Low colour stability in strawberry nectar during storage led to the study of polyphenols and polysaccharides in fruit and their potential role in colour after processing in relation with cultivar, ripening stage and harvest time. Cultivar had a significant effect on both polyphenols and cell wall polysaccharides. Anthocyanins and proanthocyanidins (PACs), including procyanidin and propelargonidin, were the main polyphenols in strawberry. Ripening and late harvest increased anthocyanin contents and reduced PACs contents whereas changes in other polyphenols were minor. Cell walls were mainly composed of cellulose, highly methylated pectins rich in arabinan and galactan side chains, and xyloglucan. Ripening and late harvest increased the loss of pectin side chains, yet these effects were cultivar-dependent. Overripe samples from late harvest exhibited greater colour stability after nectar production. Strawberries with a high content in anthocyanins, low content of PACs, and with pectins rich in galactose, were correlated with nectars of higher colour stability.

Strawberry nectars have increased colour stability when produced from overripe, darker, and redder strawberries, with a high anthocyanin concentration. The post-harvest storage of strawberries has been shown to develop these properties. Nectars are frequently produced from strawberries rejected for fresh sale due to poor colour, which are insufficiently ripe to produce colour-stable nectars. This study investigated post-harvest anthocyanin development in strawberries to improve the colour and colour stability of nectars, which is the first time these developments were studied for beverage production. Strawberries at five ripeness stages were stored at 20 °C for 1 and 2 days prior to nectar production. The anthocyanin content of nectars was determined by a pH-differential method, and the colour stability was tracked for 12 weeks using a consumer Acceptance Factor, derived from CIELAB colour components. The anthocyanin content and colour stability were highly correlated, and both were dependent on ripening, with larger increases observed in under-ripe strawberries, and small to no improvement in overripe samples. Stored partially coloured strawberries produced nectars with equivalent colour stability to non-stored strawberries of normal ripeness. This allowed strawberries that were previously unsuitable for both fresh sale and nectar production to be used as a feedstock for nectar production, reducing food waste.

Among fruits, the apple is unique for producing large amounts of the dihydrochalcone phloridzin, which, together with phloretin, its aglycone, is valuable to the pharmaceutical and food industries for its antidiabetic, antioxidant, and anticarcinogenic properties, as well as its use as a sweetener. We analysed the phloridzin concentration, total phenolic content, and antioxidant activity in the peel, flesh, seeds, juice, and pomace of 13 international and local apple varieties. In the unprocessed fruit, the seeds had the highest phloridzin content, while the highest total phenolic contents were mostly found in the peel. In processed samples, phloridzin and the total phenolic compounds especially were higher mostly in juice than in pomace. Moreover, the total phenolic content was much higher than the phloridzin content. Juice showed the highest antioxidant activity, followed by the peel and flesh. Across all samples, antioxidant activity did not directly correlate with phloridzin concentrations, suggesting that the antioxidant activity ascribed to phloridzin may need re-evaluation. In the Ferric Reducing Antioxidant Power (FRAP) assay, phloridzin only showed antioxidant activity at high concentrations when compared to its aglycone, phloretin. Considering the large amounts of apple juice produced by the juice industry, residual pomace is a promising source of phloridzin. For technical use, processing this phloridzin to phloretin would be advantageous.

The browning of fruit juices and nectars is a common issue in the beverage industry and is a particular problem in strawberry nectars, where it significantly reduces the shelf-life. Polyphenol oxidases (PPOs), which are multicopper enzymes responsible for the oxidation of a wide plethora of polyphenols in plants, have been widely assumed to be involved in the enzymatic browning of strawberry nectar. To investigate the possible involvement of PPOs, the substrate specificity of four recombinant PPOs and their gene expression pattern in 10 cultivars of Fragaria × ananassa at five ripening stages were determined. This allowed us to obtain adequate amounts of enzymes to study them independently and without interfering matrix effects. All four PPOs possess monophenolase activity, which was particularly high for PPO4. PPO3 did not show sufficient stability for the kinetic studies. The other three showed a high preference for the flavan 3-ol catechin with a 2-fold higher catalytic efficiency compared to dopamine for PPO1 and PPO2. At a neutral pH, they also showed activity with cyanidin but not with pelargonidin, which is the prevalent anthocyanidin type in strawberry. The enzymes showed a high affinity but only low turnover rates for the dihydrochalcone phloretin, resulting in an inhibitory effect that was strong enough to extend the shelf-life of the strawberry nectar by one week if phloretin was added in high concentrations (600 µM). PPO1 and PPO2 were prevalently expressed in all fruit stages. The gene expression of the four PPOs did not correlate with the color stability of the nectars of the 10 varieties and also showed a random expression pattern during fruit development. The limited activity in acidic conditions and the low substrate specificity for pelargonidin does not point to a crucial role for PPOs in the browning of strawberry nectar, but the high catalytic efficiency with catechin as a substrate could contribute to anthocyanin degradation via mechanisms such as copolymerization.