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Sensorial and chemical quality of electron beam irradiated almonds (Prunus amygdalus)
ARTICLE IN PRESS
LWT 41 (2008) 442–449 www.elsevier.com/locate/lwt
Sensorial and chemical quality of electron beam irradiated almonds (Prunus amygdalus) Paloma Sa´nchez-Bela, Isabel Egeaa, Felix Romojaroa, M. Concepcio´n Martı´ nez-Madridb, a
Department of Food Science and Technology, CEBAS-CSIC, Apdo. 4195, 3100 Murcia, Spain Departamento de Agroquı´mica y Medio Ambiente, Escuela Polite´cnica Superior, Universidad Miguel Herna´ndez, Ctra. Beniel km 3.2, 03312-Orihuela, Alicante, Spain
b
Received 24 October 2006; received in revised form 20 March 2007; accepted 20 March 2007
Abstract This work reports the effects of irradiation with accelerated electrons (0, 3, 7, and 10 kGy) on the chemical composition (water content, proteins, neutral detergent fiber, sugars, lipid content, organic acids, and color) and sensorial properties (rancidity, sweetness, off-flavors and odors, texture, and whiteness) of the shelled almond variety Guara, packaged under air atmosphere and stored for 5 months at 2071 1C. Changes observed where a decrease for glucose in samples treated at all irradiation doses. An increase of citric acid, at doses above 3 kGy and then a decrease to values similar to those of the control was observed. With respect to the sensorial analysis, there was no treatment effect on the sweetness, texture or color but there was a marked rancidity in the samples treated with 10 kGy that decreased the overall appreciation of the samples. Irradiation doses of up to 7 kGy seem to be a suitable post-harvest sanitation treatment since they did not cause significant changes in the sensorial quality or in the contents of protein, fiber, water, or lipid with respect to the control samples, both following the treatments and after 5 months of storage. r 2007 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. Keywords: Almond; Irradiation; Sugars; Organic acid; Post-harvest quality
1. Introduction In recent years, numerous scientific studies have demonstrated the beneficial effects of the consumption of nuts for human health, these being an important component of a balanced diet due to their high nutritional value. In spite of their high fat content, they possess elevated levels of monoand poly-unsaturated fatty acids and large quantities of vitamin E and fiber. Also, they represent an interesting source of vegetable proteins thanks to their amino acid composition and to the iron, calcium, and oxalic acid content. According to the estimations of the FAO (1988), around 25% of world grain production is lost due to insects, bacteria, and rodents, and a similar percentage of nut production is contaminated by mycotoxins, the most commonly contaminated being peanuts and pistachios Corresponding author. Tel.: +34 966 749 696; fax: +34 966 749 619.
E-mail address: [email protected] (M.C. Martı´ nez-Madrid).
(Hallman, 1999). The various post-harvest procedures for control of insects and mites in stored products are chemical, biological and physical control or a combination of these techniques; food irradiation has been used for treatments of disinfestation (Hoosmand & Klopsfenstein, 1995), inhibition of sprouting (Thomas, 1984), delay of fruit ripening (Baccaunaud, 1991), and treatments of pasteurization and sterilization (Scandella & Foures, 1987). In some cases, it can replace the use of chemical additives, or be used in combination with them, and turns out to be effective in the treatment of nuts. The effects of the irradiation of foods cannot be generalized if the food and the dose absorbed by it are not specified (Lacroix & Ouattara, 2000). The effects of ionizing radiation on foodstuffs can be direct or indirect. In the first case, the radiation acts directly on the matter, causing the breakdown of molecules like DNA and other food constituents. In the second case, the radiation acts through water radiolysis and, hence, the different water radiolysis products will react with other
0023-6438/$30.00 r 2007 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2007.03.015
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compounds, causing significant chemical changes, like formation of hydroperoxide and free radicals (El Assis, Huber, & Brecht, 1997). The compounds induced by these free radicals are predictable from the composition of the food, and they can be formed by other chemical reactions (Grootveld, Jain, Claxson, Naughton, & Blake, 1990), or by enzymatic action (lipoxygenases and peroxidases) (Lebovics, Gaal, Somogyi, & Farkas, 1992). Consequently, the water content of the foodstuff is one of the most important factors determining the alterations brought about by the irradiation, in relation to oxidation of lipids and fatty acids (Lebovics et al., 1992) or degradation of vitamins and pigments (Grootveld et al., 1990). The changes effect by irradiation may be desirable, for example a decrease in the degree of polymerization of polyosids, a useful phenomenon since it reduces the cooking time for starch. A decrease in the percentages of certain polysaccharides responsible for flatulence has been described also (Rao & Vakil, 1983). The literature indicates that irradiation does not cause any significant change in the proximate composition of dried fruits and nuts (Akingbohungbe, 1994). Doses sufficient to kill all infesting insects had no adverse effects on either nutritional value or the sensory quality of dried fruits and nuts (Lescano & Narvaiz, 1992). In addition to possible changes in the nutritive quality, it is necessary to consider the sensorial quality following ionization treatments; in the case of almond, the consumer demands nutritive quality, a creamy-white color, an absence of vitreous zones, pleasant sweet and oily flavors, and a complete absence of rancid flavors and smells. Previous studies of irradiated almonds (Sa´nchez-Bel, Martı´ nez Madrid, Egea, & Romojaro, 2005) showed that irradiation doses of up to 7 kGy did not produce significant changes in the fatty acid composition, the lipid oxidation or the appearance of rancidity. Following on from this, the aim of the current work was to investigate the changes produced in other aspects of the chemical composition (proteins, water content, lipid content, neutral detergent fiber, organic acids, and sugar composition), the kernel color, and the sensorial properties (rancidity, sweetness, off-flavors and odors, texture, and whiteness) of the shelled almond (Prunus amygdalus) var. Guara, packaged in an air atmosphere and stored for 5 months at 20 1C, after being treated with irradiation doses of 3, 7 or 10 kGy.
443
process as the treated bags. After ionization, the batches were stored in controlled conditions (2071 1C and 75 g/ 100 g RH) for a period of 5 months, and periodic samples were taken after 7, 14, 21, 58, and 157 days. Each sample constituted three bags (sub-samples). 2.2. Ionization treatments Irradiation was carried out by the firm IONMED Esterilizacio´n S.A. (Taranco´n, Cuenca, Spain), using a Rhodotron circular electron accelerator (I.B.A., Belgium) at an energy level of 10-MeV. The protocol and treatment conditions were those suggested by the R+D Department of IONMED. Treatment lots were arranged as a monolayer in a transporting tape leading to the electron beam. The programmed irradiation doses were 3, 7, and 10 kGy; non-irradiated samples were separated as control lots. An adequate number of radiochromic dosimeters FTW-60.0 (Far West Technology, USA) were placed randomly in both faces of the bags in order to verify the real dose absorbed by the fruits and to study the penetration of the radiation. The absorbed dose was measured at 600 nm in a Ge´nesis-5 spectrophotometer (Spectronic, USA) with an uncertainty of DAbs of 0.006 for a level of confidence of 95%. The treatment characteristics are shown in Table 1. 2.3. Lipid extraction The fat was extracted in a 6-unit extractor (Det-Gras J.P. Selecta S.A., Barcelona, Spain), using petroleum ether (40–601) as extractant; in order to avoid fat oxidation during the extraction, the ether evaporation was carried out in a vacuum. The fat content was analyzed in duplicate in each sub-sample and the results expressed as grams of lipids per 100 g of fresh weight (g/100 g FW). 2.4. Moisture content This was determined by accurately weighing ground almond samples after heating them in an oven at 105 1C for 24 h (AOAC, 2000). Two measurements were carried out Table 1 Conditions applied during the irradiation treatments and the later readings of the dosimeters in order to verify the real absorbed dose Dose (kGy)
2. Material and methods
3
7
10
5 2.15 103
4.7 4.38 103
3.47 5 103
0.41370.002 3.270.17
0.72070.001 7.170.12
1.00370.003 10.370.4
2.1. Plant material and experimental design The shelled almond (P. amygdalus) variety Guara, without tegument, was supplied by Frutos Secos el Man˜an (Pinoso, Alicante, Spain). A total of 6 kg of almonds from the same harvest was packaged in 60 high-barrier plastic bags (100 g each), 45 of them being ionized at different doses (15 bags per dose) and another 15 separated as a control and later subjected to the same preservation
Speed of the belt (m/min) Intensity of the beam (mA) Width of the electron beam (cm) Optic abs of the dosimeter Real dose (kGy)
The variability of the real dose of irradiation absorbed by the samples was less than 1% of the programmed dose applied. The dosimeters also verified the homogeneity of the dose and validated the irradiation process.
