Coffeeberry all over the world mostly gives out two beans from a single berry. in case of a peaberry, a single bean is extracted from a berry. That makes peaberry coffee beans unique.
There is no specific reason for this as it's natural mutation and just 5-10% of the coffee produced from the plantation is estimated to be pea-berry. The shape of the coffee beans are flat at the center and curved at the sides, but in the case of pea-berry, the coffee beans are round like pea as there is only single bean the berry.
Merchant and coffee traders earlier believed that these beans were not of use. They would consider these as defective until they realize that the beans were coffee beans with benefits.
The round shape can allow free movement of beans inside the drum, giving a more even roast.People also believe that the beans are denser and tastes sharper, sweeter, and juicier.
Roasting peaberry requires a slow and steady roast. The roasting process becomes complex as the beans we are dealing with are dense and have special characteristics than the other coffee beans.
The way to get these beans segregated from the entire production is bypassing all the beans through the screening size machine where these small round beans get separated from the lot and these are then sold separately at a slightly higher price.
However, the scientific reason and the best explanation we could find is:-
"The ultimate cause of a peaberry is that either the ovule fails to be fertilized, or there is a failure in the growth of the endosperm. Several factors are thought to cause these two conditions: Insufficient pollination, environmental conditions, and genetic defects. Peaberries are much more common in the extremities of the plant, where weather conditions are more severe, the journey of the nutrients is much longer, and the agents of pollination, gravity, and wind (it is estimated that insects account for only 5-10% of pollination), have a harder time reaching the stigma of the blossoms for fertilization. "
Source: Casa Brazil Coffees