Food Chemistry 117 (2009) 647–653 Contents lists available at ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem Effect of air-drying temperature on physico-chemical properties, antioxidant capacity, colour and total phenolic content of red pepper (Capsicum annuum, L. var. Hungarian) Antonio Vega-Gálvez a,*, Karina Di Scala b,c, Katia Rodríguez a, Roberto Lemus-Mondaca a,d, Margarita Miranda a, Jessica López a, Mario Perez-Won a a Department of Food Engineering, Universidad de La Serena, Avenida Raúl Bitrán s/n, 599 La Serena, Chile Food Engineering Research Group, Facultad de Ingeniería, Universidad Nacional de Mar del Plata, Juan B, Justo 4302, Mar del Plata, Argentina CONICET (Consejo Nacional de Investigaciones Científicas y Técnicas), Avenida Rivadavia 1917, Buenos Aires, Argentina d Department of Mechanical Engineering, Universidad de Santiago de Chile, Av. Lib. Bdo., O’Higgins 3363, Santiago, Chile b c a r t i c l e i n f o Article history: Received 16 December 2008 Received in revised form 4 March 2009 Accepted 17 April 2009 Keywords: Pepper Air-drying Antioxidant properties Total phenolic content Radical scavenging activity Vitamin C a b s t r a c t Red pepper has been recognised as an excellent source of antioxidants, being rich in ascorbic acid and other phytochemicals. Drying conditions, particularly temperature, leads to pepper modifications that can cause quality degradation. In this work, the effects of process temperatures between 50 and 90 °C on physico-chemical properties, rehydration, colour, texture, vitamin C, antioxidant capacity and total phenolics during the drying of red pepper were studied. The rehydration ratio decreased with temperature and the maximum water holding capacity was achieved at 50 °C. Both vitamin C content and the total phenolic content decreased as air-drying temperature decreased. The radical scavenging activity showed higher antioxidant activity at high temperatures (i.e. 80 and 90 °C) rather than at low temperatures (i.e. 50, 60 and 70 °C). Chromatic parameters (L*, a*, b*, C* and H°), non-enzymatic browning compounds and extractable colour were affected by drying temperature, which contributed to the discolouring of pepper during this process. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction The pepper (Capsicum annuum, L.), indigenous to South and Central America, has been introduced worldwide. In particular, red pepper var. Hungarian, also known as Sweet Banana, is a very important product in the manufacture of paprika in different regions of the word. This pepper is highly appreciated for its flavour and colour, being the latter the main quality attribute that determines its overall quality and consequently its final market price (Krajayklang, Klieber, & Dry, 2000). Carotenoids are responsible for the colour of pepper and their contents are related to variety, ripeness and technological factors (Deepa, Kaura, George, Singh, & Kapoor, 2007; Gnayfeed, Daood, Biacs, & Alcaraz, 2001). Carotenoids are natural pigments responsible for the diverse colours in fruits and vegetables and are abundant in peppers. The main pigments in peppers are b-carotene, lutein and capsanthin and they are predominantly provitamin A (Howard, 2001; RodriguezAmaya, Kimura, Godoy, & Amaya-Farfan, 2008; Topuz & Ozdemir, 2007). Furthermore, red pepper is an excellent source of vitamin C (Guil-Guerrero, Martinez-Guirado, Rebolloso-Fuentes, & Carri* Corresponding author. Tel.: +56 51 204305; fax: +56 51 204446. E-mail addresses: avegag@userena.cl, avegag@gmail.com (A. Vega-Gálvez). 