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per sub-sample and the results expressed as percentage of moisture per 100 g of fresh weight (g/100 g FW). 2.5. Color The color was measured by reflectance in the chopped almond using a Minolta CR-300 colorimeter calibrated with a white standard. Duplicate measurements were made for each sub-sample and the results expressed as the color coordinate L, this being the one which best shows the variations in the white tonality.
mobile phase was c(H2SO4) ¼ 5 mmol/L at a flow rate of 0.4 mL/min. The detection wavelength was 210 nm for oxalic acid and 230 nm for citric, malic, and succinic acids. The sugar and organic acid concentrations in the tissue were obtained using calibration curves for each compound. Two extractions were performed for each bag or subsample and each extract was analyzed in duplicate. For sugars, the results are expressed in grams per 100 g of fresh weight (g/100 g FW) and for the organic acids in milligrams of acid per 100 g of fresh weight (mg/100 g FW). 2.9. Sensory test methodology
2.6. Protein content The content of globular proteins, which constitute 90% of the total proteins present in almond, was measured. For this, a de-fatted sample (0.5 g) was homogenized with 50 mL of water in a Polytrons (Kinema´tica AG, Switzerland) and then centrifuged for 20 min at 10,000 g (at 20 1C). For the resulting supernatant, protein quantification was performed by the method of Lowry, Rosebrough, Farr, and Randall (1951) modified according to the protocol accompanying the Biorad DC Protein Assay kit (Biorad Laboratories, Spain); absorbance was measured at 750 nm in a UVIKON 930 spectrophotometer (Kontron Instruments, Ltd.). The protein contents of the samples were calculated using a calibration curve obtained for bovine serum albumin standards (0–1.5 mg) treated in the same way. Two extractions were carried out per subsample and each sample was analyzed in duplicate, the results being expressed as grams of protein per 100 g of fresh weight (g/100 g FW). 2.7. Fiber The determination of the fiber content was performed for the de-fatted samples, according to the methods described in AOAC (1995). The neutral detergent fiber (NDF) was measured in duplicate for each sub-sample, following the method of Van Soest and Robertson (1979). The results are expressed as grams of NDF per 100 g of fresh weight (g/100 g FW). 2.8. Sugar and organic acids content Sugars and organic acids were extracted according to the procedure of Madrid, Gabarro´n, Sa´nchez Bel, Valverde, and Romojaro (2001). The extract was filtered through a Durapores 0.45-mm HV (Millipore Corporation, USA) membrane disk and then passed through a C18 Plus SepPak cartridge (Waters Corporation, Massachusetts, USA). Quantification was carried out by HPLC, using a Shimadzu HPLC machine (Kyoto, Japan) having an ion exchange column (ION 300, Teknochroma) and two detectors: a Shimadzu Refractive Index Detector (Kyoto, Japan), at 30 1C, for detection of sugars, and a Shimadzu UV–Vis detector for organic acids (Kyoto, Japan). The
Sensory rating was conducted by a selected and trained panel comprising of seven judges with some expertise in sensory tests. The evaluation was done using 5-point structured scales, 5 being the best and 1 the worst quality. In order to evaluate the capacity for examination and the sensitivity of the tasters, sucrose was used as standard for the sweet flavor, bitter almond for the bitter flavor and oxidized oleic acid for detecting rancidity. The overall quality attribute was assessed as the measurement of the acceptability of the product by the consumer, using a scale from ‘‘very unpleasant’’ (score of 1) to ‘‘very pleasant’’ (score of 5). The selection and training of tasters, as well as the fitting-out of the tasting room, were carried out according to UNE 87-004-79 (1995). 2.10. Statistics Data were analyzed using the General Linear Model of the SPSS (version 11.0) statistical package. Analysis of variance was conducted with irradiation dose and storage time as factors. When differences were significant (po0.05), multiple comparisons were made using Tukey’s test. 3. Results and discussion Under the storage conditions assayed in this study, the water content was 3.82–3.93 g/100 g, 3.54–3.89 g/100 g, 3.71–3.87 g/100 g, and 3.61–4.15 g/100 g for the control samples and those receiving, 3, 7 or 10 kGy, respectively (Table 2), whilst the respective lipid contents were 53.67–58.64 g/100 g, 52.15–55.98 g/100 g, 50.69– 55.74 g/100 g, and 53.20–56.03 g/100 g (Table 2). The analysis of variance, employing the irradiation dose and storage time as factors, showed that none of the differences were statistically significant, for either parameter. Previous work by other groups on these parameters in other nuts also showed no effect of irradiation treatments: Al-Bachir (2004), following irradiation of walnuts at 1, 1.5 or 2 kGy, not detected significant changes in the protein composition or the contents of lipid, water or ash, whilst Inayatullah, Zeb, Ahmad, and Khan (1987) did not detect significant changes in the contents of lipid, water or ash after irradiation of soybeans with doses of up to 5 kGy.
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Table 2 Qualitative characteristics of control and irradiated almonds during the storage time Dosea (kGy)
Storageb (days)
Moisture (g/100 g FW)
Darkness
Fat (g/100 g FW)
0
0 7 21 58 157
3.8270.09 3.8270.18 3.7770.14 3.8970.20 3.9370.38
96.8570.54 97.9070.04 95.6872.88 91.4772.52 86.8172.57
53.6771.00 52.4770.01 58.6474.24 54.4970.78 56.3170.54
3
0 7 21 58 157
3.8470.15 3.8970.22 3.7270.04 3.7970.04 3.5470.01
94.3471.36 93.8571.47 90.6570.42 89.5871.02 85.6471.20
53.4972.91 55.9873.61 55.1971.78 53.6572.69 52.1575.62
7
0 7 21 58 157
3.8770.03 3.7770.09 3.7170.04 3.7270.01 3.7670.13
93.8170.44 93.9370.20 90.3470.04 86.1472.16 89.8372.26
54.1973.00 51.7374.96 54.9171.30 55.7471.06 50.6970.75
10
0 7 21 58 157
3.9370.03 3.8470.08 3.7070.02 4.1570.05 3.6170.69
91.0375.76 90.3370.90 90.0172.12 88.3972.27 87.2470.25
55.1873.66 53.2070.62 56.0371.66 55.9472.32 55.2276.76
Ns Ns Ns
Ns * Ns
Ns Ns Ns
Irradiation dose Storage time Dosestorage a.b
Statistical differences were analyzed by ANOVA (pp0.05). Ns ¼ no significance.
