Abstract
Asian white radish pieces were dried to < 10% moisture using a hot air drier, a heat pump drier and a freeze drier, and whole roots were partially dried by salting under pressure with sodium chloride. The level of the primary flavor compound, 4-methylthio-3-trans-butenyl isothiocyanate (MTBITC), decreased in all treatments. Freeze-dried white radish showed the lowest loss of about 15% MTBITC. Radish dried with hot air and heat pump driers showed increasing MTBITC loss with increasing drier temperature with a lower loss in the heat pump drier at equivalent temperatures with the rate of drying also faster with the heat pump drier. Osmotic dehydration with salt caused a substantial loss of MTBITC with the loss increasing with increasing salt concentration. Use of 5% salt resulted in the lowest loss of MTBITC while achieving an acceptable rate of water loss.
INTRODUCTION
Asian white radish, or daikon, (Raphanus sativus L.) is a popular root vegetable throughout Asia. It is processed into a range of fresh, dried, salted and pickled products and commands about 60% of the processed vegetable market in Japan where it is consumed daily.[Citation1,Citation2] The taste of raw white radish involves some irritation in the nasal cavity and a burning sensation on the tongue.[Citation3] The primary compound responsible for the characteristic sulphurous, pungent flavour and aroma has been identified as 4-methylthio-3-trans-butenyl isothiocyanate (MTBITC).[Citation4,Citation5] MTBITC is produced from the hydrolysis of 4-methylthio-3-trans-butenyl glucosinlolate by the enzyme myrosinase[Citation4] and occurs when cells in the root are disrupted, allowing the enzyme and substrate to mix.[Citation6] The drying of white radish in East Asia is traditionally performed at farm level by sun drying while reduction of water content through osmosis with high concentrations of chemical solutes is also practiced.[Citation7] Like most foods, the lack of control over ambient conditions results in variable quality for dried radish products and the use of mechanical driers is becoming more common. The only reported study of the mechanical drying of white radish was by Ahmed[Citation8] who examined drying rates in a hot air drier at 55–60°C. While osmotic drying of fruit and vegetables has been studied on various produce,[Citation9,Citation10] there are no reported studies on the effect of salting on the flavour of dried or salted white radish. MTBITC is a volatile compound and the concentration present could be expected to be affected by the drying process. This study examined the effect on the flavour compound of white radish that was dehydrated using different types of mechanical driers and by salting.
MATERIALS AND METHODS
The moisture content of Hoshiriso white radish roots was determined gravimetrically on 5 g samples that were dried to constant weight in a vacuum oven (Thermoline, Sydney) at 60°C and − 70 kPa. Roots were sliced into pieces (10 mm × 10 mm × 50 mm) with a mechanical slicer (Kroner, Germany). The moisture content of the radish pieces was reduced in a hot air drier; heat pump drier and freeze drier. The hot air drier was a 18 kW multiphase convection hot-air drier (GTD Engineering, Sydney) (internal dimensions of 46 × 81 × 79 cm) and was operated at 40, 50, 60, and 70°C. The heat pump drier was a 0.75 kW, single phase heat-pump drier (Greenhalgh, Caloundra Queensland) (internal dimensions 53 × 44 × 35 cm) and was operated at 35, 40, 45, and 50°C. Radish slices in both driers were weighed hourly and measurements continued until the product contained < 10% moisture. The freeze drier (FD3 Dynavac, Seven Hills, Sydney) (chamber dimensions of 40 mm × 40 mm × 700 mm) was operated at a pressure of 10 − 4 Pa with the condenser temperature at –45°C. Radish pieces were frozen with liquid nitrogen before placing in the drier and weight loss measurements were conducted every 24 h until the product contained < 10% moisture. The equilibrium operating temperature of the freeze drying chamber was 15°C. The data was statistically evaluated by analysis of variance and regression analysis.
For salting, a commercial procedure described by Watanabe[Citation7] was used. Whole radish roots were placed horizontally as a single layer in a plastic container (50 × 40 × 50 cm). Salt (commercial grade, Saxa, Sydney) was added at the rate of 0, 5, 15, and 25 g/100 g radish. Two cement blocks (45 × 25 × 10 cm) each weighing 13.5 kg were placed on top of the layer of roots exerting pressure evenly to assist salt distribution. The blocks were left in place for 72 h at a temperature of 20°C after which time roots were placed in fresh flowing water for 30 min to remove excess salt and the moisture content of the roots determined.
