Journal of Food Engineering 101 (2010) 273–280
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Journal of Food Engineering
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Infrared heating for dry-roasting and pasteurization of almonds
Jihong Yang a,b, Gokhan Bingol c, Zhongli Pan b,c,*, Maria T. Brandl d, Tara H. McHugh c, Hua Wang a a Northwest Agricultural and Forestry University, Yangling, Shaanxi 712100, China b Biological and Agricultural Engineering Department, University of California, One Shields Avenue, Davis CA 95616, USA c Processed Foods Research Unit, Agricultural Research Service, US Department of Agriculture, 800 Buchanan St., Albany CA 94710, USA d Produce Safety and Microbiology Research Unit, Agricultural Research Service, US Department of Agriculture, Albany CA 94710, USA article info abstract
Article history: The use of infrared (IR) heating for improving the microbial safety and processing efficiency of dry- Received 2 February 2010 roasted almonds was investigated. Almonds were roasted at 130, 140 and 150 °C with three different Received in revised form 19 April 2010 methods: IR roasting, sequential infrared and hot air (SIRHA) roasting, and traditional hot air (HA) roast- Accepted 4 July 2010 ing. The heating rate and pasteurization efficacy of almonds under different roasting methods and tem- Available online 8 July 2010 peratures were evaluated. Pediococcus sp. NRRL B-2354 was used as a surrogate for Salmonella enterica Enteriditis PT 30 for evaluating the pasteurization efficacy of different processing methods and condi- Keywords: tions. When SIRHA roasting at 130, 140 and 150 °C roasting temperatures was used to produce medium Almond roasted almonds, 4.10-, 5.82- and 6.96-log, bacterial reductions were achieved with 38%, 39% and 62% Dry-roasting Hot air time saving compared to HA roasting at each temperatures, respectively. The decimal reduction time Infrared of the bacteria at all roasting temperatures were calculated for SIRHA roasting as 8.68, 3.72 and Pasteurization 1.42 min, respectively, with a correlation coefficient greater than 0.92 and the thermal resistance con- Salmonella stant was found as 25.4 °C. The total color change followed zero-order reaction kinetics and the activation energies were 73.58, 52.15 and 67.60 kJ/mol for HA, IR and SIRHA roasting, respectively. No significant difference (p > 0.05) was observed in sensory quality of medium roasted almonds processed with differ- ent roasting methods. We conclude that the SIRHA roasting is a promising new method for the produc- tion of dry-roasted pasteurized almonds. Published by Elsevier Ltd.
1. Introduction take 30–40 and 10–15 min, respectively, to obtain a light to med- ium roasted product (Anon, 2007b). There are two major concerns Almonds are the California’s largest tree nut crop in total dollar related with HA roasting processes. Firstly, they might not be able value and acreage. In 2007, California almond growers produced to meet pasteurization requirement (Anon, 2007c; Centrella, 2007; 1.377 thousand million pounds of almonds (Anon, 2008a; Deng Issacs et al., 2005; Marks et al., 2007) and secondly, they necessi- et al., 2007). However, outbreaks of Salmonella enteritidis was asso- tate relatively long processing times, thereby increasing processing ciated with almonds and a total of 168 cases were reported in the costs. Therefore, there is a need to develop new processing meth- past several years (Danyluk et al., 2006; Issacs et al., 2005; Lapsley, ods that can produce roasted almonds while meeting the pasteur- 2005; Perren, 2008). In order to improve the safety of almond con- ization requirements and reducing processing time. sumption, the US Department of Agriculture and the Almond Board Infrared (IR) radiation is an energy in the form of electromag- of California (ABC) have mandated that all processed almonds sold netic wave and is more rapid in heat transfer than convection on the US domestic market must be treated to achieve a 4-log and conduction mechanisms. IR heating has been found to be more reduction of Salmonella enterica population sizes. (Anon, 2007a, effective compared to conventional heating (Hebbar et al., 2004; 2008b). Pan et al., 2008; Shi et al., 2008; Zhu and Pan, 2009). Huang Currently, the almond industry uses hot air (HA) roasting pro- (2004) used IR heating for surface pasteurization of turkey frank- cesses with temperatures