However, the storage conditions have been found to affect the water content, particularly when assaying different temperatures and packaging atmospheres, which shows the need to optimize these conditions for each type of sample (Al-Bachir, 2004). In our study, the lipid and moisture contents of the samples were practically constant for all the treatments applied, and during the whole period of storage, supporting the idea that irradiation treatments do not influence these parameters (at least, at the doses applied) and suggesting also that the storage conditions assayed in our study were ideal for this plant material. On the other hand, the color of the samples showed a gradual darkening during storage, irrespective of the treatment which they received. The initial color values were 96.85 for the control and 94.34, 93.81, and 91.03, respectively, for the samples treated with 3, 7 or 10 kGy. These values remained virtually constant until the 8th week of storage; from this point there was a trend towards lower values until the end of storage, when the values were 86.81, 85.64, 89.83, and 87.24 for the control and the 3, 7, and 10 kGy treatments, respectively. The degree of darkening was of a similar magnitude for all the samples assayed, with no statistically significant differences (p40.05) being observed between treatments (Table 2). However, there were significant differences according to the storage time: irrespective of the treatment, the samples underwent a process of gradual darkening during the post-harvest storage, even though the conditions can be considered
optimal, which depended above all on the initial composition of the sample, this being a characteristic of the variety involved. The color change is fundamentally the result of respiration activity in combination with enzymatic reactions, which are the reason for sugar consumption— leading to the formation of colored reaction intermediates. Thus, the results are in agreement with the hypothesis, put forward by other authors, that the color variation of nuts after their harvest is a direct function of the starting plant material and of the specific growing conditions, such as the climatic conditions or vegetative cycle, and is independent of the treatments applied post-harvest and the subsequent storage conditions (Kazantzis, Nanos, & Stavroulakis, 2003). 3.1. Effect of irradiation on the fiber and protein content The fiber content of the samples remained relatively stable during the storage period; a value of 4.26–6.10 g/100 g for the control and 4.09–6.45 g/100 g, 4.04–5.72 g/100 g, and 3.80–6.04 g/100 g for the samples irradiated at 3, 7, or 10 kGy, respectively (Table 3). These values show that the irradiation doses used did not modify the fiber content, neither immediately following the treatments nor after a storage period of more than 5 months, at 20 1C. Regarding the protein content, there was a slight and not significant (p40.05) decrease following the treatments, for the three doses used; the values being 24.16–28.37 g/100 g for samples
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Table 3 Fiber and protein content of control and irradiated almonds during the storage time Dosea (kGy) Storageb (days) NDF (g/100 g) Protein content (g/100 g) 0
0 7 21 58 157
5.5870.91 4.8170.28 5.6470.80 6.1070.08 5.1370.45
24.1671.97 25.7672.87 27.1470.92 25.6871.60 28.3770.49
3
0 7 21 58 157
4.9870.25 6.4570.59 4.1770.72 5.2170.19 4.8770.37
21.5870.66 25.8871.97 23.4570.85 24.4973.09 24.7971.08
7
0 7 21 58 157
4.4370.82 5.4770.56 4.6970.11 4.7770.99 5.7270.49
23.7474.38 23.3872.69 25.0973.37 23.1572.45 21.8772.94
10
0 7 21 58 157
6.0470.44 5.0570.11 3.8070.62 4.0870.64 4.5770.92
20.4470.02 24.3074.50 25.3871.24 23.7773.68 23.3870.08
Ns Ns Ns
Ns Ns Ns
Irradiation dose Storage time Dosestorage
certain functional groups, like hydroxyl, and the formation of compounds such as gluconic acid and 2-deoxy-gluconolactone (Von Sonntag, 1980). The main products identified following the irradiation of sugars are acids, short-chain carbonyl derivatives, and modified sugars, including sugars with two carbonyl groups when dealing with samples in the presence of oxygen (Sendra et al., 1996). Fig. 1 shows the modifications in the sucrose and glucose contents after the irradiation and subsequent storage. All of the irradiated samples exhibited a decline in sucrose concentration, until the end of storage, whilst, from 2 months onwards, there was a stabilization in the control values (Fig. 1A). The irradiation treatments brought about an important decrease in the glucose content, independent of the dose applied (Fig. 1B). After this initial decline, the glucose level did not change significantly until day 71 of storage, from when there was a further drop for both irradiated and control samples. The analysis of variance for these results, employing irradiation dose and storage time as factors, showed significant differences in the glucose content among the irradiation doses of 3, 7, and 10 kGy. On the other hand, there were no significant differences with respect to the glucose and sucrose contents during the storage. Tukey’s test, when used to indicate the irradiation
3.2. Effect of irradiation on sugar content During the irradiation of sugars, the action of free radicals produced by water can bring about oxidation of
6
4
2
0 0.6 Glucose g/100 g FW
irradiated at 3 kGy, 21.58–25.88 g/100 g for those receiving 7 kGy, and 21.87–25.09 g/100 g at 10 kGy, compared with 20.44–25.38 g/100 g for the control samples (Table 3). However, when the analysis of variance was performed, these differences were not statistically significant, for the treatment or storage time or their interaction (dosestorage). The radiolytic compounds that can form, after an irradiation treatment, from the compounds already present in the foodstuff depend directly on the water content; because of this, when nuts or dehydrated materials are irradiated, the expected modifications are much less. But, the damage inflicted by irradiation on the peptide bond depends, in addition to the degree of hydration of the material, greatly on the oxygen content, since this bond is highly stable and is not normally broken by the irradiation doses generally applied to foodstuffs (Krumhar & Berry, 1990). Thus, given that the moisture content of the samples in this study was very low (3–4 g/100 g), it was to be expected that both the protein and fiber contents would remain largely unaltered throughout the assay, for all the treatments applied.
Sucrose g/100 g FW
a.b Statistical differences were analyzed by ANOVA (pp0.05). Ns ¼ no significance.
a
0.4
0.2
b 0.0 0
20
40
60
80 100 120 Days of storage
140
160
180
Fig. 1. Changes in the contents of (A) sucrose and (B) glucose during the storage period for the (K) control samples and those irradiated at (J) 3 kGy, (.) 7 kGy or (,)10 kGy: a and b represent the homogeneous subgroups generated by Tukey’s test, using the applied irradiation dose as the factor, at the 95% confidence level (pp0.05).
