At the beginning of the ripening process the sugar/acid ratio is low, because of low sugar content and high fruit acid content, this makes the fruit taste sour. During the ripening process the fruit acids are degraded, the sugar content increases and the sugar/acid ratio achieves a higher value.
In the USA, a minimum TSS/acid maturity ratio of between 7 to 9:1 is typically desired for oranges and mandarins. For grapefruit between 5 to 7:1 is the standard for marketable produce.
In tropical climates harvesting of oranges commences when the minimum TSS/acid ratios are reached and the fruit has a green-yellow color on no more than 25% of its surface.
In Mediterranean climates, fruit is harvested when the appropriate TSS/acid ratio is reached and the fruit is orange on its entire surface. Exceptions in fruit color may be made in early fruit harvests; this fruit may be 'degreened' in ethylene chambers.
The TSS/acids ratio is determined by simple division. For example: if the oBrix value is 12 and the total acid value is 1.0%, the ratio would be 12. If juice had a oBrix value of 12 and an acid value of 0.8%, the ratio would be 15.
The TSS/Acid ratio is reduced by high rates of nitrogen and potassium, and to a lesser extent, magnesium.
Together with a slight effect on TSS content, high nitrogen rates will reduce the TSS/acid ratio. It is important not to use excessive N or harvest will be delayed.
Potassium increases the content of organic acids in fruit juice. This increase in acidity may also reduce the TSS/Acid ratio. Thus, the correct K regime is important and will differ for the production of sweet oranges, mandarins, or acid lemons.
Boron, copper and iron may cause an increase in the TSS/Acid ratio. These trials illustrate the importance of copper in fruit quality and maturity.