0308-8146/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2009.04.066 que-Pérez, 2006; Topuz & Ozdemir, 2007) and polyphenols, particularly flavonoids, quercetin and luteolin (Chuah et al., 2008; Materska & Perucka, 2005). The antioxidant activity of phenolics is mainly due to their redox properties which allow them to act as reducing agents, hydrogen donors, single oxygen quenchers and metal chelators (Deepa et al., 2007). Thus, all the mentioned compounds show antioxidant activity as potential action against certain cancers, stimulate the immune system, prevent cardiovascular diseases and delay the aging process, amongst other biological activities (Chuah et al., 2008; Podsedek, 2007). Dehydration is one of the most widely used methods for fruits and vegetables preservation. Its main objective is the removal of water to the level at which microbial spoilage and deterioration reactions are minimised. However, it is well known that during hot-air drying, vegetables undergo physical, structural, chemical and nutritional changes that can affect quality attributes like texture, colour, flavour, and nutritional value (Di Scala & Crapiste, 2008). Amongst others, the acceptability of dried products depends mainly on their structural properties, like texture, which is one of the attributes used by consumers in judging their quality. Destruction of the cellular system is one of the most important physical and structural changes that occur during drying (Crapiste, 648 A. Vega-Gálvez et al. / Food Chemistry 117 (2009) 647–653 2000; Yadollahinia, Latifi, & Mahdavi, 2009). Moreover, dehydrated pepper must be rehydrated for consumption of prepared foods like soups, sauces, pizzas and other foodstuffs. Rehydration behaviour has been considered as a measure of the induced damage in the material during drying, such as integrity loss and reduction of hydrophilic properties, which decrease the rehydration ability (Marques, Prado, & Freire, 2009). Theoretically, if there are no adverse effects on the integrity of the tissue structure, it should absorb water to the same moisture content as the initial product before drying (Senadeera, Bhandari, Young, & Wijesinghe, 2000, chap. 6). Rapid and complete rehydration is very important for dried products. Rehydration capacity is affected significantly by drying conditions, pretreatments prior to drying and textural characteristics of dried products. Likewise, drying can diminish the osmotic properties of cell walls; as a result, an increase in water absorption and volume occurs due to the swelling of hydrophilic materials such as starch, cellulose, and pectic materials (KaymakErtekin, 2002). Rehydration is maximised when structural disruption at the cellular level is minimized (Crapiste, 2000). The basic requirements for a vegetable dryer are that it must achieve the required amount of drying in a reasonable time, obtain a product of acceptable quality, minimising operative costs (Kiranoudis, Maroulis, Marinos-Kouris, & Tsamparlis, 1997). Besides, the increasing demand for high-quality shelf-stable dried products requires the optimisation of the drying process conditions, especially temperature, with the purpose of accomplishing not only the efficiency of the process but also the final quality of the dried product (Banga, Balsa-Canto, Moles, & Alonso, 2003). Thus, from an engineering point of view, the control of this variable is a challenging problem that demands the evaluation of many interconnected non-linear mass and heat transport phenomena in a system with variable properties, shrinkage and modifications of quality attributes (Crapiste, 2000). Therefore, the aim of this work was to study the effect of airdrying temperature on physico-chemical properties, rehydration ability (rehydration ratio (RR) and water holding capacity), colour development, texture (firmness) and antioxidant capacity (phenolic and ascorbic acid content) that occurred during the drying process of red pepper var. Hungarian. odologies followed the recommendations of the Association of Official Analytical Chemists (AOAC (1990)). The pH was measured using an EXTECH Instruments microcomputer pH-vision 246072 (Waltham, Massachusetts, USA); the level of titratable acidity was expressed as malic acid. The water activity (aw) was measured at 25 °C by means of a water activity instrument (Novasina, model TH-500, Pfäffikon, Lachen, Switzerland). Soluble solids were measured using a refractometer (ABBE, 1T, Tokio, Japan) which measures refraction indices both of solid and liquid samples in a fast and accurate way and its scale ranges from 0.0 to 95 °Brix. All measurements were done in triplicate. 