Apples are one of the most widely consumed fruits and are a good source of phytochemicals. Epidemiological studies have showed that the consumption of apples can reduce risk of some cancers, cardiovascular disease, asthma, and diabetes. One way to increase fresh fruit consumption is processing fruit into fresh-cut product. However, fresh-cut processing induces mechanical stresses to fruit tissues that result in tissue enzymatic browning, which makes the product unsuitable for the consumer.
Different treatments have been applied to extend the shelf life of fresh-cut apples, mainly dipping in solutions of a wide range of anti-browning chemical agents. Among these, calcium ascorbate (CaAsc) has been found to be the most effective anti-browning agent and can be marketed as a minimal chemical input.
Scientists from University of Cartagena (Spain), The New Zealand Institute for Plant & Food Research Ltd, and University of Tasmania (Australia) have investigated whether the hot water treatment (HWT) could be recommended as a method in combination with CaAsc dips to help maintain the sensory quality of fresh-cut apples and to extend the shelf-life.
For the study, fresh-cut 'Braeburn' apple slices were dipped into cold water (4°C for 2 min) or hot water (HWT, 48°C or 55°C for 2 min) followed by dips into 0 or 6% w/v aqueous calcium ascorbate (CaAsc, 2 min, 0°C) and stored in air up to 28 d at 4°C. Microbial counts, changes in browning and sensory acceptance were measured to indicate changes in quality.
CaAsc dips had a strong impact reducing the browning through increasing the flesh luminosity and hue angle. The use of 6% CaAsc in fresh-cut apples extended the shelf-life from less than 7 days to 14 days. Immediately after CaAsc treatment, ascorbic acid content (AA) content was 5 fold higher (0.25–1.25 g/kg) than those not dipped into CaAsc. However, the combination of HWT treatments and CaAsc dips led to seven fold increased levels of AA inside the apple tissue (0.25–1.85 g/kg) and consequently increased the antioxidant activity.
HWT did not increase the AA content when not combined with CaAsc dips. The HWT followed by CaAsc dip extended the shelf-life to 21 days compared to 14 days for samples not heated but only dipped into CaAsc.
Shelf life was ultimately limited by sensory quality. At day 28, total plate counts were reduced from 5.3 log cfu/g (untreated slices) to 4.6 log cfu/g in the 6% CaAsc dips and further to 3.9 log cfu/g with the combination of HWT and CaAsc dip.
Changes in the content of phenolic compounds with time, HWT and CaAsc dip were generally not significant except for slightly increased quercetin and phloridzin levels and decreased p-coumaric and procyanidins over time.
Scientists conclude that the combination of HWT at 48°C for 2 min followed by 6% CaAsc dip was the best for preserving the eating quality of apple slices and extending the shelf life from 14 d to up to 21 d at 4°C. HWT for 2 min at 48°C was preferable to 55°C with no significant benefit of the higher temperature that would also have increased energy cost and could induce a superficial damage in the apple flesh.
Results of the study are available online from August 15th at:
http://www.sciencedirect.com/science/article/pii/S0925521415300508
Source: Encarna Aguayo, Cecilia Requejo-Jackman, Roger Stanley, Allan Woolf, 'Hot water treatment in combination with calcium ascorbate dips increases bioactive compounds and helps to maintain fresh-cut apple quality', 2015, Postharvest Biology and Technology, Vol. 110, pages 158–165.
Contacts:
Encarna Aguayo
Postharvest and Refrigeration Group. Department of Food Engineering, Technical University of Cartagena
Paseo Alfonso XIII 48
30203 Cartagena
Murcia, Spain
Email: encarna.aguayo@upct.es