ranging from 129.5 °C (265°F) to furters that were contaminated with potentially fatal Listeria mon- 154.5 °C (310°F). At the lowest and highest temperatures, it may ocytogenes and showed that 4.5-log reduction in population size of the pathogen could be achieved in 103 s. By using decimal reduc- tion time, James et al. (2002) predicted that 6-log reduction of S. * Corresponding author at: Biological and Agricultural Engineering Department, enteritidis could be achieved on the exterior of eggs with a 30-s University of California, One Shields Avenue, Davis CA 95616, USA. Tel.: +1 530 752 4367; fax: +1 530 752 2640. IR exposure without adversely damaging the contents. In a E-mail address: [email protected] (Z. Pan). previous study (Brandl et al. 2008), we investigated the effect of
0260-8774/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.jfoodeng.2010.07.007 274 J. Yang et al. / Journal of Food Engineering 101 (2010) 273–280
IR heating on S. enteritidis population sizes on raw almond kernels center of the kernels were measured using T-type thermocouples and concluded that IR heating technology has the potential to be (TT-T-36-SLE, Omega Engineering Inc., Stamford, CT, USA; response used as an effective non-chemical and dry treatment for decontam- time0.15s).Datawererecordedusingadatalogger(HH147,Omega ination of this product. Engineering Inc., Stamford, CT, USA). The reported surface tem- Besides shortening the duration of roasting, infrared roasted perature profiles are the average temperatures of top and bottom products have been reported to have superior sensory and chemi- surface of the almonds. All experiments were replicated three cal qualities compared to conventional roasting. Park et al. (2009) times. demonstrated that physicochemical qualities of green tea can be increased using infrared roasting. Kumar et al. (2009) reported that 2.2.1. Hot air roasting method IR roasted groundnuts had better product quality compared to For HA roasting, a custom-built laboratory scale forced convec- drum- and sand-roasted samples and that the time required for tion hot air oven was used. The dimension of the roasting oven was IR roasting was 33% and 60% less compared to sand- and drum- 50 50 50 cm (length width height). The stainless steel roasting, respectively. frame of the oven was insulated with glass wool to minimize heat It has been stated that for many roasted products the color loss. Ten gram of almonds were spread in a single layer on a stain- changes are mainly related to non-enzymatic browning since the less steel wire mesh and a T-type thermocouples (TT-T-36-SLE, enzymes are denatured due to high temperature (Demir et al., Omega Engineering Inc., Stamford, CT, USA; response time 0.15 s) 2002; Kahyaoglu and Kaya, 2006; Özdemir and Devres, 2000). Ozd- was placed on the mesh wire close to almonds and was connected emir and Devres (2000) observed that there were significant differ- to an on–off temperature controller. The air in the oven was heated ences between the L*- and b*-values of ground hazelnuts without and blown by a Leister CH-6060 hot air gun (Leister Process Tech- skin and skins of the whole nuts. Demir et al. (2002) later con- nologies, Kaegiswil, Switzerland) that was connected to the firmed that the color response of nut without skin was higher than controller. the whole kernel with skin since the nut-meat is more sensitive to temperature change during roasting. Therefore, ground almond 2.2.2. Infrared roasting method powder was used to monitor L*a*b* color components in this study. A laboratory scale double-sided IR heating device (Catalytic Based on the capabilities of IR heating to pasteurize and dry- Industrial Group Inc., Independence, KS, USA) (Fig. 1) was used to roast with superior quality, IR technology could be a good candi- conduct IR roasting tests. The equipment had two emitters with sur- date for dry-roasting and pasteurization of almonds. Thus, the face area of 30 60 cm for each emitter. The almond samples were objectives of this study were (i) to determine the appropriate IR placed on a metal screen tray (15 15 cm) located 35 cm from the heating conditions to minimize the roasting time during IR and upper emitter and 38 cm from the lower emitter. The average IR SIRHA processes, and (ii) to evaluate and compare the pasteuriza- intensity was approximately 5000 W/m2, which was measured tion efficacy of the IR, SIRHA and