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dose effect, produced two sub-groups with means that were significantly different: the first comprised the control and the second the irradiation treatments (3, 7 or 10 kGy)— thus, there were no significant differences between the three doses (Fig. 1Ba and Bb). These results demonstrate the limited influence of the irradiation treatments on the sugar levels in the almonds. This could be explained by the low moisture content of this type of material, since it has been shown that the levels of radiolytic products originating from the sugars present in irradiated foodstuffs are much lower when the moisture content is low (St-Le`be, Raffi, & He´non, 1982). The irradiation of carbohydrates provokes the rupture of the ether bonds between hexose residues in high-molecularweight carbohydrates, as well as the dehydration of monosaccharides (Siddhuraju, Makkar, & Becker, 2002), so that the content of monosaccharides should rise as a result of this process. The fact that this did not happen could be due to their formation being compensated for by their conversion into dehydrated products such as furfurals, which were not measured in this study. 3.3. Effect of irradiation on the organic acids content The main organic acids found were citric, malic, oxalic, and succinic. Their concentrations, and the treatment effects, can be seen in Fig. 2. For oxalic acid (Fig. 2B),
there was little variation during the entire storage period, for all of the irradiation doses assayed; its concentration being around 12 mg/100 g FW. Malic acid showed an analogous behavior, with values around 5 mg/100 g FW (Fig. 2D). Contrary to this, citric acid had a different profile (Fig. 2A): for doses of 7 kGy or above, it showed an initial increase which was maintained for 3–4 weeks, before declining to reach values similar to those of the control and 3-kGy treatment. One notable point is the dramatic decrease in the succinic acid level (Fig. 2C), from the time of treatment and for all irradiation doses, such that in certain cases there were only trace amounts present. 3.4. Effect of irradiation on the sensory testing To ascertain whether the irradiation treatments affected the organoleptic properties of the almonds, a sensorial analysis was performed after 121 days of storage. The evaluation was made using 5-point structured scales, 5 being the best and 1 the worst quality. The quality attributes were assessed as the acceptability of the product by the consumer, using a scale from ‘‘very unpleasant’’ (score of 1) to ‘‘very pleasant’’ (score of 5). Figs. 3 and 4 show the numerical values assigned by the tasting panel to each of the attributes analyzed. In general, a decrease in quality was found, both for the control and the irradiated
20
20 15
15
a
10
10
5
5
b
0
Oxalic acid mg/100 g FW
25 a
0
25
25 a
20
20
15
15 10
10 b
5
5
Malic acid mg/100 g FW
Citric acid mg/100 g FW
25
Succinic acid mg/100 g FW
447
0
0 0
20
40
60
80 100 120 140 160 0 Days
20
40
60
80 100 120 140 160 Days
Fig. 2. Changes in the principal organic acids, (A) citric, (B) oxalic, (C) succinic, and (D) malic, present in the almond during storage, for the (K) control samples and those irradiated at (J) 3 kGy, (.) 7 kGy or (,) 10 kGy: a and b represent the homogeneous sub-groups generated by Tukey’s test, using the applied irradiation dose as the factor, at the 95% confidence level (pp0.05).
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appreciation (around 1), which are statistically different to the values obtained for control samples and those irradiated at 3 or 7 kGy.
4.5 a
4.0
a a
3.5
Scores
3.0
Acknowledgments
b
2.5
This work was financed by the Spanish Ministry of Science and Technology (Ministerio de Ciencia y Tecnologı´ a) (Project ‘‘1FD 1997-1005-C04-01’’). The authors are grateful to the company IONMED Esterilizacio´n S.A. (Taranco´n, Cuenca, Spain), for the ionization of the samples, and to the OPFS ‘‘El Man˜an’’ (EP Pinoso, Alicante, Spain), for providing plant material.
2.0 1.5 1.0 0.5 0.0 Sweetness
Color
Texture
Fig. 3. Scores given by the tasting panel for the attributes of sweetness, color, and texture for the (’) control samples and those irradiated at (O) 3 kGy, (&) 7 kGy or ( ) 10 kGy: a and b represent the homogeneous subgroups generated by Tukey’s test, using the applied irradiation dose as the factor, at the 95% confidence level (pp0.05).
5.5 5.0 4.5 4.0
a a
a a
Score
3.5
a
3.0
b
a
2.5 b
2.0 1.5 1.0 0.5 0.0 Rancidity
Overall quality
Fig. 4. Scores given by the tasting panel for the attributes of rancidity and overall quality for the (’) control samples and those irradiated at (O) 3 kGy, (&) 7 kGy or ( ) 10 kGy: a and b represent the homogeneous subgroups generated by Tukey’s test, using the applied irradiation dose as the factor, at the 95% confidence level (pp0.05).
samples—for the latter, marked differences existed between samples receiving 3 or 7 kGy and those irradiated with 10 kGy. A prolonged storage of 5 months at 20 1C affected negatively all of the samples, since none of the sensorial attributes studied reached the maximum value of 5, considered optimal from the point-of-view of the consumer. In Fig. 3, the scores obtained for sweetness, color, and texture are given. However, the score was significant only for color measurement. The rancidity and overall appreciation behaved differently. Fig. 4 shows values for samples treated at doses of 3 or 7 kGy slightly lower (around 0.4) than for the control samples. The panel of tasters did not find perceptible differences among the three samples regarding the rancid flavor. On the contrary, the dose of 10 kGy showed a completely different profile, with very low values for rancidity (around 2) and overall
References Akingbohungbe, A. E. (1994). Pilot scale trials of irradiation preservation of maize, kola nuts and dried ash, and preliminary market test. Ile Ife, Nigeria: Obafemi Awolowo University (21pp.). Al-Bachir, M. (2004). Effect of gamma irradiation on fungal load, chemical and sensory characteristics of walnuts (Juglans regia L.). Journal of Stored Products Research, 40(4), 355–362. AOAC Method 991.43. (1995). Total, insoluble and soluble dietary fibber in food—Enzymatic–Gravimetric Method, MES-TRIS buffer. Official methods of analysis (16th ed.). Gaithersburg, MD: AOAC International. AOAC Method 950.46b. (2000). Moisture content, Official methods of analysis (17th ed.). Gaithersburg, MD: AOAC International. Baccaunaud, M. (1991). Effects l0 ionisation sur les fruits et le´gumes destine´s a´ la consommation en frais. In J. P. Vasseur (Ed.), L0 Ionisation des Produits Alimentaires (pp. 330–349). Paris, France: Lavoisier. El Assis, N., Huber, D. J., & Brecht, J. K. (1997). Irradiation induced changes in tomato fruit and pericarp firmess, electrolyte efflux and cell wall enzyme activity as influenced by ripening stage. Journal of the American Society for Horticultural Science, 122, 100–106. Grootveld, M., Jain, R., Claxson, A. W. D., Naughton, D., & Blake, D. R. (1990). The detection of irradiated foodstuffs. Trends in Food Science and Technology, 1, 7–14. Hallman, G. J. (1999). Ionizing radiation quarantine treatments against tephritid fruit flies. Postharvest Biology and Technology, 16, 93–106. Hoosmand, H., & Klopsfenstein, C. F. (1995). Effects of gamma irradiation on mycotoxin disappearance and amino acid contents of corn, wheat, and soybeans with different moisture contents. Plant Foods Human Nutrition, 47(3), 227–238. Inayatullah, H., Zeb, A., Ahmad, M., & Khan, I. (1987). Effect of gamma irradiation on physico-chemical characteristics of soybean. Nucleus Karachi, Pakistan, 24, 31–34. Kazantzis, I., Nanos, G. D., & Stavroulakis, G. G. (2003). Effect of harvest time and storage conditions on almond kernel oil and sugar composition. Journal of the Science of Food and Agriculture, 83, 354–359. Krumhar, K. C., & Berry, J. W. (1990). Effect of antioxidant and conditions on solubility of irradiated food proteins in aqueous solutions. Journal of Food Science, 55(4), 1127–1132. Lacroix, M., & Ouattara, B. (2000). Combined industrial processes with irradiation to assure innocuity and preservation of food products. Food Research International, 1–6. Lebovics, V. K., Gaal, O., Somogyi, L., & Farkas, J. (1992). Cholesterol oxides in gamma-irradiated spray-dried egg powder. Journal of the Science of Food and Agriculture, 60(2), 251–254. Lescano, G., & Narvaiz, P. (1992). Shelf life extension of sweet maize by gamma irradiation. La alimentacio´n latinoamericana, 192, 58–65. Lowry, O. H., Rosebrough, N. J., Farr, A. L., & Randall, R. J. (1951). Protein measurement with the folin phenol reagent. Journal of Biological Chemistry, 193, 265–275.