2.3. Rehydration analysis The dried pepper slabs were placed in distiled water at 40 °C for 6 h, using a solid to liquid ratio of 1:50. The samples were then removed, drained for 30 s, and weighed. All measures were done in triplicate. The rehydration ratio (RR) was calculated according to Eq. (1) and expressed as grams of water absorbed per gram dry matter. The water holding capacity (WHC) was determined by centrifuging the rehydrated samples at 3500g for 15 min at 20 °C in tubes fitted with a centrally placed plastic mesh which allowed water to drain freely from the sample during centrifugation. The water holding capacity was calculated from the amount of water removed following Eq. (2), according to Vega-Gálvez, Lemus Mondaca, Bilbao-Sáinz, Fito, and Andrés (2008b): W reh  X reh  W dried  X dried W dried  ð1  X dried Þ W reh  X reh  W l  100 WHC ¼ W reh  X reh RR ¼ ð1Þ ð2Þ where Wreh is the weight of the sample after the rehydration process, Xreh is the corresponding moisture content on a wet basis, Wdried is the weight of the sample after the drying process, Xdried is the corresponding moisture content on a wet matter and Wl is the weight of the drained liquid after centrifugation. 3. Quality parameters 3.1. Surface colour measurement 2. Materials and methods 2.1. Sample preparation and drying process Red peppers were grown and harvested in Salamanca, Chile, and stored at 4 °C before processing for a maximum time period of five days. The samples were selected visually by colour, size and freshness, and with no sign of mechanical damage. Then, they were cut into slabs of 4.0 ± 0.2 mm in thickness. The samples were dried in a pilot-scale convective dryer at five inlet temperatures 50, 60, 70, 80 and 90 °C (Vega-Gálvez, Lemus-Mondaca, Bilbao-Sainz, Yagnam, & Rojas, 2008a). The air flow rate was 2.0 ± 0.1 m/s. Each experiment was carried out in triplicate. 2.2. Physico-chemical analysis The crude protein content was determined using the Kjeldahl method with a conversion factor of 6.25 (AOAC No. 960.52). The lipid content was analysed gravimetrically following Soxhlet extraction (AOAC No. 960.39). The crude fibre was estimated by acid/ alkaline hydrolysis of insoluble residues (AOAC No. 962.09). The crude ash content was estimated by incineration in a muffle furnace at 550 °C (AOAC No. 923.03). The available carbohydrate was estimated by difference. The equilibrium moisture content was determined by means of AOAC method No. 934.06. All meth- Surface colour of the samples was measured using a colorimeter (HunterLab, model MiniScanTM XE Plus, Reston, VA, USA). Colour was expressed in CIE L* (whiteness or brightness), a* (redness/ greenness) and b* (yellowness/blueness) coordinates, standard illuminant D65 and observer 10° (Vega-Gálvez et al., 2008b). Five replicate measurements were performed and results were averaged. In addition, colour intensity (Chroma), total colour difference (DE) and hue angle were calculated using the following Eqs. (3)– (5), where Lo, ao and bo are the control values for peppers (Sigge, Hansmanw, & Joubert, 2001). Chroma ¼ ða2 þ b Þ0:5 h i  DE ¼ ða  a0 Þ2 þ ðb  b0 Þ2 þ ðL  L0 Þ2 0:5 2 1   Hue angle ¼ tg ðb =a Þ ð3Þ ð4Þ ð5Þ 3.2. Extractable colour The determination was carried out according to the methodology proposed by the American Spice Trade Association (ASTA (1995)). Rehydrated red pepper slabs (0.5 g) were kept in about 50 ml acetone (Sigma Chemical CO., St. Louis, MO, USA) in 100 ml screw-cap jars maintained in the dark for 16 h at ambient temperature. An aliquot of this solution was used for the A. Vega-Gálvez et al. / Food Chemistry 117 (2009) 647–653 spectrophotometric measurement (SpectronicÒ 20 GenesysTM, Illinois, USA) at 460 nm. ASTA units were calculated as follows: Cv ¼ A  16; 4  IF W reh ð6Þ where A is the absorbance of the acetone extract; IF is the instrument correction factor calculated from a pattern solution of potassium dichromate and Wreh is the sample weight. The colour loss value (%) was calculated according to the following equation: ð%ÞColour loss value ¼ Cv o  Cv T  100 Cv o ð7Þ Cvo is the ASTA colour value of the fresh pepper and CvT is the ASTA colour value of the rehydrated peppers dried at different temperatures. 