The content of calcium oxalate and calcium in powder from Opuntia ficus-indica (nopal pad) was investigated. Atomic absorption spectrometry was used to evaluate the calcium and calcium oxalate contents. The results suggest a positive relationship between calcium content and the maturity stages of nopal pads. For pads that were from 40 to 135 days old, the range was from 17.9 to 30.7 mg/g dry matter: the older the nopal, the higher the calcium content. The oxalate content also shows an apparent cyclic tendency during the period of study; nopal pads aged from 40 to 135 days of maturity showed an oxalate range from 11.5 to 4.3 mg/g dry matter. Scanning electron microscopy demonstrated that oxalates were present in the samples, and that they were located mainly in the parenchyma tissue, with druse morphology. Energy-dispersive X-ray spectroscopy analysis confirmed their composition as calcium oxalates. The X-ray analysis showed that there were different contents of calcium oxalates at different phases of maturity. This result is in agreement with data obtained by using atomic absorption spectroscopy. In addition, the molar ratio between oxalate and calcium in all states of maturity was lower than 1, with a range from 0.348 to 0.067. This finding therefore suggests that the bioavailability of this mineral has not been compromised. The results indicate that nopal powder from more mature nopal pads can be a natural source of calcium that might be included in human diets.
The accumulation of calcium shows that there is a positive relationship with the time of maturity. When the maturity time increases, the concentration of calcium increases as well, and this is an important finding suggesting that the nopal pads in advance maturity stages can be an important sources of calcium. The accumulation of oxalates depends partially on the maturity of the nopal pads, but shows a complex relationship in which environ- mental factors, including climatic conditions, soil type, needs of the plant and variety, probably play an important role. However, the empirical equations obtained can help to estimate the best time for cutting the nopal pads, namely when they have the highest concentration of calcium as well as the lowest oxalates.