The MTBITC content of the radish was determined by the method developed by Okano, Asano and Ishii.[Citation5] Dried samples were placed in a blender (341b199 Waring, New Hartford CN), with the volume of water that had been lost during drying to ensure the same ratio of water to dry material was maintained as in the fresh sample. Juice from the reconstituted or fresh sample was squeezed by hand from the pulp. Hexane (5 ml) was added to a sample of juice (5 ml) and placed into a sealed 12 ml vial that was shaken vigorously for 40 s. on a Vortex mixer. The vial was centrifuged at 4000 rpm for 10 min, and the solvent fraction removed with a Pasteur pipette into a sealed vial and stored at − 20°C until analysis. An aliquot (2.5 μL) was injected into a flame ionisation gas chromatograph (Varian 3400, Walnut Creek CA) using a 2 meter stainless steel column of SE-30 (8%) on DCMS-WAKS, Chromosorb 100–120 (Alltech, Sydney). The column temperature was programmed for 1 min at 135°C then increased at 20°C/min to 180°C. Operating parameters were injector temperature 185°C, detector temperature 200°C, nitrogen carrier gas 30 mL/min, hydrogen flow rate 30 mL/min and air 200 mL/min. The retention time of MTBITC was about 4.2 minutes. MTBITC in the samples was quantified against a standard curve prepared with a working standard of MTBITC extracted from white radish and having a purity of 94% as determined by mass spectrometry.[Citation11]
RESULTS AND DISCUSSION
Effect of Mechanical Drying
The loss of MTBITC in white radish pieces dried to < 10% moisture content by hot air, heat pump and freeze driers is shown in . The heat pump drier resulted in a significant linear loss of MTBITC (y) in the dried radish as the temperature (x) increased from 35° to 50°C (y = − 2.4 x + 163; P < 0.05) with almost half the MTBITC lost at 50°C. The hot air drier also resulted in a significant linear loss (y = 1.5 x – 11; P < 0.05) of MTBITC between 40° and 70°C with > 90% of MTBITC lost at 70°C. At equivalent temperatures, there was a significantly higher (P < 0.05) loss of MTBITC in radish dried by hot air than with the heat pump. In addition, the drying time in the heat pump drier was shorter than in the hot air drier, being 9 h shorter when both driers were operated at 40°C and 3 h shorter at 50°C. The equilibrium temperature in the freeze drier chamber was 15°C and resulted in the amount of MTBITC lost during freeze drying being about 15%, which was significantly less (P < 0.01) than all temperature regimes used for the heat pump and hot air driers. However, the time to freeze dry the radish slices was much longer at > 48 hours, Freeze drying is not considered to be a commercially feasible option for a relatively low cost raw material such as white radish due to the slow drying time as well as the relatively the high cost of freeze drying. Heat pump drying therefore appears to be the preferred drying due to the dual advantage of a better retention of MTBITC and a shorter drying time than hot air drying. The greater retention of MTBITC in heat pump drying was presumed due to the lower total heat loading applied to the radish slices and hence a lower rate of evaporation from the tissues and degradation in the slices. This explanation would also apply to the low loss of MTBITC in freeze drying.
Table 1 Loss of MTBITC and drying time of white radish pieces after drying at various temperatures in a hot air, heat pump, and freeze drier
Effect of Osmotic Drying
The loss of MTBITC in radish roots after salting under pressure was found to increase with increasing amount of applied salt (). There was a significant linear relationship between loss of MTBITC (y) and salt concentration (x) (y = 160x + 11; P < 0.05) with about half the MTBITC lost with 25% salt compared to a 15% decrease without salt. As would be expected, there was a significant increase in water loss with the use of salt P < 0.001) with the water loss increasing from about 3% with unsalted radish roots to about 46% with 5% salt. However, the relationship of water loss and salt level was not linear with the increase in water loss rising only to 54% with 25% salt. While osmotic treatment of fruits and vegetables is generally considerd to give better retention of flavor,[Citation10] it can be speculated that the greater loss of MTBITC could be due to the movement of the flavour constituent along with other water soluble solutes from cells at higher salt contents.
Table 2 Effect of osmotic drying on loss of MTBITC in white radish roots by application of sodium chloride under 12 kg pressure for 72 h at 20°C
CONCLUSIONS
There is a substantial loss of MTBITC during the mechanical drying of white radish that has considerable implications for the eating quality of processed radish. While freeze drying of radish pieces results in the lowest loss of MTBITC, use of a heat pump drier is considered to be more commercially feasible in terms of cost and throughput. While heat pump driers are more expensive than hot air driers, use of a heat pump drier is preferred to a hot air drier due to dried output having a much lower loss of MTBITC and faster drying time at equivalent temperatures. The operating temperature selected will depend on balancing product quality with plant throughput and will be affected by the size of radish pieces being dried. Drying through the salting of whole radish roots also results in loss of MTBITC. The lowest level of salt used in this study of 5% would seem to be preferred to minimise MTBITC loss while still achieving an acceptable level of water loss in the 72-h pressure treatment. Although different batches of radish were used in the two trials, the use of 5% salt with whole radish roots resulted in a similar loss of MTBITC as radish pieces dried with a heat pump drier at 35°C.
ACKNOWLEDGMENTS
The authors wish to thank the Australian Rural Industries Research and Development Corporation (RIRDC) for financial support.
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