3.3. Determination of non-enzymatic browning index The methodology applied for determination of non-enzymatic browning compounds (NEB) solubilised in the rehydration water was that proposed by Vega-Gálvez et al. (2008b). The rehydration water was first clarified by centrifugation at 3200g for 10 min. The supernatant was diluted with an equal volume of ethanol (Sigma Chemical CO., St. Louis, MO, USA) at 95% and centrifuged again at 3200g for 10 min. The browning index (absorbance at 420 nm) of the clear extracts was determined in quartz cuvettes using a spectrophotometer (SpectronicÒ 20 GenesysTM, Illinois, USA). All measurements were done in triplicate. 3.4. Determination of vitamin C Vitamin C (AA) was determined based upon the quantitative discolouration of 2,6-dichlorophenol indophenol (Merck KgaA, Darmstadt, Germany) titrimetric method as described in AOAC methodology No. 967.21 (AOAC, 2000). Comparative evaluations of vitamin C stability in fresh and rehydrated pepper were carried out, where 5.0 ± 0.1 g of each sample was weighed, crushed and diluted in 1 l distiled water. The vitamin C content was expressed as mg AA retained/100 g dry matter. All measurements were done in triplicate. 3.5. Determination of total phenolic content Total phenolic content (TPC) was estimated as gallic acid equivalents (GAE) as described by Folin–Ciocalteau’s (FC) method with modifications (Chuah et al., 2008). An aliquot (0.5 ml) of the pepper extract solution is transferred to a glass tube; 0.5 ml of reactive FC is added after 5 min; 2 ml of Na2CO3 (200 g/l) are added and shaken. After 15 min of incubation at ambient temperature, 10 ml of ultra-pure water was added and the formed precipitate was removed by centrifugation during 5 min at 4000g. Finally, the absorbance was measured in an spectrophotometer (SpectronicÒ 20 GenesysTM, Illinois, USA) at 725 nm and compared to a GA calibration curve. Results were expressed as mg acid gallic/100 g dry matter. All reagents were purchased from Merck (Merck KGaA, Darmstadt, Germany), and all measurements were done in triplicate. 3.6. Determination of DPPH radical scavenging activity Free radical scavenging activity of the samples was determined using the 2,2,-diphenyl-2-picryl-hydrazyl (DPPH) method (Turkmen, Sari, & Velioglu, 2005) with some modifications. Different dilutions of the extracts were prepared in triplicate. An aliquot of 2 ml of 0.15 mM DPPH radical in ethanol was added to a test tube 649 with 1 ml of the sample extract. The reaction mixture was vortexmixed for 30 s and left to stand at room temperature in the dark for 20 min. The absorbance was measured at 517 nm, using a spectrophotometer (SpectronicÒ 20 GenesysTM, Illinois, USA). The spectrophotometer was equilibrated with 80% (v/v) ethanol. Control sample was prepared without adding extract. All solvents and reagents were purchased from Sigma (Sigma Chemical CO., St. Louis, MO, USA). Total antioxidant activity (TAA) was expressed as the percentage inhibition of the DPPH radical and was determined by the following equation: ð%Þ TAA ¼  1  Abssample  100 Abscontrol ð8Þ where TAA is the total antioxidant activity and Abs is the absorbance. IC50, which is the concentration required to obtain a 50% antioxidant capacity, is typically employed to express the antioxidant activity and to compare the antioxidant capacity of various samples. IC50 was determined from a graph of antioxidant capacity (%) against amount of extract (mg). 