Midwinter is prime citrus season for both the San Joaquin Valley and Southern California districts, with an abundance of excellent mandarins, oranges, tangelos and lemons. The one laggard is conventional grapefruit, which, as grown in these two areas will be too sour for most palates for a couple of months or more. By compensation, we have three fine locally adapted grapefruit-like hybrids, Oroblanco, Melogold and Cocktail "grapefruit," which are at their peak right now.
The Oroblanco is the most widely grown and flavorful of the three. It originated in the 1950s, when R.K. Soost and J.W. Cameron, citrus breeders at UC Riverside, recognized that regular grapefruit does not get enough heat in California, outside the desert, to sweeten in the main winter citrus season.
In what turned out to be an inspired choice, they crossed Siamese Sweet pummelo, an acidless form of the parent species of the grapefruit, with Duncan grapefruit, which is the seedy, white-fleshed variety from which all other commercial grapefruit originated, mostly by mutation. Siamese Sweet itself is insipid, but as a parent it lowered the acidity of its daughter varieties, so that their flavor balance is ideal under conditions of moderate heat; and Duncan contributed much of its intense, distinctive taste, which is rightly considered by experts to be the finest of all grapefruit.
In addition, the breeders deliberately used a selection of Duncan that was a tetraploid, with twice the usual number of chromosomes. As a hybrid of this and a regular diploid pummelo, the Oroblanco, introduced in 1980, was a triploid, with three sets of chromosomes, and this genetic unevenness causes it to be seedless. Or close to it. Sometimes it contains tiny aborted seeds, which are usually flicked aside when the fruit is eaten.
As one drawback, the Oroblanco inherited a thick rind from its pummelo parent, so that the edible "fruit ball" inside can be relatively small. This is therefore one of the few citrus fruits for which it is advisable to look for large or at least medium-size specimens, and particularly ones that feel heavy in the hand; these will have a larger fruit ball. Also, look for smoother, flatter fruit, rather than fruit with a "sheep nose" at top.
The best way to eat an Oroblanco is to slice off the top and the bottom horizontally, cutting just deep enough to reveal the juicy pulp. Then cut the fruit in half longitudinally, slicing along the axis, trying to align the knife along segment walls. Because these membranes contain naringin, a flavonoid compound responsible for much of the bitter taste in grapefruit, while the flesh is delightfully lacking in it, it is worth the trouble to fillet an Oroblanco. To do so, slice along each membrane wall, pulling off each segment if you wish, or leaving it attached to the rind, which provides a convenient handle for eating the flesh.
Alternatively, you can eat an Oroblanco, like a grapefruit, with a spoon.
Oroblancos, which are grown on 541 acres in California, are harvested from November to January in the San Joaquin Valley. Fruits from this area can have acceptable flavor when harvested early, when the rind is light green, but the best flavor comes from more mature fruits with a pale yellow or slightly darker golden color.
The season from the intermediate inland areas of Southern California, such as Riverside and north San Diego County, starts in January and runs well into the spring. Peak quality from most of these local groves is right now, from late January to early March. For reasons that I have never been able to fully identify (some combination of growing conditions, rootstock and soil, perhaps), the Oroblancos grown by Bob Polito in Pauma Valley and Valley Center offer a perfect balance of sweetness and acidity, firm but juicy flesh, and an intense, very pleasant flavor. He sells them at the Venice farmers market on Friday, at Santa Monica on Wednesday and at La Cienega on Thursday.
The sister of Oroblanco, Melogold, which resulted from the same cross and was released in 1986, has a larger fruit with a thinner rind but a somewhat less attractive flavor, more like a pummelo, and slightly less sweet. It is grown on 135 acres around the state and is a bit later in season than Oroblanco, February to April in the main citrus growing areas of Southern California. It's also more likely to develop off-flavors when overmature, so locally grown fruits are best eaten in mid- to late winter.
The third and most unusual of these varieties is the so-called Cocktail grapefruit, which originated at UC Riverside as a cross of Siamese Sweet and Frua, an otherwise unremarkable early-maturing mandarin ("frua" is Esperanto for "early"). It has sweet, mild-flavored, very tender and juicy flesh, which is light orange and has a flavor combining both pummelo and mandarin.