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The effects of the different water sources used for irrigation of guava crops on the nutritional properties of guava crops are shown in Table 4. Guava crops harvested from the farm that uses treated waste water for irrigation of guava crops were observed to contain higher ascorbic acid content, total polyphenols content and total antioxidant activity. Ascorbic acid content was observed to be higher by up to 21% in the guava crops. Ascorbic acid (vitamin C) is one of the most abundant antioxidants in plants and is a cofactor of many plant dioxygenases, and hence is an important factor in grading the quality of fruits (Mahieddine et al. 2011). Guavas with higher ascorbic acid content are therefore given higher preference. Besides a higher ascorbic acid content, guavas harvested from the farm irrigated with treated waste water were also observed to contain significantly higher total polyphenols and total antioxidant activity. The total polyphenol content was up to 12% higher, while total antioxidant activity was higher by up to 18%. There were no significant differences in the DPPH scavenging activity of the guavas irrigated with different water sources. Polyphenols are yet another attribute that is highly desired by consumers, which contributes to the quality of fruits. Polyphenols are known to possess biological properties and exhibit anticancer, antioxidant, antiviral and anti-inflammatory actions (Borchardt et al. 2008), whilst playing an important role in plants by acting as a chemical defence of plants against predators and in plant-plant interferences (Bhattacharya et al. 2010). As for the mineral content, lower calcium, magnesium and potassium contents were observed in guavas harvested from the farm that uses treated waste water for irrigation. This corresponds with the lower level of minerals in the treated waste water used for irrigation. Accordingly, calcium, magnesium and potassium contents were higher by up to 7%, 41% and 2% respectively in guavas irrigated with lake water. No significant differences were observed between the total nitrogen and phosphorus content of the guavas irrigated with different water sources.

Table 4

Effects of different water sources used for irrigation of guava crops on the nutritional properties of the guava fruit

 TWWLW
Ascorbic acid content (mg AA/100 g) 6.8 ± 0.31a 5.6 ± 0.21b 
Total polyphenol content (mg GAE/100 ml) 122.82 ± 3.01a 110.06 ± 2.74b 
DPPH scavenging activity (μg AAE/ml) 9.51 ± 0.04a 9.55 ± 0.02a 
Total antioxidant activity (mg AAE/g) 1240.06 ± 49.06a 1047.65 ± 40.14b 
Calcium (mg/g) 15.5 ± 0.1a 16.6 ± 0.2b 
Magnesium (mg/g) 12.8 ± 0.93a 18.1 ± 0.75b 
Potassium (mg/g) 177 ± 0.15a 180 ± 0.25b 
Total nitrogen (mg/g) 0.08 ± 0.01a 0.08 ± 0.01a 
Phosphorus (mg/g) 9.8 ± 0.23a 8.9 ± 0.18b 
 TWWLW
Ascorbic acid content (mg AA/100 g) 6.8 ± 0.31a 5.6 ± 0.21b 
Total polyphenol content (mg GAE/100 ml) 122.82 ± 3.01a 110.06 ± 2.74b 
DPPH scavenging activity (μg AAE/ml) 9.51 ± 0.04a 9.55 ± 0.02a 
Total antioxidant activity (mg AAE/g) 1240.06 ± 49.06a 1047.65 ± 40.14b 
Calcium (mg/g) 15.5 ± 0.1a 16.6 ± 0.2b 
Magnesium (mg/g) 12.8 ± 0.93a 18.1 ± 0.75b 
Potassium (mg/g) 177 ± 0.15a 180 ± 0.25b 
Total nitrogen (mg/g) 0.08 ± 0.01a 0.08 ± 0.01a 
Phosphorus (mg/g) 9.8 ± 0.23a 8.9 ± 0.18b 

Values followed by different letters within the same column are significantly different for each fruit (p < 0.05) (n = 30). TWW: treated waste water; LW: lake water.

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