WITH the world's highest per capita consumption of oranges, Hongkong is a natural port of call for Mr Russell Hanlin, president of Sunkist, the largest orange co-operative. The average Hongkong consumer eats about 24 kilograms of oranges a year, against about nine kilograms per person in the United States. ''They may not explain it in the terms that a PhD would, but over hundreds of years of watching, Chinese people have understood the nutritional value of this fruit,'' said Mr Hanlin, who after 42 years at Sunkist still relishes his daily orange. ''There is absolutely nothing you can do as good for your health as eating an orange,'' he added, exuding a fair measure of good health himself despite his tough schedule which allows him only two days in the territory. Mr Hanlin is in Hongkong to celebrate the 100th anniversary of Sunkist, the first agricultural co-operative to reach this age. He is also eyeing the 1.2 billion potential orange-eaters over the border, where he says the potential is ''astonishing''. It would be an appropriate move: some botanists believe the orange originated in China. But it now lags behind, both in production and quality. Yet every country with a substantial Chinese population was near the top of Sunkist's per capita sales table. The Chinese believed that oranges were neither ''hot'' nor ''cold'' and therefore particularly good for children, he added. Last year, Sunkist's turnover was US$1 billion from 220,000 acres on 6,500 citrus ranches. It produced 85 million boxes, each weighing about 18 kilograms, enough to fill the streets of Central shoulder-high. Hongkong fruit-hawkers, usually the subject of numerous complaints, got top marks from Mr Hanlin, who said he wished the big US supermarket chains were as interested in keeping his oranges in good condition.