Cas is a small, round fruit in the guava family, native to Central America and most closely associated with Costa Rica, where it’s a cultural staple. Its scientific name is Psidium friedrichsthalianum, and it’s sometimes called Costa Rican guava or sour guava. If you’ve never heard of it, that’s because cas rarely leaves the region. It’s beloved locally but almost unknown internationally.
What Cas Looks, Tastes, and Feels Like
Cas resembles a small, pale guava. The skin is thin and semi-smooth, ripening from pale green to yellow. Inside, the flesh is cream-colored to yellow, firm and crisp when young, softening as it matures. The fruit contains small seeds similar to those in common guava.
The flavor is what sets cas apart from its guava relatives. It’s intensely tart, comparable to lemon, with a subtle guava-like undertone. Some people enjoy eating cas straight off the tree, but for most, it’s too sour to eat plain. That extreme acidity is actually the point: it makes cas ideal for beverages and recipes where sugar balances out the tartness, creating a flavor profile you can’t get from sweeter fruits.
Where Cas Grows
Cas is native to Central America, with Costa Rica as its heartland. The trees thrive in tropical climates at various elevations, and you’ll find them in home gardens, small farms, and rural roadsides throughout the country. The fruit is available year-round in Costa Rica, though production peaks twice: once from winter through early spring and again from late summer into early fall.
Outside Central America, cas is extremely difficult to find. The fruit doesn’t ship well, and there’s no significant export market for fresh cas. You won’t see it in most U.S. or European grocery stores. Occasionally, frozen pulp appears in Latin American specialty shops, but fresh cas remains a fruit you mostly have to travel to taste.
How Costa Ricans Use Cas
The most iconic preparation is fresco de cas, a chilled fruit drink served at homes, restaurants, and roadside stands across Costa Rica. It’s one of the country’s signature “frescos naturales,” the fresh fruit beverages that accompany everyday meals.
Making it is simple: wash four or five cas fruits, blend them with about a liter of water (being careful not to pulverize the seeds, which turn bitter), sweeten to taste, and pour over plenty of ice. The result is a refreshing, tangy drink that balances sour and sweet. Some people strain it for a smoother texture, while others prefer it with pulp.
Beyond the classic fresco, cas shows up in jams, jellies, sorbets, and desserts throughout Costa Rican cooking. Its high acidity makes it a natural fit anywhere you’d use citrus or tart berries. Cas marmalade is a common way to preserve the fruit’s flavor beyond its short shelf life after picking.
Nutritional Profile and Antioxidants
Like other guava-family fruits, cas is rich in vitamin C and dietary fiber. But researchers have found that its real nutritional story lies in its unusually high concentration of plant compounds with antioxidant and anti-inflammatory properties.
A study published in the journal Food Chemistry analyzed cas pulp and identified nine distinct compounds, including several well-documented antioxidants like ellagic acid, quercetin, and myricetin. These are the same types of protective compounds found in berries, pomegranates, and green tea, but cas appears to contain them in notable concentrations. In lab testing, two of these compounds reduced inflammatory markers more effectively than cortisol, a benchmark anti-inflammatory substance.
The same research explored whether cas extracts could counteract the kind of cellular damage caused by cigarette smoke. The fruit’s compounds inhibited the production of specific inflammatory proteins involved in chronic lung conditions. While this doesn’t mean drinking fresco de cas treats lung disease, it does suggest the fruit packs a more potent antioxidant punch than its obscurity might imply.
How to Pick and Store Cas
If you’re lucky enough to encounter fresh cas, selecting ripe fruit is straightforward. Look for fruits that have turned from green to a uniform pale yellow. The skin should feel smooth and slightly give under gentle pressure. Green, rock-hard fruits are unripe and will be mouth-puckeringly sour even by cas standards. Overripe fruits feel mushy and begin to brown.
Fresh cas has a short window once picked. At room temperature, ripe fruits last only a few days before deteriorating. Refrigeration extends this to roughly a week. For longer storage, many Costa Rican households blend the pulp and freeze it in bags or ice cube trays, keeping the flavor available for frescos and recipes for months.
Growing a Cas Tree
Cas trees are tropical plants that need warm temperatures and consistent moisture. They grow well at a range of elevations in their native habitat and prefer well-drained soils. The trees are moderate in size, making them manageable for home gardens in appropriate climates. In the United States, cas can grow in southern Florida and Hawaii, where conditions approximate its Central American range.
The trees are relatively low-maintenance compared to other tropical fruit species and have some natural resistance to pests, partly because of the same bitter, tart compounds that give the fruit its signature flavor. From planting, expect several years before a young tree produces meaningful fruit. Once established, a healthy cas tree can be quite productive, often yielding more fruit than a single household can consume fresh, which is part of why blending and freezing the pulp is so common in Costa Rica.