The combined effect of ultrasound and enzymatic treatment on the nanostructure, carotenoid retention and sensory properties of ready-to-eat carrot chips

https://doi.org/10.1016/j.lwt.2016.11.085Get rights and content

Highlights

  • Both ultrasound and pectin lyase influenced the nanostructure of carrot tissue.

  • Ultrasound altered the uronic acid content in a different way to enzyme.

  • Ultrasound impacted the shape of carotenoid agglomeration in tissue substructure.

  • Both ultrasound and enzyme can improve the colour of dried carrot.

  • Applying both ultrasound and pectin lyase gave the most preferred vivid colour.

Abstract

Fat-free crispy carrot snacks, could be an interesting proposal for consumers eager to increase their vegetable consumption, if the carotenoids naturally occurring in carrots could be retained during the drying processes used in their production. In an attempt to enhance carotenoid retention during the snacks production, ultrasound (US) and enzymatic treatment (EN) were applied as pre-treatment operations. Carrot slices were first sonicated for 20 min (25 kHz, 0.4 W/cm2), and then soaked in a light sucrose solution (110 g/L) with or without pectin lyase enzyme (2 g/L), and finally were conventionally dried to a crispy state. Final product quality was assessed, paying attention to the carotenoid content, and changes in molecular pectin structure. Both the applied pre-treatments caused transformations of diluted alkali soluble pectin fraction in the form of shortening the skeleton length. Moreover, a decrease in the diameter of pectin polymers was noticed for samples treated with ultrasound, whereas enzyme action resulted opposite alteration. The US turned out to be a less effective factor in enhancing carotenoid retention of dried carrot chips than the EN; however, their simultaneous application not only had a positive effect on carotenoid retention, but also led to a higher sensory appreciation of the product colour.

Introduction

Ready-to-eat dried fruit and vegetable chips have roused the interest of food technologists for many years, as the conjunction of attractive sensory properties with a high concentration of phytonutrients in a dried tissue matrix, provided a unique opportunity to offer a product with functional properties in an appealing form (Vinson et al., 2005, Konopacka et al., 2010b, Dueik et al., 2013). Among different vegetable species, carrots are considered to be one of the most promising raw materials to exploit in the production of crispy chips (Skrede et al., 1997, Sulaeman et al., 2001, Albertos et al., 2016). Due to their naturally high carotenoid and dietary fibre content, carrot snacks could be an interesting proposal for consumers eager to increase their vegetable consumption. Generally two kinds of technology can be used to transform carrot slices into crunchy snacks: frying in oil in a reduced pressure environment (Sulaeman et al., 2001, Albertos et al., 2016) and drying, either by means of a hot air drying method (Plocharski and Konopacka, 2002, Dueik et al., 2013) or a microwave-vacuum technique (Cui et al., 2004, Lin et al., 1998 .).

In all the above cases the carrot slices are subjected to thermal processes, leading to a strong dehydration of the tissue matrix, which may cause major carotenoid degradation, mainly due to β-carotene isomerization and oxidation reactions that can substantially decrease biological activity in the final products (Hiranvarachat et al., 2011, Dueik et al., 2013). The current trend for a healthy diet, eliminating any fried salty snacks, induced the efforts of researchers towards developing and improving the technology of dried vegetable snack production (Konopacka et al., 2010b; Hiranvarachat et al., 2011). The hot air drying method, relatively cheap and widely available without excessive cost investments, easily leads to carrot quality degradation, including colour, aroma, as well as flavour losses (Dueik et al., 2013). These alterations are not accepted by consumers, who are not willing to sacrifice the sensory attributes in their quest for a healthy diet (Sijtsema, Jesionkowska, Symoneaux, Konopacka, & Snoek, 2012), thus many technological proposals have been put forward, to enable the production of dried carrot chips with visually pleasing attributes, whilst retaining the required high carotenoid content (Cui et al., 2004). Among the different solutions, the most popular are blanching and different variants of osmotic treatment, all which lead to an enhancement of taste and an increase in texture crispness (Górnicki and Kaleta, 2007, Hiranvarachat et al., 2011, Konopacka et al., 2010b). One of the possible methods to enhance both the required aspects was described by Konopacka, Dyki, and Seroczyńska (2010a), where a short treatment of raw carrot in an enzyme preparation of pectolytic activity substantially diminished carotenoid losses, not only during hot air drying, but also during subsequent storage. The same research team suggested that an enzymatic pre-treatment with a preparation of pectin lyase activity can be recommended for other carotene-rich vegetables, and especially for winter squash (Konopacka et al., 2010b). Recent literature describing the possible positive effect of ultrasound energy on carotenoid retention during plant material drying, could also point to a new technological option for the production of dried carrot snacks with substantially improved bioactivity (Rawson, Tiwari, Tuohy, O'Donnell, & Brunton, 2011). It was proven that optimized ultrasound treatment of carrot slices (25 °C, 3–10 min, 20 kHz, 0.39–0.95 W/ml) could be used instead of traditional blanching (85 °C, 3 min) (Gamboa-Santos et al., 2013). By using the mentioned conditions, the authors were able to produce dried carrot of higher carotenoid retention (including falcarinol of anticancerogenic properties) than in the case of those subjected to regular blanching. Moreover, the ultrasound treatment did not influence the sensory profile of the final product.

Taking into consideration the above mentioned positive effect of ultrasound treatment on the bioactivity of carrot tissue, as well as recent literature reports promoting acoustic energy as a feasible method for accelerating fruit dehydration processes (Siucińska & Konopacka, 2014), an investigation was undertaken to assess the usefulness of ultrasound treatment (US) as a tool for enhancing the quality of dried carrot chips. It was decided that this method be compared with the effectiveness of enzymatic treatment (EN), previously described by Konopacka (2006). The effects of both pre-treatment methods (US and EN), taking into account various aspects of the quality characteristics of dried carrot snacks, including nanostructure, carotenoid content, as well as sensory perception of colour attractiveness were considered. Special attention was given to the analysis of possible synergy within the compared technological operations.

Section snippets

Raw material and processing

The carrot variety ‘Belgrado’ was obtained in March 2015 from the storage facility of a commercial farm dealing with vegetable production, located in central Poland. Up to the processing date (last week of April), the raw material was stored in experimental cold storage (1 °C, 95% RH). On the day of processing, the carrot roots were warmed up to room temperature, washed, peeled and cut into 2 mm slices. Prior to ultrasound treatment, freshly cut slices were immediately placed in polypropylene

Uronic acid content in pectin fractions

In Table 2 the uronic acid content in particular pectin fractions isolated from carrot chips is presented. Both ultrasound and pectin lyase, applied as the slices pre-treatment before drying, significantly changed the uronic acid distribution between the fractions. The uronic acid content in particular pectin fractions of the untreated dried carrot chips was as follows: WSP – 40.4, CSP – 129.5, DASP – 113.8 μg/mg AIR. The KOH extracted fractions and insoluble pectin fraction contained a total

Conclusions

  • 1.

    Both US and EN pre-treatment of carrot slices caused transformations of the diluted alkali soluble pectin fraction in the form of shortening the skeleton length. Moreover, a decrease in the diameter of pectin polymers was noticed for samples treated with ultrasound, whereas enzyme action resulted in a substantial increase of the diameter of polymers.

  • 2.

    The results of cytological studies of chromoplasts of the dried carrot slices indicate that in the range of applied ultrasound energy, the acoustic

Acknowledgements

This work had been prepared within the ‘BIOSUSZ’ PBS Project financed by the National Centre for Research and Development (PBS1/A8/13/2012).

References (27)

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