Impact of frozen storage on polyacetylene content, texture and colour in carrots disks

https://doi.org/10.1016/j.jfoodeng.2011.09.003Get rights and content

Abstract

This study investigated the effect of freezing method (slow or blast freezing) with or without blanching during storage at −20 °C on the levels of three polyacetylenes, falcarinol (FaOH), falcarindiol (FaDOH), falcarindiol-3-acetate (FaDOAc) in carrot disks. The quality of the carrot disks was also assessed using instrumental texture and colour measurements. Blast frozen carrot disks retained higher amounts of polyacetylenes compared to their slow frozen counterparts. Whilst the levels of retention of total polyacetylenes was higher in unblanched than blanched disks prior to freezing there was a sharp decrease in the levels of polyacetylenes in unblanched frozen carrots during the storage period for 60 days at −20 °C. FaDOH was observed to be the most susceptible to degradation during frozen storage of unblanched carrot disks, followed by FaOH and FaDOAc. The changes in the level of polyacetylenes during storage were adequately described by using Weibull model. The texture and colour were also found to decrease during frozen storage compared to fresh carrots.

Highlights

► Quick freezing process enhances polyacetylenes retention rates in carrot. ► Blanching prior to freezing aids in retaining polyacetylenes during frozen storage. ► Weibull modelling is useful in demonstrating changes of polyacetylenes in carrot.

Introduction

Refrigeration and freezing of vegetables and its products is of huge economic importance for both unprocessed and processed foods (Campañone et al., 2002). In fact freezing is one of the most accepted and widely used food preservation techniques for long term preservation of food and helps in retention of key quality parameters such as sensory attributes and nutritive properties of food products over long storage periods (Fennema, 1977, Oliveira and Oliveira, 1999). Freezing retards chemical and biochemical reactions, microbial growth, water evaporation or any other process that may reduce quality or product shelf-life (Campañone et al., 2002, George, 1993). However, freezing can reduce the loss of some bioactive compounds as demonstrated for some better known bioactive compounds such as ascorbic acid, carotenoids etc. For example, Favell (1998) reported a decrease in ascorbic acid due to the frozen storage of carrots for 12 months. Several studies reported the decrease in ascorbic acid (Howard et al., 1999), and carotenoids (Howard et al., 1999, Park, 1987) due to freezing of carrots.

Pre-treatments such as minimal processing, freezing may act as stress upon the plant matrices. Hence, can affect the metabolic activity in the plant cell and induce changes in the content and profile of polyacetylenes in the frozen samples (Koidis et al., 2011). The effect of freezing methods (slow freezing, blast freezing) on polyacetylene has not been reported to date though some studies on freezing effect on polyacetylene was conducted by Hansen et al. (2003) and Kidmose et al. (2004). Polyacetylenes are phytoalexines, i.e. compounds synthesized by the plants in response to stress, such as fungal infections, water stress etc. Lund and White (1990) demonstrated that water stress had profound effects on the total polyacetylene content of carrots resulting in reduced levels of the three major polyacetylenes (FaOH, FaDOH, FaDOAc) and formation of other minor polyacetylenic compounds. The nutritional significance of polyacetylenes lies in their cytotoxicity against colon cancer cells, human gastric adenocarcinoma cells, leukemia (L-1210), mouse melanoma (B-16), and mouse fibroblast-derived tumor cells. (Christensen and Brandt, 2006, Kobaek-Larsen et al., 2005).

In addition, the sensorial quality of frozen food can be influenced by the freezing method employed. For example, slow freezing may cause extensive structural damage due to the formation of larger ice crystals (George, 1993). Whereas, rapid freezing using cryogens induces cracking owing to the initial decrease of volume due to cooling and the subsequent increase in volume due to freezing (Kalichevsky et al., 1995). Subsequent thawing of slow frozen food products retards re-entry of extracellular ice into the cells and may cause extensive drip loss and texture softening (Cheftel et al., 2000). Whilst the retention of bioactive compounds during frozen storage is of paramount importance there is little point in retaining these compounds if the eating quality of foodstuff is not also maintained. Therefore the aim of the present study was to determine the effect of slow freezing and blast freezing with and without blanching, followed by frozen storage on the polyacetylene content and quality (colour and texture) of carrot disks.

Section snippets

Chemicals

Acetonitrile (ACN), ethyl acetate and water were of HPLC grade (99.9%) and obtained from Fischer’s (Dublin, Ireland). Diatomaceous earth was obtained from Dionex (Dionex Camberley, Surrey, UK).

Sample preparation

Fresh carrots (Daucus carota, cv. Nazri) were obtained from a local wholesaler (Donelly’s, Dublin) and stored at 4 °C for a minimum of 24 h prior to processing. Carrots with no visible damage and of root size 15.5 ± 1.5 cm for carrots were selected for the processing. After hand peeling, carrots were sliced

Effect of freezing rate on the levels of polyacetylenes

Mean levels of the 3 major polyacetylenes [falcarinol (FaOH), falcarindiol (FaDOH), and falcarindiol-3-acetate (FaDOAc)] in the fresh carrots disks were found to be 245.2, 100.5 and 449.8 μg/g dry basis respectively (Table 1). The levels reported were within the range of those reported by Zidorn et al. (2005) for FaDOH and FaDOAC. However, the level of falcarinol was slightly higher compared to that reported by Zidorn et al. (2005), but in the range of that reported by Kreutzmann et al. (2008).

Conclusion

This study shows that both blanching and freezing methods had significant impact on polyacetylenes content in carrot. During frozen storage polyacetylenes were found to decrease with storage time irrespective of freezing method employed however the losses in the samples which were blanched and stored were significantly reduced. Among freezing treatments, blast freezing resulted in higher retention of polyacetylenes and other quality factors such as colour and texture compared to slow freezing.

Acknowledgement

This project is funded under the Food Institutional Research Measure (FIRM, ref. 363 06/RTD/AFRC/518) by the Irish Department of Agriculture, Food and Fisheries (DAFF).

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