Effect of pH on the chemical stability of carotenoids in juice

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 %.