3.7. Determination of firmness The property firmness, i.e. the maximum force applied to puncture the pepper tissue, was measured as an indicator of texture. Firmness of samples was measured using a Texture Analyzer (Texture Technologies Corp., TA, XT2, Scardale, NY, USA). The puncture diameter was 2 mm, with a travel distance of 20 mm and 1.7 mm/s test speed. The maximum force was measured by making one puncture in each rehydrated pepper sample, using 10 slabs per treatment. The mean value of maximum firmness for each treatment was then calculated and the results were expressed as N/mm. 3.8. Statistical analysis The effect of air-drying temperature on each quality parameter was estimated using StatgraphicsÒ Plus 5 (Statistical Graphics Corp., Herndon, VA, USA). The results were analysed by an analysis of variance (ANOVA). Differences amongst the media were analysed using the least significant difference (LSD) test with a significance level of a = 0.05 and a confidence interval of 95% (p < 0.05). In addition, the multiple range test (MRT) included in the statistical program was used to demonstrate the existence of homogeneous groups within each of the parameters. 4. Results and discussion 4.1. Effect on physico-chemical properties Proximate analysis of red pepper (on 100 g of fresh weight) presented an initial moisture content of 89.40 ± 1.40 g; crude protein (nitrogen x 6.25) of 1.20 ± 0.10 g; total lipids of 0.53 ± 0.04 g; crude fibre of 1.20 ± 0.10; crude ash of 2.07 ± 0.16 g; available carbohydrates (by difference) of 4.82 ± 0.44 g. Table 1 shows the mean values and standard deviations of the moisture content, water activity, soluble solids, % acidity and pH of both fresh and dry-rehydrated samples. Significant differences were found between temperature and the properties mentioned (p < 0.05). A maximum value of moisture content of 10.63 ± 0.16 g water/g dry matter was observed at 70 °C; however, products with lower moisture contents were obtained at 50 and 60 °C as well as at 80 and 90 °C due to long drying times and high temperatures, respectively. Values of water activity, which is an indicator of water availability, were high for all the samples, as expected for rehydrated samples. Soluble solids and % acidity exhibited a 650 A. Vega-Gálvez et al. / Food Chemistry 117 (2009) 647–653 Table 1 Physico-chemical properties of fresh and dry-rehydrated red pepper as function of air-drying temperature. Sample Condition Water content (g water/g dry matter) aw a Fresh 50 °C 60 °C 70 °C 80 °C 90 °C Soluble solids (°Brix) 0.975 ± 0.002 0.975 ± 0.001 0.981 ± 0.001 0.979 ± 0.002 0.981 ± 0.001 0.982 ± 0.003 4.52 ± 0.28 8.75 ± 0.75b 9.01 ± 0.05b 10.63 ± 0.16c 8.72 ± 0.21b 8.88 ± 0.66b 11.33 ± 0.29 4.2 ± 0.30b 3.8 ± 0.30b 2.7 ± 0.30c 2.8 ± 0.30c 2.5 ± 0.50c Acidity (%) a pH a 4.72 ± 0.01a 5.29 ± 0.04b 4.97 ± 0.05c 5.13 ± 0.06d 4.96 ± 0.04c 4.96 ± 0.11c 0.358 ± 0.004 0.113 ± 0.009b,c 0.138 ± 0.009c,e 0.108 ± 0.009b 0.128 ± 0.009d,e 0.123 ± 0.009c,d Different letters in the same column indicate that the values are significantly different (p < 0.05). Rehydration process depends on structural changes in vegetal tissues and cells of food material during drying, which produces shrinkage and collapse (Kaymak-Ertekin, 2002). Fig. 1 presents the behaviour of the rehydration ratio (RR) as well as the water holding capacity (WHC) for each air-drying temperature studied. The WHC decreased as the air temperature increased (p < 0.05). The maximum WHC was 47.4 ± 2.8 (g retained water/100 g water) at 50 °C which implies that this drying temperature causes tissue structure damage; thus, the pepper dehydrated at this temperature retained a great amount of water. On the other hand, samples dried at 80 and 90 °C have reduced their WHC, thereby preventing the complete rehydration of the dried product. Similar investigations reported that drying temperature is the main factor affecting the WHC (Vega-Gálvez et al., 2008b). In the same figure, RR was affected by the drying temperatures, since absorbed water decreased with temperature. However, RR showed no significant differences (p > 0.05). The lowest RR value was 4.24 ± 0.12 (g absorbed water/g d.m.) at 90 °C, this could be explained due to cellular structure damage resulting in modifications of osmotic properties of the cell as well as lower diffusion of water through the surface during rehydration (Kaymak-Ertekin, 2002). Moreover, pretreatments with saline or sweet solutions such as CaCl2 or saccharose allow to maintain the initial texture, leading to cellular structure stability (Lewicki, 2006; Papageorge, McFeeters, & Fleming, 2003). 