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The popularity of white asparagus (Asparagus officinalis L.) spears is mainly due to its unique flavour and its high concentration of health promoting compounds. Physical elicitors that activate plant defence systems are known to cause an increase of antioxidant compounds in plant tissue, such as phenolic compounds, and their corresponding enzymes (e.g. phenylalanine ammonia- lyase, [PAL, EC 4.3.1.5], peroxidase [POD, EC 1.11.1.7]). The objective of this work was to evaluate the impact of UV-B on PAL and POD kinetics, as well as to characterize the resulting changes in flavonoid composition in the different morphological sections (apical and basal) of harvested asparagus spears. For this purpose, spears of the cultivar ‘Gijnlim’ were irradiated with either 0.54 kJ m− 2 or 1.08 kJ m− 2 of UV-B. In order to follow the temporal dynamics of the UV-B treatments, analysis were performed after 2 h and 22 h of adaptation. Results revealed changes in the polyphenol composition, and in the activity of related enzymes (PAL, POD), in the different morphological parts of asparagus spears as mediated by UV-B dosage at the different adaptation times. The flavonol quercetin-4′-O-monoglucoside showed the most pronounced UV-B response because its concentration increased with increasing UV-B dosage. Furthermore, POD activity of both segments tended to increase with higher UV-B dosage. In contrast, PAL activity in the base segment tended to decrease with higher UV-B irradiation, while PAL activity in tips remained unaffected. However, PAL and POD activities were mainly influenced by the adaptation time. While PAL decreased with the duration of adaptation time, POD showed an opposite behaviour. The presented results revealed the complexity of the antioxidant defence system of asparagus, which is discussed in detail.

The effects of direct-electric-current (DC) on secondary plant compounds and antioxidant activity in harvested tomato fruits were investigated. A new technology was developed to supply DC to the tomatoes. Different intensities of DC (100–500 mA) with varied application times (15–60 min) were applied during postharvest. Almost all DC treatments significantly affected the secondary metabolism, resulting in an accumulation of carotenoids, phenolic compounds, and antioxidant activity in tomatoes. In the present study, optimal DC treatments were found in order to obtain the highest content of all investigated secondary plant compounds. After an adaptation time (AT) of 2 h, the maximum contents of lycopene (122.4%), s-carotene (140.4%), total phenol (120.0%), and antioxidant activity (126.5%) were attained with a DC treatment of 500 mA for 15 min. The results changed after an adaptation time of 24 h (AT 24 h), where the highest contents of lycopene (128.7%), s-carotene (129.6%), total phenol (113.6%), and antioxidant activity (120.9%) were obtained using the same DC treatment (500 mA), but with an application time of 30 min. Therefore, the application of DC in harvested tomato fruits may be appropriate to improve the health-promoting properties of tomatoes.

There are many reports on the potential consequences of UV-B radiation on plants, but there is a rather limited understanding of the effect on secondary plant metabolites, e.g. phenolic compounds and volatiles, at all. The popularity of highbush blueberries (Vaccinium corymbosum L.) is mainly due to its unique flavour and its high content of bioactive compounds, i.e. phenolic compounds. However, information on UV-B elicitor mediated changes on secondary plant metabolites on blueberries is scanty. In the present study, blueberry fruits were harvested and exposed to UV-B radiation with different dosage and adaptation times. With regard to volatile secondary metabolites, C6-aldehydes, terpenes and ketones, an increase of the relative peak area was observed after both UV-B treatments (0.075 and 0.15 Wh/m2 = low [L] and high [H] dosage, respectively). Furthermore, there was a strong influence of the adaptation time. Increasing relative peak areas were determined already after a short adaptation time (2 h) at both, low and high UV-B dosages, but after 24 h adaptation time relative peak areas decreased significantly. However, alcoholic compounds, as degradation products of aldehydes, showed opposite results. In contrast, the non-volatile phenolic compounds revealed a continuously increase with UV-B intensity.

The phenolic profile and quantitative composition of blueberries as well as the corresponding antioxidant activity of blueberries is well documented. Unfortunately, little is reported on the development of phenolic compounds and antioxidant activity during fruit maturation and ripening. In the present study, the total phenolic content and main phenolic compounds of four highbush blueberry cultivars (Vaccinium corymbosum L.) were analyzed at five stages of maturation and ripening. Antioxidant activity was screened with electron spin resonance spectrometry and trolox equivalent antioxidant capacity (TEAC) assay. An adequate picture of phenolic compounds developed during maturation and ripening was determined using HPLC-DAD. Anthocyanins of all varieties increased during successive harvest stages; meanwhile flavonols and hydroxycinnamic acids decreased from unripe green to ripe blue stage of berry ripening. Blueberry antioxidant activity, as well as total phenolic content tended to decrease during ripening.