Fechas

It is also necessary to consider how well the phytochemicals are retained during storage and whether the processing method or storage conditions influence this at all. Some of the researchers discussed previously extended their investigations of blueberry juice manufacture to include a shelf life study.

Brownmiller et al (2008) evaluated storage effects on blueberry juice stored in 6 oz (168 ml) glass bottles at 25°C over a six month period. Measurements at 1, 3 and 6 months showed that monomeric anthocyanin levels decreased significantly. This degradation was linear over the storage period, and the clarified juice had consistently lower levels of anthocyanins than the non-clarified juice. After six months non-clarified juice contained 23% of the original anthocyanin content and clarified juice contained 15%. However, the antioxidant capacity was practically competely retained in the stored samples and polymeric colour values increased. Therefore, Brownmiller et al (2008) assumed that the monomeric anthocyanin loss was due to the polymerisation of anthocyanins which were still able to contribute to the antioxidant capacity. An additional conference paper by Brownmiller et al (2009) reported that during storage at 25°C better retention was seen for flavonols and chlorogenic acid as compared with anthocyanins and procyanidins.

Srivastava et al (2007) stored samples of two blueberry cultivars in 30 ml glass bottles for sixty days at four different temperatures (-20, 6, 23, and 35°C). At the end of the storage period samples stored in frozen and chilled temperatures (-20 and 6°C) had consistently higher antioxidant capacities and anthocyanin and phenolic levels than samples stored in ambient and

higher temperatures (23 and 35°C). For example at day zero juice produced from the Tifblue cultivar was reported to have an antioxidant capacity of 17,000 μmol/L, an anthocyanin content of 340 mg/L and a phenolic content of 900 mg/L. At the end of the storage period at 35°C these levels dropped to 13,500 μmol/L, 180 mg/L and 400 mg/L whereas juice stored at -20°C had much higher levels: 16,800 μmol/L, 270 mg/L and 650 mg/L. As can be seen from these data during storage the antioxidant activity did not decrease proportionally as much as anthocyanin or phenolic levels, again supporting the suggestion that products from anthocyanin degradation are in fact polymerised anthocyanins. The researchers also suggest that oxidation could have contributed to some of this degradation on the basis of the paper by Kalt et al (2000).

The stability of individual anthocyanin compounds during storage reported by Srivastava et al (2007) was consistent with chemical structure. Malvidin and petunidin glycosides which have a single hydroxyl group and two methoxy groups on their B phenolic ring, making them less reactive, appeared to be the most stable upon storage. Conversely the delphinidin glycosides with three hydroxyl groups were seen to be the most unstable. Degradation of phenolic acids and flavonoids were also reported by Srivastava et al (2007). Of the eight flavonoids and phenolic acids measured only the catechins were present in amounts comparable to the anthocyanins at 350 mg/kg. However, their levels had decreased by 85 – 90% at the end of the storage period for all storage temperatures. The absolute decrease of the other compounds was much less significant as the concentrations of these compounds were ten fold lower initially. However, quercetin and ellagic acid did appear to benefit from a low storage temperature. The authors concluded that there was a significant advantage in storing the juice at low temperatures.

Vitamin C Retention

Vitamin C is one of the most abundant antioxidants in blueberries aside from the phenolic phytochemicals. Harb et al (2010) reported that fresh blueberries contained 6.3 mg/g of vitamin C. Although it is documented to contribute relatively insignificantly to the antioxidant capacity of blueberries (Prior et al, 1998) it is still a recognised nutrient and therefore potentially important to maintain, particularly from a marketing perspective. Vitamin C is readily oxidised, especially in the presence of metal ions such as copper and iron. Heat and light

accelerate the process, while additional factors such as oxygen concentration, pH and water activity strongly influence the rate of the reaction (Fennema, 1996). Fennema (1996) outlines several possible mechanisms of vitamin degradation but states that vitamin C activity is compromised in all resulting degradation compounds.

Harb et al (2010) evaluated changes in vitamin C in fresh blueberries during storage. They reported that a dramatic loss occurred under all storage conditions, which was attributed to changes in gene expression between freshly harvested and stored fruit. Degradation ranged between 60 and 40% after 3 weeks chilled storage for the various treatments. The least degradation of vitamin C was observed with low O2 and high CO2 level (up to 18%). Conversely

Kalt et al (1999) found no significant change in vitamin C content for highbush blueberries stored at 0, 10, 20, and 30°C for up to 8 days.

From a processing perspective Kalt et al (2005) reported that because of its high water solubility the largest losses of vitamin C generally occur in processes involving water. For example blanching was shown to decrease vitamin C levels by 50% in some fresh fruits and vegetables, although the remaining vitamin C was then more stable upon subsequent storage. They suggested that, again, the heat treatment could inactivate native plant enzymes, such as ascorbate oxidase which is capable of breaking down vitamin C. The high water solubility of vitamin C would also mean that leaching of water during thawing could be particularly detrimental to levels in the juice. In terms of the effect of heat, Villota & Hawkes (2007) reported that although an increasing temperature is generally shown to increase the breakdown of vitamin C, sub-freezing temperatures may also accelerate degradation. However, unfortunately none of the blueberry juice experiments discussed here included a measurement of vitamin C. Therefore it is difficult to evaluate the potential overall effect of juice processing. However, vitamin C is easily added back into processed foods in order to meet consumer expectations.