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.
Because it is quite seedy and mature fruits are so soft that they can be a challenge to pack and ship commercially, Cocktail grapefruit was not officially released when it was first grown in the 1980s; it "escaped" from scientific trials and became popular among local and farmers market growers. The name refers to its size, which can be as small as that of an orange, although it can also be as big as a large grapefruit.
The Cocktail season in Southern California runs from January through spring, but from most areas the fruit is at its best right now, delightfully sweet but with a pleasing tinge of acidity and adequately firm. Some vendors sell them as late as April, but at that time they can be insipid in flavor and puffy and soft in texture.
Large fruits can be filleted like an Oroblanco, while smaller specimens makes a superbly sweet and refreshing juice.
Valentine, a cousin of these varieties, is worth noting even though the fruits are not yet much available. It resembles a low-acid blood grapefruit and resulted from a cross made by Soost and Cameron of Siamese Sweet pummelo on one side and a cross of Dancy mandarin and Ruby blood orange on the other. It inherited large size and low acidity from the pummelo, some of the complex floral taste of the Dancy and juicy red flesh, pigmented with anthocyanins, from the blood orange.
Ottillia "Toots" Bier, who worked at the Citrus Variety Collection, nicknamed the variety "Valentine" because it ripens around Valentine's Day in Riverside and because, when cut in half and turned upside down, it resembles a vibrant red heart. Budwood was just released to nurseries in 2009, and it remains to be seen whether the variety will be productive in a commercial setting, but a few specialty citrus growers have planted trees, and the fruits should start showing up at farmers markets in a year or two.