as an indication of what is achievable with good practice in a hot season, not a guarantee.
Application in practice
Growers using registered particle-film products generally work to a pattern like this:
| Stage | Practice | Why |
|---|---|---|
| First application | Ahead of the first forecast heat event, not after it | The film is preventive; damage already done cannot be reversed |
| Build-up | Two applications close together to establish continuous cover | A single pass rarely achieves a coherent film |
| Maintenance | Reapply after significant rain and through rapid fruit growth | Rain removes film; fruit expansion stretches and thins it |
| Coverage | High water volumes, attention to the sun-exposed side of the canopy | Uncovered surface is unprotected surface |
| Removal | Packhouse washing / brushing | Residual film affects appearance at point of sale |
The practical objections growers raise are real and worth naming: the white residue makes visual pest scouting and maturity assessment harder, equipment needs thorough cleaning, and packhouse removal adds a step. These are manageable, but they belong in the decision.
Grade requirements and the regulatory position
Particle-film kaolin is not simply any china clay. The properties that matter are:
- High brightness and whiteness — reflectance is the entire mechanism, so brightness is the primary functional specification.
- Fine, controlled particle size — fine enough to form a continuous coherent film and to stay in suspension through the tank and boom; coarse fractions settle out and block nozzles.
- Low abrasiveness and low grit — spray pumps and nozzles are expensive.
- Low iron — iron drives colour away from the neutral white that reflects best.
- Chemical inertness and appropriate purity for a material going onto a food crop.
On the regulatory point, we want to be unambiguous. Any product applied to a crop in Australia must be registered with the APVMA. Registered particle-film products are formulated materials — they include wetting and dispersing agents, without which raw mineral powder will not suspend, spray or adhere. PIME supplies the mineral raw material to formulators and manufacturers; we do not supply a registered crop-protection product, and growers should purchase a registered product through the normal channels.
If you are a formulator developing or supplying into this category and need kaolin graded to a particle-film specification, the kaolin product page sets out our available grades, and we can grade against your brightness and PSD targets.
Frequently asked questions
How much does kaolin reduce fruit sunburn?
Published trial work reports reductions in sunburn incidence in the range of roughly 50 to 60% in high-temperature seasons, with USDA Agricultural Research Service studies reporting sunburn loss reductions of up to 50% in hot trial years. The figure varies considerably with season, cultivar, canopy management and spray coverage — it is a risk-reduction measure rather than an elimination, and the benefit is largest in exactly the hot seasons where losses would otherwise be worst.
Does the white film reduce photosynthesis or delay ripening?
The film is engineered to be selective: it reflects infrared and ultraviolet radiation while transmitting a large share of photosynthetically active radiation. Well-applied films therefore have modest effects on whole-canopy photosynthesis, and by reducing heat stress they can protect photosynthetic capacity during extreme heat. Effects on maturity and colour development vary by crop and cultivar — in some red apple cultivars heavy films have been associated with reduced red colour development, so timing and rate need local calibration.
Which crops is kaolin particle film used on in Australia?
It is most established on apples and pears, where sunburn is a major source of downgrade, and is also used on citrus, stone fruit and wine grapes. In viticulture the interest is in protecting fruit from heat damage and limiting berry temperature during heatwaves. The same reflective film also has a documented repellent and oviposition-deterrent effect on some insect pests, which is why it appears in organic orchard programmes.
Can I buy kaolin from PIME and spray it directly on my crop?
No. PIME supplies the mineral raw material to formulators and manufacturers, not a registered crop-protection product. Any product applied to a crop in Australia must be registered with the Australian Pesticides and Veterinary Medicines Authority (APVMA), and registered particle-film products are formulated with wetting and dispersing agents that untreated mineral powder does not contain — without them the material will not suspend, spray or adhere properly. Growers should buy a registered product; formulators seeking the raw mineral should contact us.