ARTICLE IN PRESS P. Sa´nchez-Bel et al. / LWT 41 (2008) 442–449 Madrid, R., Gabarro´n, M. J., Sa´nchez Bel, P., Valverde, M., & Romojaro, F. (2001). La calidad de la almendra espan˜ola. III Hidratos de carbono, Fibra y Composicio´n Mineral. Cuadernos de Fitopatologı´a, 69, 110–116. Rao, V. S., & Vakil, U. K. (1983). Effects of gamma-irradiation on flatulence-causing oligosaccharides in green gram (Phaseolus areus). Journal of Food Science, 48, 1791–1795. Sa´nchez-Bel, P., Martı´ nez Madrid, C., Egea, I., & Romojaro, F. (2005). Oil quality and sensory evaluation of almonds stored after electron bean proceeding. Journal of Agricultural and Food Chemistry, 53, 2567–2573. Scandella, D., & Foures, M. (1987). Effect de l0 ionisation sur le maintien de la qualite´ des le´gumes de quatrie`me gamme. Industries Agricoles et Alimentaires, 104, 911–916. Sendra, E., Capellas, M., Guamis, B., Felipe, X., Mor-Mur, M., & Pla, R. (1996). Revisio´n: Irradiacio´n de alimentos. Aspectos generales. Food Science and Technology International, 2, 1–11.
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Siddhuraju, P., Makkar, H. P. S., & Becker, K. (2002). The effect of ionising radiation on antinutritional factors and the nutritional value of plant materials with reference to human and animal food. Food Chemistry, 78, 187–205. St-Le`be, L., Raffi, J. & He´non, Y. (1982). Le traitement ionisant des denre´es alimentaires. Rapport R-5182. Commissariat a` l0 Energie Atomique. Thomas, P. (1984). Radiation preservation of food of plant origin. 2 Onions and other bulb crops. CRC. Critical Reviews in Food Science and Nutrition, 21, 95–136. UNE 87-004-79. (1995). Ana´lisis sensorial. Guı´a para la instalacio´n de una sala de cata. Asociacio´n Espan˜ola de Normalizacio´n y Certificacio´n. Madrid: Espan˜a. Van Soest, P. J., & Robertson, J. B. (1979). The detergent System of analysis and it application to human foods. EEC. Dietary Fiber. Von Sonntag, C. (1980). Free radical reactions of carbohydrate as studied by radiation techniques. Advances in Carbohydrate Chemistry and Biochemistry, 37, 7–11.
LWT 41 (2008) 442–449 www.elsevier.com/locate/lwt
Sensorial and chemical quality of electron beam irradiated almonds (Prunus amygdalus) Paloma Sa´nchez-Bela, Isabel Egeaa, Felix Romojaroa, M. Concepcio´n Martı´ nez-Madridb, a
Department of Food Science and Technology, CEBAS-CSIC, Apdo. 4195, 3100 Murcia, Spain Departamento de Agroquı´mica y Medio Ambiente, Escuela Polite´cnica Superior, Universidad Miguel Herna´ndez, Ctra. Beniel km 3.2, 03312-Orihuela, Alicante, Spain
b
Received 24 October 2006; received in revised form 20 March 2007; accepted 20 March 2007
Abstract This work reports the effects of irradiation with accelerated electrons (0, 3, 7, and 10 kGy) on the chemical composition (water content, proteins, neutral detergent fiber, sugars, lipid content, organic acids, and color) and sensorial properties (rancidity, sweetness, off-flavors and odors, texture, and whiteness) of the shelled almond variety Guara, packaged under air atmosphere and stored for 5 months at 2071 1C. Changes observed where a decrease for glucose in samples treated at all irradiation doses. An increase of citric acid, at doses above 3 kGy and then a decrease to values similar to those of the control was observed. With respect to the sensorial analysis, there was no treatment effect on the sweetness, texture or color but there was a marked rancidity in the samples treated with 10 kGy that decreased the overall appreciation of the samples. Irradiation doses of up to 7 kGy seem to be a suitable post-harvest sanitation treatment since they did not cause significant changes in the sensorial quality or in the contents of protein, fiber, water, or lipid with respect to the control samples, both following the treatments and after 5 months of storage. r 2007 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. Keywords: Almond; Irradiation; Sugars; Organic acid; Post-harvest quality
1. Introduction In recent years, numerous scientific studies have demonstrated the beneficial effects of the consumption of nuts for human health, these being an important component of a balanced diet due to their high nutritional value. In spite of their high fat content, they possess elevated levels of monoand poly-unsaturated fatty acids and large quantities of vitamin E and fiber. Also, they represent an interesting source of vegetable proteins thanks to their amino acid composition and to the iron, calcium, and oxalic acid content. According to the estimations of the FAO (1988), around 25% of world grain production is lost due to insects, bacteria, and rodents, and a similar percentage of nut production is contaminated by mycotoxins, the most commonly contaminated being peanuts and pistachios Corresponding author. Tel.: +34 966 749 696; fax: +34 966 749 619.
E-mail address: [email protected] (M.C. Martı´ nez-Madrid).
