Inside the coffee fruit, two seeds typically develop. As the seeds enlarge and mature, they push against each other. The result of this pushing is the flat face of a coffee seed. On every coffee tree, a percentage of the cherries contain only a single seed. With no opposing seed in the cherry, the lone seed has no flat face. Rather, it is entirely round, almost pealike. This seed is called a peaberry. The percentage of peaberries on a given tree varies, but most of the time it is 4 to 8 percent. There have been several reports, however, of trees that produce percentages from 30 to 35 percent.

Why peaberries occur is not known, though many scientists over the years have speculated on a variety of possibilities. These include genetic factors, plant age, climatic conditions, poor pollination, and nutritional deficiencies. Scant research exists that examines any of these potential influences. Ultimately, some kind of malfunction occurs at the cellular level, which prevents the growth of the seed. The malfunction could occur prior to fertilization. For example, the pollen tube—an organ that grows from a pollen grain after it has landed on a stigma and whose purpose is to deliver its gamete to the receiving gamete in the flower’s ovule—might be disrupted, preventing it from delivering its package. Another possibility is that the gamete reaches its destination, but either the female egg or the ovule itself are inviable, preventing fertilization. Alternatively, fertilization may occur without incident, but the zygote or ovule aborts, leaving an empty chamber behind.

What has become clear is that there is a strong genetic component to peaberries.

Offspring can produce different percentages of peaberries than their parents. Irradiating seeds with neutrons or X-rays, then letting them grow into plants, increases the percentage of peaberries. Even manually cross-pollinating flowers decreases the occurrence of peaberries.

Peaberries have captured the imagination of coffee drinkers who seem quite happy to pay a premium for them and roasters are just as happy to supply them. This suggests that there is something different or special about the physiology, biochemistry, or taste of peaberries. Unfortunately, there isn’t much research on the subject. Peaberries germinate just as often as their flat-faced brethren. There are studies that show some biochemical differences in the seeds when they are unroasted, but those differences largely disappear after roasting. As for taste, no research, complete with statistical analysis, could be found comparing flat-faced seeds to corresponding peaberries. In the literature where their taste is discussed, peaberries are considered to taste the same or inferior to flat-faced seeds, though the research was just anecdotal. 

An important consideration with peaberries and taste is how they respond to roasting. A round, somewhat uniform shape will interact with heat differently than an asymmetric shape. If the heat transfer during roasting is different between the two shapes, resulting in different roast profiles, then a taste difference could arise. If this is the case, then the taste difference is an artifact of roasting, not the internal characteristics of the seed. I hypothesize, then, that the taste difference would be fairly small and nothing of the scale usually touted by retailer or consumers.

With the potential of no important difference, do peaberries warrant their higher price?

As peaberries do only occur in low percentages, they are rare; typical supply-demand curves would suggest a higher price. In addition, while farmers have no control over their occurrence, their maximum potential yield is never reached. Each percentage increase of peaberries results in a 0.5 percent decrease in potential yield. This is just a numerical difference. Peaberries tend to be smaller and weigh less than most flat-faced seeds, making the yield, as measured by weight, even lower! Thus, farmers have a sense of being penalized by nature and are keen to make up for the economic loss. Finally, at mills where the peaberries are removed manually, there is an added cost of labor for that effort.

Although, large mills with lots of equipment typically have machines that sort coffee by size, separating out the peaberries, and these mills incur no additional cost or effort.

Ultimately, it doesn’t matter if there is a statistically significant difference in the biochemistry or taste of peaberries or whether they cost more to produce. If consumers continue to pay a premium for them and believe them to be better, then they are better, at least in the mind of the buyer. 

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