**5. Sugars and organic acid composition**

Citric and malic acids are the major organic acids present in prickly pear fruit pulp and peel. Other organic acids, such as oxalic, tartaric, quinic, shikimic and fumaric acids, are present in traces. Citric acid ranges from 1.60 to 3.20 g L<sup>−</sup><sup>1</sup> in fruit peel and shows values from 0.30 to 1.61 in pulp g L<sup>−</sup><sup>1</sup> [9]. Malic acid shows concentrations between 1.04 and 2.20 g L<sup>−</sup><sup>1</sup> in peel and 1.20 and 2.10 g L<sup>−</sup><sup>1</sup> in pulp. However, cladodes show higher values of these acids (71.8 g L<sup>−</sup><sup>1</sup> of malic acid and 37.7 g L<sup>−</sup><sup>1</sup> of citric acid) and also contain succinic acid (43 g L<sup>−</sup><sup>1</sup> ) [9]. This is due to the CAM metabolism of *O. ficus-indica*, especially in the cladodes. Organic acids are accumulated in the vacuole during night and suffer a reciprocal reserve carbohydrates accumulation during the daytime phase [27].

Organic acids in fruits are in lower concentration in comparison with cladodes; however, fruits, especially pulp, are characterized by high sugar content. Some authors [9] studied the concentration of glucose and fructose in fruits and their results show that glucose predominates over fructose in both fruit peel and pulp (123 g L<sup>−</sup><sup>1</sup> of glucose and 91 g L<sup>−</sup><sup>1</sup> of fructose in pulp versus 91 g L<sup>−</sup><sup>1</sup> of glucose and 52 g L<sup>−</sup><sup>1</sup> of fructose in fruit peel). However, other studies [28] show that glucose, fructose and sucrose concentration is higher in fruit peel than in pulp. These results indicate that concentration of sugars may depend on the cultivars.

Sugar concentration in prickly pear fruit makes it a good source of energy and a natural source of sweetness for food preparations. Besides, fructose contributes to the typical sweet taste of this fruit, due to its high wetness compared with glucose and sucrose [29].