(Hallman, 1999). The various post-harvest procedures for control of insects and mites in stored products are chemical, biological and physical control or a combination of these techniques; food irradiation has been used for treatments of disinfestation (Hoosmand & Klopsfenstein, 1995), inhibition of sprouting (Thomas, 1984), delay of fruit ripening (Baccaunaud, 1991), and treatments of pasteurization and sterilization (Scandella & Foures, 1987). In some cases, it can replace the use of chemical additives, or be used in combination with them, and turns out to be effective in the treatment of nuts. The effects of the irradiation of foods cannot be generalized if the food and the dose absorbed by it are not specified (Lacroix & Ouattara, 2000). The effects of ionizing radiation on foodstuffs can be direct or indirect. In the first case, the radiation acts directly on the matter, causing the breakdown of molecules like DNA and other food constituents. In the second case, the radiation acts through water radiolysis and, hence, the different water radiolysis products will react with other
0023-6438/$30.00 r 2007 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2007.03.015
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compounds, causing significant chemical changes, like formation of hydroperoxide and free radicals (El Assis, Huber, & Brecht, 1997). The compounds induced by these free radicals are predictable from the composition of the food, and they can be formed by other chemical reactions (Grootveld, Jain, Claxson, Naughton, & Blake, 1990), or by enzymatic action (lipoxygenases and peroxidases) (Lebovics, Gaal, Somogyi, & Farkas, 1992). Consequently, the water content of the foodstuff is one of the most important factors determining the alterations brought about by the irradiation, in relation to oxidation of lipids and fatty acids (Lebovics et al., 1992) or degradation of vitamins and pigments (Grootveld et al., 1990). The changes effect by irradiation may be desirable, for example a decrease in the degree of polymerization of polyosids, a useful phenomenon since it reduces the cooking time for starch. A decrease in the percentages of certain polysaccharides responsible for flatulence has been described also (Rao & Vakil, 1983). The literature indicates that irradiation does not cause any significant change in the proximate composition of dried fruits and nuts (Akingbohungbe, 1994). Doses sufficient to kill all infesting insects had no adverse effects on either nutritional value or the sensory quality of dried fruits and nuts (Lescano & Narvaiz, 1992). In addition to possible changes in the nutritive quality, it is necessary to consider the sensorial quality following ionization treatments; in the case of almond, the consumer demands nutritive quality, a creamy-white color, an absence of vitreous zones, pleasant sweet and oily flavors, and a complete absence of rancid flavors and smells. Previous studies of irradiated almonds (Sa´nchez-Bel, Martı´ nez Madrid, Egea, & Romojaro, 2005) showed that irradiation doses of up to 7 kGy did not produce significant changes in the fatty acid composition, the lipid oxidation or the appearance of rancidity. Following on from this, the aim of the current work was to investigate the changes produced in other aspects of the chemical composition (proteins, water content, lipid content, neutral detergent fiber, organic acids, and sugar composition), the kernel color, and the sensorial properties (rancidity, sweetness, off-flavors and odors, texture, and whiteness) of the shelled almond (Prunus amygdalus) var. Guara, packaged in an air atmosphere and stored for 5 months at 20 1C, after being treated with irradiation doses of 3, 7 or 10 kGy.
443
process as the treated bags. After ionization, the batches were stored in controlled conditions (2071 1C and 75 g/ 100 g RH) for a period of 5 months, and periodic samples were taken after 7, 14, 21, 58, and 157 days. Each sample constituted three bags (sub-samples). 2.2. Ionization treatments Irradiation was carried out by the firm IONMED Esterilizacio´n S.A. (Taranco´n, Cuenca, Spain), using a Rhodotron circular electron accelerator (I.B.A., Belgium) at an energy level of 10-MeV. The protocol and treatment conditions were those suggested by the R+D Department of IONMED. Treatment lots were arranged as a monolayer in a transporting tape leading to the electron beam. The programmed irradiation doses were 3, 7, and 10 kGy; non-irradiated samples were separated as control lots. An adequate number of radiochromic dosimeters FTW-60.0 (Far West Technology, USA) were placed randomly in both faces of the bags in order to verify the real dose absorbed by the fruits and to study the penetration of the radiation. The absorbed dose was measured at 600 nm in a Ge´nesis-5 spectrophotometer (Spectronic, USA) with an uncertainty of DAbs of 0.006 for a level of confidence of 95%. The treatment characteristics are shown in Table 1. 2.3. Lipid extraction The fat was extracted in a 6-unit extractor (Det-Gras J.P. Selecta S.A., Barcelona, Spain), using petroleum ether (40–601) as extractant; in order to avoid fat oxidation during the extraction, the ether evaporation was carried out in a vacuum. The fat content was analyzed in duplicate in each sub-sample and the results expressed as grams of lipids per 100 g of fresh weight (g/100 g FW). 2.4. Moisture content This was determined by accurately weighing ground almond samples after heating them in an oven at 105 1C for 24 h (AOAC, 2000). Two measurements were carried out Table 1 Conditions applied during the irradiation treatments and the later readings of the dosimeters in order to verify the real absorbed dose Dose (kGy)
2. Material and methods
3
7
10
5 2.15 103
4.7 4.38 103
3.47 5 103
0.41370.002 3.270.17
0.72070.001 7.170.12
1.00370.003 10.370.4
2.1. Plant material and experimental design The shelled almond (P. amygdalus) variety Guara, without tegument, was supplied by Frutos Secos el Man˜an (Pinoso, Alicante, Spain). A total of 6 kg of almonds from the same harvest was packaged in 60 high-barrier plastic bags (100 g each), 45 of them being ionized at different doses (15 bags per dose) and another 15 separated as a control and later subjected to the same preservation
Speed of the belt (m/min) Intensity of the beam (mA) Width of the electron beam (cm) Optic abs of the dosimeter Real dose (kGy)
The variability of the real dose of irradiation absorbed by the samples was less than 1% of the programmed dose applied. The dosimeters also verified the homogeneity of the dose and validated the irradiation process.
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per sub-sample and the results expressed as percentage of moisture per 100 g of fresh weight (g/100 g FW). 2.5. Color The color was measured by reflectance in the chopped almond using a Minolta CR-300 colorimeter calibrated with a white standard. Duplicate measurements were made for each sub-sample and the results expressed as the color coordinate L, this being the one which best shows the variations in the white tonality.
mobile phase was c(H2SO4) ¼ 5 mmol/L at a flow rate of 0.4 mL/min. The detection wavelength was 210 nm for oxalic acid and 230 nm for citric, malic, and succinic acids. The sugar and organic acid concentrations in the tissue were obtained using calibration curves for each compound. Two extractions were performed for each bag or subsample and each extract was analyzed in duplicate. For sugars, the results are expressed in grams per 100 g of fresh weight (g/100 g FW) and for the organic acids in milligrams of acid per 100 g of fresh weight (mg/100 g FW). 2.9. Sensory test methodology
2.6. Protein content The content of globular proteins, which constitute 90% of the total proteins present in almond, was measured. For this, a de-fatted sample (0.5 g) was homogenized with 50 mL of water in a Polytrons (Kinema´tica AG, Switzerland) and then centrifuged for 20 min at 10,000 g (at 20 1C). For the resulting supernatant, protein quantification was performed by the method of Lowry, Rosebrough, Farr, and Randall (1951) modified according to the protocol accompanying the Biorad DC Protein Assay kit (Biorad Laboratories, Spain); absorbance was measured at 750 nm in a UVIKON 930 spectrophotometer (Kontron Instruments, Ltd.). The protein contents of the samples were calculated using a calibration curve obtained for bovine serum albumin standards (0–1.5 mg) treated in the same way. Two extractions were carried out per subsample and each sample was analyzed in duplicate, the results being expressed as grams of protein per 100 g of fresh weight (g/100 g FW). 2.7. Fiber The determination of the fiber content was performed for the de-fatted samples, according to the methods described in AOAC (1995). The neutral detergent fiber (NDF) was measured in duplicate for each sub-sample, following the method of Van Soest and Robertson (1979). The results are expressed as grams of NDF per 100 g of fresh weight (g/100 g FW). 2.8. Sugar and organic acids content Sugars and organic acids were extracted according to the procedure of Madrid, Gabarro´n, Sa´nchez Bel, Valverde, and Romojaro (2001). The extract was filtered through a Durapores 0.45-mm HV (Millipore Corporation, USA) membrane disk and then passed through a C18 Plus SepPak cartridge (Waters Corporation, Massachusetts, USA). Quantification was carried out by HPLC, using a Shimadzu HPLC machine (Kyoto, Japan) having an ion exchange column (ION 300, Teknochroma) and two detectors: a Shimadzu Refractive Index Detector (Kyoto, Japan), at 30 1C, for detection of sugars, and a Shimadzu UV–Vis detector for organic acids (Kyoto, Japan). The
Sensory rating was conducted by a selected and trained panel comprising of seven judges with some expertise in sensory tests. The evaluation was done using 5-point structured scales, 5 being the best and 1 the worst quality. In order to evaluate the capacity for examination and the sensitivity of the tasters, sucrose was used as standard for the sweet flavor, bitter almond for the bitter flavor and oxidized oleic acid for detecting rancidity. The overall quality attribute was assessed as the measurement of the acceptability of the product by the consumer, using a scale from ‘‘very unpleasant’’ (score of 1) to ‘‘very pleasant’’ (score of 5). The selection and training of tasters, as well as the fitting-out of the tasting room, were carried out according to UNE 87-004-79 (1995). 2.10. Statistics Data were analyzed using the General Linear Model of the SPSS (version 11.0) statistical package. Analysis of variance was conducted with irradiation dose and storage time as factors. When differences were significant (po0.05), multiple comparisons were made using Tukey’s test. 3. Results and discussion Under the storage conditions assayed in this study, the water content was 3.82–3.93 g/100 g, 3.54–3.89 g/100 g, 3.71–3.87 g/100 g, and 3.61–4.15 g/100 g for the control samples and those receiving, 3, 7 or 10 kGy, respectively (Table 2), whilst the respective lipid contents were 53.67–58.64 g/100 g, 52.15–55.98 g/100 g, 50.69– 55.74 g/100 g, and 53.20–56.03 g/100 g (Table 2). The analysis of variance, employing the irradiation dose and storage time as factors, showed that none of the differences were statistically significant, for either parameter. Previous work by other groups on these parameters in other nuts also showed no effect of irradiation treatments: Al-Bachir (2004), following irradiation of walnuts at 1, 1.5 or 2 kGy, not detected significant changes in the protein composition or the contents of lipid, water or ash, whilst Inayatullah, Zeb, Ahmad, and Khan (1987) did not detect significant changes in the contents of lipid, water or ash after irradiation of soybeans with doses of up to 5 kGy.
