What Determines If Citrus Fruit is Mature?

The factors that are taken into consideration in determining if citrus fruit is mature and ready to be shipped are sugar content, juice content, and acid content. The fruit must also be degreened sufficiently to meet established color standards although the color of the fruit is not an indication of whether or not it is mature.

Sugar Content (more formally known as Brix or TSS 'Total Soluble Solids')

The most common method for determining Brix or TSS is simply to use a Brix scale hydrometer. The hydrometer is placed into a quantity of juice and it floats and measures the specific gravity of the juice. The hydrometer is calibrated to read in degrees of Brix. The measured results must then be corrected for temperature to arrive at the true Brix reading.

Another tool that can be employed to measure Brix or TSS is a refractometer. A refractometer is an optical instrument that measures the amount of light refracted in a liquid.

About 85 percent of the total soluble solids (TSS) of citrus juice are sugars. As a result, determining what the TSS of the juice is will give a very good indication of the sugar content of the juice.

Standards for sugar content will change as the season progresses and will be different for each variety of fruit. As the season progresses and the fruit matures on the tree, the standards for sugar content are raised. If a crop does not meet the established sugar standards it cannot be shipped.

Juice Content (% Juice)

A test is also conducted to determine what percentage of the fruit is juice. The inspector will weigh the fruit sample that he is testing, which will consist of a number of pieces of fruit that he takes as a sample as the fruit is running through the packing house.

He will then juice the fruit with a specially designed juice machine. All citrus packing facilities in Florida are required to utilize the same type of machine to insure uniformity of testing.

The juice is then weighed. The inspector will then arrive at a % juice value by dividing the weight of the juice by the weight of the fruit sample before juicing and multiply by 100 to arrive at a percentage.

The juice percentage is then compared to established standards that change throughout the season. If the standards are not met, the fruit cannot be packed or shipped.

Acid Content (citric acid content)

The test to determine the acid content of citrus fruit involves adding an indicator solution (most often phenolphthalein) to a measured amount of citrus fruit. The inspector will then measure the amount of sodium hydroxide required to neutralize the solution (and turn it pink). The acidity (also described as titratable acidity) is measured in ml. Again, specific standards must be met or the crop cannot be packed.

Sugar (Brix):Acid ratio

Immature fruit normally has a low sugar to acid ratio because immature fruit will normally have a low sugar content and a high acid content. That is why immature fruit will normally taste very sour. As the season continues and the fruit matures, the sugar content will normally increase and the acid content will diminish. This will result in a HIGHER sugar to acid ratio.