Balancing Total Soluble Solids and Acidity Ratio in Stone Fruit

The TSS content of the fruit is usually obtained from assessing the oBrix of the fruit. The TSS or ‘sugar content’ measures and includes the carbohydrates, organic acids, proteins, fats and minerals of the fruit. It represents from 10-20% of the fruit's fresh weight and increases as fruit matures to produce a less acidic, sweeter fruit.

During the ripening process stone fruit acids are degraded, the sugar content increases and the sugar/acid ratio reaches a higher value. A high level of acidity – common in under-ripe fruit – makes the fruit taste sour. Conversely, over-ripe fruits have very low levels of fruit acid and therefore lack characteristic flavour.

It is important that the grower aims to produce an acceptable balance of TSS and fruit acidity. The TSS/Acid ratio is a key characteristic determining the taste, texture and feel of fruit segments. It contributes towards giving many fruits their characteristic flavour. It is also an indicator of commercial and sensory ripeness.

Appropriate crop nutrition programs help to manage the TSS/Acidity balance in the fruit.

Crop Nutrition and TSS/Acidity in Stone Fruit

Nitrogen

Increasing nitrogen supply can increase fruit acidity and also improve vitamin C content.

However, overuse of nitrogen can reduce fruit colour and TSS contents. This is due to the improved yield and larger fruit size, which dilutes the level of TSS in the fruit.

Potassium

Potassium has a major role to play in improving the TSS and sugar levels in the fruit.

The content of organic acids in the fruit is also increased by potassium. This increase in acidity may also reduce the TSS/Acid ratio. Thus, a correct potassium regime is important to manage the TSS/acid ratio, giving desirable fresh fruit of good taste and flavour.

Calcium

Calcium can be used to manipulate the TSS/Acid ratio in fruits through increases in TSS contents, influencing fruit taste and flavour.

Iron

Use of iron may also cause an increase in the TSS/Acid ratio. Iron effectively increases the level of sugars in the fruit. Trials with iron-chelate sprays on plums show increases where iron is deficient.