4.3. Antioxidant activity 6.0 0.6 A 0.5 A A 5.0 4.0 A a a,b 0.5 a,b a,b 0.4 b B 0.4 3.0 0.3 50 60 70 80 Air-drying temperature (ºC) 90 WHC(g retained water/ g water) RR (g absorbed water/g dry matter) Fig. 2 shows the vitamin C as well as the total phenolic content (TPC) for the fresh and dry-rehydrated samples during the five dry- Fig. 1. Effect of air-drying temperature on the rehydration ratio (RR) and the water holding capacity (WHC) for dry-rehydrated pepper samples. Identical letters above the bars indicate no significant difference. 200 A 160 a 1200 120 800 80 B B 40 c b B B B c,d c,d d 70 80 90 0 400 Total phenolic content (mg galic acid/100 g dry matter) 4.2. Effect on water holding capacity and rehydration ratio 1600 Vitamin C (mg /100 g dry matter) decreasing tendency in all the different treatments respect to the fresh sample. In the same table, pH increased slightly from its original value, showing the same tendency of acidity. Similar results were found by Miranda, Maureira, Rodriguez, and Vega-Gálvez (2009), working with dehydrated Aloe Vera. 0 Fresh 50 60 Air-drying temperature (ºC) Vitamin C Total phenolic content Fig. 2. Effect of air-drying temperature on vitamin C and total phenolic content of fresh and dry-rehydrated pepper samples. Identical letters above the bars indicate no significant difference. ing experiments. The initial content of vitamin C in peppers was 188.2 ± 4.5 mg ascorbic acid/100 g fresh sample, which is within the ranges found in other studies for red peppers (75–277 mg/ 100 g fresh weight; Castro et al., 2008). It can be seen that an increase in drying temperature has an important effect on vitamin C (p < 0.05), thus, a maximum loss of 98.2% vitamin C in samples dried at 90 °C is observed. This could be explained due to irreversible oxidative processes either during drying or rehydration water lixiviation of this water-soluble vitamin (Sigge et al., 2001; VegaGálvez et al., 2008b). In addition, vitamin C is considered as an indicator of the quality of food processing due to its low stability during thermal processes (Podsedek, 2007). Similar results were obtained by other authors working with peppers (Di Scala & Crapiste, 2008; Sigge et al., 2001). Pretreatments like blanching or additives like SO2 and CaCl2 can improve the retention of this vitamin (Vega-Gálvez et al., 2008b). Besides vitamin C, foods of plant origin also supply our diet with other antioxidants in large amounts: carotenoids and phenolic compounds which constitute one of the major groups of compounds acting as primary antioxidants or free radical terminators (Tabart, Kevers, Pincemail, Defraigne, & Dommes, 2009). The initial phenolic content was 1359 ± 148 g galic acid/g fresh sample. Sweet peppers contain numerous phenolic compounds, and not all of the genotypes may contain a similar profile or relative proportions of these compounds within the profile. Differences in these profiles may subsequently result in complex changes in antioxidant activity or other bioactivities (Deepa et al., 2007). It can be observed (Fig. 2) that an increase in drying temperature has an important effect on the total phenolic content (p < 0.05). The formation of phenolic compounds at high temperatures (i.e. 90 °C) might be because of the availability of precursors of phenolic molecules by non-enzymatic interconversion between phenolic molecules (Que, Mao, Fang, & Wu, 2008). The radical scavenging activity was investigated based on airdrying temperature (p < 0.05) as observed in Fig. 3, where dehydration at high temperatures (i.e. 80 and 90 °C) shows higher antioxidant activity rather than at low temperatures (i.e. 50, 60 and 651 IC50 - Concentration (ug/mg sample) A. Vega-Gálvez et al. / Food Chemistry 117 (2009) 647–653 3000 b b 2500 b b 2000 b 1500 1000 500 0 a Fresh 50 60 70 80 90 Air-drying temperature (ºC) Fig. 3. Effect of air-drying temperature on DPPH free radical scavenging activity of fresh and dry-rehydrated pepper samples. Identical letters above the bars indicate no significant difference. 