The brix of sweet fruit

Buying local is good for the environment and for your taste buds. When in season, I enjoy apples and peaches from the nearby Virginia farms. Produce from Pennsylvania farms — only a few hours away — arrives rich and flavorful. But it is now winter, local trees and fields lay barren, and only a few energy-sucking greenhouses produce anything. I will trade carbon emissions from many other activities before I give up those fruits and vegetables flown in from far and warmer places.

Fruits from far away need to survive the transport. Producers choose varieties not for flavor, but for appearance after transport. The long haul also raises the prices: US$1.29 for one navel orange, $6.15/kg of apples, or $12.10/kg of guavas. Before paying this much I would like to have some idea of the quality of what I am getting. I have peeled too many navel oranges only to discover a dry and tasteless fruit.

Then I came across this sign in a supermarket:

Fruits at Wegmans

See the Brix 16 sign? The Brix gives you an idea of how sweet the fruit is. The amount of sugar in a fruit increases as it ripens, and as most fruits are done growing in size by the time they start to ripen, this means that the ratio of sugar to liquid in the fruit increases as it ripens. The individual concentration of the different sugars (fructose, sucrose, etc) does not matter as much as the total sugar concentration for the flavor of the fruit, and degrees Brix (°Bx) measure the total amount of sugar in a standard amount of water. If in a drop of the fruit’s juice there are 200 milligrams of solids and 800 milligrams of water, then the juice is at 200/(200+800) = 20 °Bx concentration.

A small microscope- or telescope-looking device called a refractometer is used to measure the Brix concentration. A small amount of the juice of the fruit is placed in the refractometer, a number read based on the edge of a shadow seen through the eyepiece. The refractometer was invented by Ernst Karl Abbé (he worked with Carl Zeiss, the famed optical instrument maker) and put to use to detect the dilution of milk by water and of butter by cheaper oils.

Old refractometer

(I copied this image from an out-of-copyright book, but lost the reference. If you recognize it, let me know.)

Two of the supermarkets I shop at display Brix signs. I spotted a man with a refractometer at the supermarket and he explained that they are planning to add the brix number to all fruit signs. Different fruits have different Brix concentrations once ripe, so we will need to learn to read the signs. I’ve collected some Brix values for fruits from some food science papers:

fruit ok  good  great 
Apple (Gala)101214
fruit ok   good  great 
Passion fruit???

Question marks stand for values I have missed. I did not measured these myself, but my plan is to find an affordable refractometer and check these numbers over the year. Some regions will not produce very sweet fruit: California figs when great have 20°Bx, while Tunisian figs will start at 22 and may even hit 30°Bx.

Brix gives you an idea of the sweetness of the fruit. But other tastes, such as the acidity, contribute to the richness in flavor, but that will be another post.