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Table 2 Qualitative characteristics of control and irradiated almonds during the storage time Dosea (kGy)
Storageb (days)
Moisture (g/100 g FW)
Darkness
Fat (g/100 g FW)
0
0 7 21 58 157
3.8270.09 3.8270.18 3.7770.14 3.8970.20 3.9370.38
96.8570.54 97.9070.04 95.6872.88 91.4772.52 86.8172.57
53.6771.00 52.4770.01 58.6474.24 54.4970.78 56.3170.54
3
0 7 21 58 157
3.8470.15 3.8970.22 3.7270.04 3.7970.04 3.5470.01
94.3471.36 93.8571.47 90.6570.42 89.5871.02 85.6471.20
53.4972.91 55.9873.61 55.1971.78 53.6572.69 52.1575.62
7
0 7 21 58 157
3.8770.03 3.7770.09 3.7170.04 3.7270.01 3.7670.13
93.8170.44 93.9370.20 90.3470.04 86.1472.16 89.8372.26
54.1973.00 51.7374.96 54.9171.30 55.7471.06 50.6970.75
10
0 7 21 58 157
3.9370.03 3.8470.08 3.7070.02 4.1570.05 3.6170.69
91.0375.76 90.3370.90 90.0172.12 88.3972.27 87.2470.25
55.1873.66 53.2070.62 56.0371.66 55.9472.32 55.2276.76
Ns Ns Ns
Ns * Ns
Ns Ns Ns
Irradiation dose Storage time Dosestorage a.b
Statistical differences were analyzed by ANOVA (pp0.05). Ns ¼ no significance.
However, the storage conditions have been found to affect the water content, particularly when assaying different temperatures and packaging atmospheres, which shows the need to optimize these conditions for each type of sample (Al-Bachir, 2004). In our study, the lipid and moisture contents of the samples were practically constant for all the treatments applied, and during the whole period of storage, supporting the idea that irradiation treatments do not influence these parameters (at least, at the doses applied) and suggesting also that the storage conditions assayed in our study were ideal for this plant material. On the other hand, the color of the samples showed a gradual darkening during storage, irrespective of the treatment which they received. The initial color values were 96.85 for the control and 94.34, 93.81, and 91.03, respectively, for the samples treated with 3, 7 or 10 kGy. These values remained virtually constant until the 8th week of storage; from this point there was a trend towards lower values until the end of storage, when the values were 86.81, 85.64, 89.83, and 87.24 for the control and the 3, 7, and 10 kGy treatments, respectively. The degree of darkening was of a similar magnitude for all the samples assayed, with no statistically significant differences (p40.05) being observed between treatments (Table 2). However, there were significant differences according to the storage time: irrespective of the treatment, the samples underwent a process of gradual darkening during the post-harvest storage, even though the conditions can be considered
optimal, which depended above all on the initial composition of the sample, this being a characteristic of the variety involved. The color change is fundamentally the result of respiration activity in combination with enzymatic reactions, which are the reason for sugar consumption— leading to the formation of colored reaction intermediates. Thus, the results are in agreement with the hypothesis, put forward by other authors, that the color variation of nuts after their harvest is a direct function of the starting plant material and of the specific growing conditions, such as the climatic conditions or vegetative cycle, and is independent of the treatments applied post-harvest and the subsequent storage conditions (Kazantzis, Nanos, & Stavroulakis, 2003). 3.1. Effect of irradiation on the fiber and protein content The fiber content of the samples remained relatively stable during the storage period; a value of 4.26–6.10 g/100 g for the control and 4.09–6.45 g/100 g, 4.04–5.72 g/100 g, and 3.80–6.04 g/100 g for the samples irradiated at 3, 7, or 10 kGy, respectively (Table 3). These values show that the irradiation doses used did not modify the fiber content, neither immediately following the treatments nor after a storage period of more than 5 months, at 20 1C. Regarding the protein content, there was a slight and not significant (p40.05) decrease following the treatments, for the three doses used; the values being 24.16–28.37 g/100 g for samples
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Table 3 Fiber and protein content of control and irradiated almonds during the storage time Dosea (kGy) Storageb (days) NDF (g/100 g) Protein content (g/100 g) 0
0 7 21 58 157
5.5870.91 4.8170.28 5.6470.80 6.1070.08 5.1370.45
24.1671.97 25.7672.87 27.1470.92 25.6871.60 28.3770.49
3
0 7 21 58 157
4.9870.25 6.4570.59 4.1770.72 5.2170.19 4.8770.37
21.5870.66 25.8871.97 23.4570.85 24.4973.09 24.7971.08
7
0 7 21 58 157
4.4370.82 5.4770.56 4.6970.11 4.7770.99 5.7270.49
23.7474.38 23.3872.69 25.0973.37 23.1572.45 21.8772.94
10
0 7 21 58 157
6.0470.44 5.0570.11 3.8070.62 4.0870.64 4.5770.92
20.4470.02 24.3074.50 25.3871.24 23.7773.68 23.3870.08
Ns Ns Ns
Ns Ns Ns
Irradiation dose Storage time Dosestorage
certain functional groups, like hydroxyl, and the formation of compounds such as gluconic acid and 2-deoxy-gluconolactone (Von Sonntag, 1980). The main products identified following the irradiation of sugars are acids, short-chain carbonyl derivatives, and modified sugars, including sugars with two carbonyl groups when dealing with samples in the presence of oxygen (Sendra et al., 1996). Fig. 1 shows the modifications in the sucrose and glucose contents after the irradiation and subsequent storage. All of the irradiated samples exhibited a decline in sucrose concentration, until the end of storage, whilst, from 2 months onwards, there was a stabilization in the control values (Fig. 1A). The irradiation treatments brought about an important decrease in the glucose content, independent of the dose applied (Fig. 1B). After this initial decline, the glucose level did not change significantly until day 71 of storage, from when there was a further drop for both irradiated and control samples. The analysis of variance for these results, employing irradiation dose and storage time as factors, showed significant differences in the glucose content among the irradiation doses of 3, 7, and 10 kGy. On the other hand, there were no significant differences with respect to the glucose and sucrose contents during the storage. Tukey’s test, when used to indicate the irradiation
3.2. Effect of irradiation on sugar content During the irradiation of sugars, the action of free radicals produced by water can bring about oxidation of
6
4
2
0 0.6 Glucose g/100 g FW
irradiated at 3 kGy, 21.58–25.88 g/100 g for those receiving 7 kGy, and 21.87–25.09 g/100 g at 10 kGy, compared with 20.44–25.38 g/100 g for the control samples (Table 3). However, when the analysis of variance was performed, these differences were not statistically significant, for the treatment or storage time or their interaction (dosestorage). The radiolytic compounds that can form, after an irradiation treatment, from the compounds already present in the foodstuff depend directly on the water content; because of this, when nuts or dehydrated materials are irradiated, the expected modifications are much less. But, the damage inflicted by irradiation on the peptide bond depends, in addition to the degree of hydration of the material, greatly on the oxygen content, since this bond is highly stable and is not normally broken by the irradiation doses generally applied to foodstuffs (Krumhar & Berry, 1990). Thus, given that the moisture content of the samples in this study was very low (3–4 g/100 g), it was to be expected that both the protein and fiber contents would remain largely unaltered throughout the assay, for all the treatments applied.