70 °C). This behaviour could be related to drying process at low temperatures, which implies long drying times that may promote a decrease of antioxidant capacity (Garau, Simal, Roselló, & Femenia, 2007). Furthermore, generation and accumulation of Maillardderived melanoidins having a varying degree of antioxidant activity could also enhance antioxidant properties at high temperatures (i.e. 80 and 90 °C) (Miranda, Maureira, Rodriguez, & Vega-Gálvez, 2009; Que et al., 2008). Increasing correlation between antioxidant activity and total phenolic content has been reported during food dehydration (Deepa et al., 2007). However, data on the effects of drying on TPC and antioxidant activity of vegetables are conflicting due to several factors, like drying method, type of extraction solvent, antioxidant assays used as well as interactions of several antioxidant reactions (Manzocco, Calligaris, Mastrocola, Nicoli, & Lerici, 2001; Que et al., 2008). 4.4. Effect on colour Fig. 4 presents the colour changes of pepper related to both non-enzymatic browning (NEB) and the ASTA colour value. It can be observed that an increase in temperature led to an important formation of brown products. This could be explained due to an increase in the kinetic reaction rate that shows a maximum NEB value of 0.19 ± 0.01 Abs/g dry matter at 90 °C. Furthermore, it is well known the effect of water on chemical reactions, via aw or by plasticising amorphous systems (dehydrated systems) since the inhibitory effect of water seems to be a decisive factor in the NEB reaction rate (Acevedo, Schebor, & Buera, 2008). In addition, the 700 0.25 C NEB (Ab/g dry matter) 0.20 A D e development of the Maillard reaction frequently occurs in concomitance with other events, which can contribute to change both colour and the overall antioxidant capacity of pretreated foods (Manzocco et al., 2001). For example, addition of SO2 can be applied during drying as an inhibitor of browning due to its capacity of both disinfectant and antioxidant on essential compounds (Miranda, Berna, Salazar, & Mulet, 2009). Moreover, Fig. 4 shows the profiles of the ASTA colour for the different drying temperatures. The ASTA colour for the fresh samples was 222.83 ± 3.06 ASTA units/ g dry matter. When the results were compared, it could be observed that the highest air-drying temperature (90 °C, 380.90 ± 2.60 ASTA units/g dry matter) presented the lowest value of colour showing a maximum at 70 °C (568.90 ± 6.90 ASTA units/g dry matter). ASTA colour content is mainly attributable to endogenous carotenoids such as capxantine, capsorubin, b-carotene and others (Vega-Gálvez et al., 2008b). In consequence, the behaviour of this parameter is directly related to deterioration in these pigments due to high drying temperatures (Ergunes & Tarhan, 2006). Furthermore, discolouration of carotenoids during processing might occur through enzymatic or nonenzymatic oxidation (Rodriguez-Amaya et al., 2008). Both NEB and ASTA colour presented significant differences (p < 0.05). Fig. 5 shows the average values of the chromatic coordinates L*, a* and b*, for the fresh and dry-rehydrated samples of pepper. It can be observed that coordinate L* presents the lowest values 27.70 ± 2.65 at high temperatures (90 °C) indicating that fresh peppers presented a darker colour compared to the dry-rehydrated samples. For drying temperatures from 50 to 80 °C, the differences in lightness are not so evident (p < 0.05). Although there is a hypothesis related to rehydrated products with high water content having high L* values, in this case it was not confirmed. Rehydrated peppers become darker probably due to a larger extension of the Maillard reaction, especially at higher drying temperatures (Acevedo et al., 2008; Manzocco et al., 2001). Pretreatments, like addition of different compounds, can enhance the chromatic coordinates (Garau et al., 2007; Sigge et al., 2001). Modifications in coordinate a* (greenness–redness) for fresh and dry-rehydrated samples are also presented in Fig. 5, where there was a decrease of this coordinate (13% at 90 °C) respect to fresh samples (p > 0.05). This could be explained due to the presence of carotenoids as well as other components (vitamins, carbohydrates, aminoacids, etc.) in the pepper affecting the final product colour (Miranda, Maureira, Rodriguez, & Vega-Gálvez, 2009). Coordinate b* (blueness–yellowness) showed a slight increase in its value (15% at 90 °C) as temperature increased (Fig. 5) as a result of generation of brown products due to non-enzymatic reactions (p > 0.05). Since total colour difference (DE) is a function of the three CIE L* a* b* coordinates (Eq. (3)), changes from 600 50 B a 500 E 0.15 40 a,b 400 30 0.10 a a a a a a b a,b a a,b b a,b a,b a 300 c 20 d 200 a 10 b 0.05 a a 100 0 0.00 0 50 60 70 80 90 Air-drying temperature (ºC) Fig. 4. Effect of air-drying temperature on non-enzymatic browning and ASTA color for dry-rehydrated pepper samples. Identical letters above the bars indicate no significant difference. Fresh 50 60 70 80 90 Air-drying temperature (ºC) L* a* b* Fig. 5. Effect of air-drying temperature on the chromatics coordinates (L*, a* and b*) of fresh and dry-rehydrated pepper samples. Identical letters above the bars indicate no significant difference. 652 A. Vega-Gálvez et al. / Food Chemistry 117 (2009) 647–653 5.78 ± 2.91 to 7.79 ± 2.16 were estimated for 50 and 90 °C, respectively. Similar results were obtained by Miranda, Maureira, Rodriguez, and Vega-Gálvez (2009), where high DE values were found at high drying temperatures due to the effect of high temperatures on heat-sensitive components like proteins and carbohydrates, amongst others. The saturation index or chroma (C*) and the hue angle (Ho), as shown in Fig. 6, provide more information about the spatial distribution of colours than direct values of tristimulus measurements (Sigge et al., 2001). The values of C* and Ho are shown in Fig. 6, where it is observed that both indices are affected by temperature in opposite ways (p > 0.05) for both fresh and dry-rehydrated samples. Estimated chroma values retained the 96% of the fresh pepper, the hue angle showed an increase of 23% compared to fresh sample indicating discolouration of the original pepper colour. 4.5. Effect on firmness Firmness is one of the most desirable attributes in fresh as well as rehydrated peppers (Castro et al., 2008). The behaviour of this physical property affected by drying temperature is illustrated in Fig. 7. It can be observed that air-drying temperature has a negative effect on this textural property presenting a maximum decrease of 50% respect to the fresh sample at 70 °C (p < 0.05). This tissue firmness reduction could be explained due to changes in the plant cell wall that occurred during processing at high temperatures (Papageorge, et al., 2003; Castro et al., 2008; Vega-Gálvez et al., 2008b). This behaviour can be minimized adding calcium 60 which has been reported to maintain firmness by crosslinking cell wall and middle lamella pectins, stabilizing cell membranes, and/ or affecting cell turgor potential (Castelló, Igual, Fito, & Chiralt, 2009; Vega-Gálvez et al., 2008b). 5. Conclusions The effect of air-drying temperature on physico-chemical properties, firmness, rehydration, colour and antioxidant activity due to total phenolic content and ascorbic acid of red pepper during hot air-drying between 50 and 90 °C was investigated. The rehydration ratio (RR) decreased with temperature showing a lower RR of 4.24 ± 0.12 (g absorbed water/ g d.m.) at 90 °C. The maximum water holding capacity was 47.4 ± 2.8 (g retained water/100 g water) at 50 °C. Moreover, both vitamin C content and total phenolic content decreased as air-drying temperature increased. A maximum loss of 98.2% in vitamin C was observed in samples dried at 90 °C. The radical scavenging activity showed higher antioxidant activity at high temperatures (80–90 °C) respect to low temperatures (50–70 °C). Non-enzymatic compounds increased with temperature. ASTA colour was affected by temperature and presented the lowest colour value at 90 °C. 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