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        Effect of pH on the chemical stability of carotenoids in juice
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        Effect of pH on the chemical stability of carotenoids in juice
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Carotenoids are abundantly present in fruits and vegetables and are considered beneficial for human health. Although carotenoids are inherently unstable and degradation occurs during food preparation and storage, bioavailability of β-carotene from processed foods, such as carrot juice, can be up to 70 % higher as from raw carrots( 1 ). Thus, well-chosen food processing techniques can increase health benefits of fruit and vegetables. During juice and smoothie preparation, organic acids are released from sheered plant cells( 2 ). Carotenoids are sensitive to environmental factors such as light, temperature, oxygen as well to acidic conditions( 3 ). This study investigated the effect of pH on carotenoid stability in juice.

Freshly prepared carrot juice (pH 6·07) was adjusted to pH 8, 7, 6, 5, 4 and 3 using citric acid or NaOH and stored at 4 °C for 4 days. Juice samples were freeze-dried and total carotenoids extracted and quantified as previously described( 4 ). Neutral and slightly basic conditions (pH 8 and 7) reduced total carotenoid content by 26 % (p < 0·05) and acidic conditions (pH 6, 5, 4 and 3) increased the measured total carotenoid content in carrot juice by 18 %, 22 %, 27 % and 22 % respectively (p < 0·05) (Figure 1).

Fig. 1. Carotenoid content in refrigerated and pH adjusted fresh carrot juice.

When fresh fruit and vegetable juices (carrot, lemon, orange and apple) were blended to attain a pH of 3·15, 3·98 and 4·95, no carotenoid degradation was observed after 8 day storage at 4 °C. A pH of 3·15 however resulted in a 16 % reduction of total carotenoids (p < 0·05) (Figure 2).

Fig. 2. Carotenoid content in fresh and refrigerated fruit & carrot juice.

These findings indicate that carotenoids in fresh juices were sensitive to pH. However, observed carotenoid degradation in fruit and vegetable juice was small (16–25 %). The apparent increase of total carotenoid content (~20 %) in acidified carrot juice may be due to enhanced solubility of crystallized carotenoids present in the vacuoles of plant material( 5 ). While our findings confirm the notion that carotenoids are pH sensitive all observed variation were less than 25 %.

1. Skinner, M & Hunter, D. (2013).
2. Meléndez-Martínez, A, Escudero-Gilete, M, Vicario, I, et al. (2009) 3, Eur Food Res Technol, 527532.
3. Shi, J, Xue, S, Wang, B, et al. (2015) 2, LWT - Food Sci and Tech, 9991008.
4. Tan, KW, Graf, BA, Mitra, S, et al. (2015) 7, Plos One, e0133445.
5. Ahn, J & Kim, K. (2015), Enzyme and Microbial Tech, 77:2937.