Sucrose g/100 g FW
a.b Statistical differences were analyzed by ANOVA (pp0.05). Ns ¼ no significance.
a
0.4
0.2
b 0.0 0
20
40
60
80 100 120 Days of storage
140
160
180
Fig. 1. Changes in the contents of (A) sucrose and (B) glucose during the storage period for the (K) control samples and those irradiated at (J) 3 kGy, (.) 7 kGy or (,)10 kGy: a and b represent the homogeneous subgroups generated by Tukey’s test, using the applied irradiation dose as the factor, at the 95% confidence level (pp0.05).
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dose effect, produced two sub-groups with means that were significantly different: the first comprised the control and the second the irradiation treatments (3, 7 or 10 kGy)— thus, there were no significant differences between the three doses (Fig. 1Ba and Bb). These results demonstrate the limited influence of the irradiation treatments on the sugar levels in the almonds. This could be explained by the low moisture content of this type of material, since it has been shown that the levels of radiolytic products originating from the sugars present in irradiated foodstuffs are much lower when the moisture content is low (St-Le`be, Raffi, & He´non, 1982). The irradiation of carbohydrates provokes the rupture of the ether bonds between hexose residues in high-molecularweight carbohydrates, as well as the dehydration of monosaccharides (Siddhuraju, Makkar, & Becker, 2002), so that the content of monosaccharides should rise as a result of this process. The fact that this did not happen could be due to their formation being compensated for by their conversion into dehydrated products such as furfurals, which were not measured in this study. 3.3. Effect of irradiation on the organic acids content The main organic acids found were citric, malic, oxalic, and succinic. Their concentrations, and the treatment effects, can be seen in Fig. 2. For oxalic acid (Fig. 2B),
there was little variation during the entire storage period, for all of the irradiation doses assayed; its concentration being around 12 mg/100 g FW. Malic acid showed an analogous behavior, with values around 5 mg/100 g FW (Fig. 2D). Contrary to this, citric acid had a different profile (Fig. 2A): for doses of 7 kGy or above, it showed an initial increase which was maintained for 3–4 weeks, before declining to reach values similar to those of the control and 3-kGy treatment. One notable point is the dramatic decrease in the succinic acid level (Fig. 2C), from the time of treatment and for all irradiation doses, such that in certain cases there were only trace amounts present. 3.4. Effect of irradiation on the sensory testing To ascertain whether the irradiation treatments affected the organoleptic properties of the almonds, a sensorial analysis was performed after 121 days of storage. The evaluation was made using 5-point structured scales, 5 being the best and 1 the worst quality. The quality attributes were assessed as the acceptability of the product by the consumer, using a scale from ‘‘very unpleasant’’ (score of 1) to ‘‘very pleasant’’ (score of 5). Figs. 3 and 4 show the numerical values assigned by the tasting panel to each of the attributes analyzed. In general, a decrease in quality was found, both for the control and the irradiated
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Fig. 2. Changes in the principal organic acids, (A) citric, (B) oxalic, (C) succinic, and (D) malic, present in the almond during storage, for the (K) control samples and those irradiated at (J) 3 kGy, (.) 7 kGy or (,) 10 kGy: a and b represent the homogeneous sub-groups generated by Tukey’s test, using the applied irradiation dose as the factor, at the 95% confidence level (pp0.05).
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appreciation (around 1), which are statistically different to the values obtained for control samples and those irradiated at 3 or 7 kGy.
4.5 a
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Acknowledgments
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This work was financed by the Spanish Ministry of Science and Technology (Ministerio de Ciencia y Tecnologı´ a) (Project ‘‘1FD 1997-1005-C04-01’’). The authors are grateful to the company IONMED Esterilizacio´n S.A. (Taranco´n, Cuenca, Spain), for the ionization of the samples, and to the OPFS ‘‘El Man˜an’’ (EP Pinoso, Alicante, Spain), for providing plant material.
2.0 1.5 1.0 0.5 0.0 Sweetness
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Fig. 3. Scores given by the tasting panel for the attributes of sweetness, color, and texture for the (’) control samples and those irradiated at (O) 3 kGy, (&) 7 kGy or ( ) 10 kGy: a and b represent the homogeneous subgroups generated by Tukey’s test, using the applied irradiation dose as the factor, at the 95% confidence level (pp0.05).
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2.0 1.5 1.0 0.5 0.0 Rancidity
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Fig. 4. Scores given by the tasting panel for the attributes of rancidity and overall quality for the (’) control samples and those irradiated at (O) 3 kGy, (&) 7 kGy or ( ) 10 kGy: a and b represent the homogeneous subgroups generated by Tukey’s test, using the applied irradiation dose as the factor, at the 95% confidence level (pp0.05).
samples—for the latter, marked differences existed between samples receiving 3 or 7 kGy and those irradiated with 10 kGy. A prolonged storage of 5 months at 20 1C affected negatively all of the samples, since none of the sensorial attributes studied reached the maximum value of 5, considered optimal from the point-of-view of the consumer. In Fig. 3, the scores obtained for sweetness, color, and texture are given. However, the score was significant only for color measurement. The rancidity and overall appreciation behaved differently. Fig. 4 shows values for samples treated at doses of 3 or 7 kGy slightly lower (around 0.4) than for the control samples. The panel of tasters did not find perceptible differences among the three samples regarding the rancid flavor. On the contrary, the dose of 10 kGy showed a completely different profile, with very low values for rancidity (around 2) and overall
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