HA roasting methods. using an Ophir FL205A Thermal Excimer Absorber Head (Ophir Optronics Inc., Wilmington, MA, USA). The equipment was operated 2. Materials and methods by computer software (Testpoint, Capital Equipment Corp., Billerica, MA) to control temperature and collect temperature data. 2.1. Almonds 2.2.3. Sequential infrared and hot air roasting method Almonds of the variety Nonpareil with size 27/30 CPO (counts Although a lower intensity infrared radiation will increase the per ounce) were provided by the Almond Board of California (Mod- temperature of the almonds more slowly than higher radiation esto, CA). Raw almonds, which had not been subjected to any pre- intensity, the objective during the SIRHA process was to heat the al- vious pretreatments, were used for roasting tests in this study. monds as rapid as possible to target temperature and then maintain However, to assure that the almonds had a low background bacte- the temperature with hot air. Therefore, a pilot scale double-sided rial load, almonds that were previously pasteurized with Polypro- IR heating device (Catalytic Industrial Group Inc., Independence, pylene Oxide (PPO) (with no detectable PPO residue) were used for KS, USA) with high radiation intensity (approximately 11,000 W/ 2 pasteurization tests. The almonds were sorted to remove any dam- m ) was used. The device has four emitters, two at the top and aged kernels and then were stored in plastic bags at 4 °C before two at the bottom, with a total heating area of 270 61 cm. When being used for experiments. The initial moisture content of raw al- placed on the metal screen tray (15 15 cm), the almond sample monds was 4.6% (w.b.), the average weight of raw almonds was was located at 16 and 12.5 cm from the top and bottom emitters, 1.04 ± 0.07 g and average dimensions of the raw almond kernels respectively. Once the almonds reached the desired roasting were 7.8 ± 0.4, 12.3 ± 0.3, and 22.2 ± 1.2 mm in thickness, width, temperatures, they were transferred to the laboratory hot air roast- and length, respectively. er in less than 10 s.
2.2. Roasting methods Catalytic IR Emitter Gas Flow Regulator Value Natural Gas In To determine the roasting efficiency and pasteurization efficacy of the three roasting methods, roasting temperatures convention- Thermometer Data Logger ally practiced in the industry, namely, 130, 140 and 150 °C, were selected. The roasting temperatures refer to the almond surface Almond Samples temperature for IR and SIRHA roasting and the air temperature for HA roasting. At the end of each roasting process, almonds were Metal Screen Tray naturally cooled to ambient temperature and then were stored in Ziplock bags at 4 °C for quality evaluations. All experiments were done in triplicate. Data Acquisition For evaluating the temperature profiles of almonds with roast- ing by three different heat sources, the temperatures of the kernels underneath the skin at both the top and bottom surfaces and at the Fig. 1. Diagram of IR heating system for almond roasting. J. Yang et al. / Journal of Food Engineering 101 (2010) 273–280 275
2.3. Determination of degree of roasting the samples were allowed to reach equilibrium temperature at ambient temperature. Roasting significantly alters almond kernel color and this is fre- In order to recover Pediococcus sp. cells from control and treated quently used in the industry as an indicator to specify the desired almonds, 10 almond kernels were placed in 20 ml Butterfield’s degree of roasting (light, medium or dark). The color of roasted al- phosphate buffer (BPB) in a stomacher bag containing a filter mem- monds was measured using a Minolta Chroma Meter CR-200 brane (Sterile Two-Chamber Filter Bag, Labplas Inc., Ste-Julie, Can- (Minolta Co., Ltd., Ramsey, New Jersey, USA) in CIE L*a*b* color ada). The almonds were processed in a pulsifier (PUL 100, Microgen space. Ten almond kernels were ground (Waring Commercial Hea- Bioproducts Ltd., Surrey, UK) at the highest power for 1 min to dis- vy-Duty Blender 38BL19L, Waring Laboratory and Science, Torring- lodge the bacterial cells from the almonds according to the proce- ton, CT, USA) and 8 g of ground sample was put into a 5-cm dure described by Wu et al. (2003). The resulting suspension was diameter plastic Petri dish. The reported color values are the aver- diluted and plated with a spiral plater (Autoplate 4000; Spiral bio- age values of 5 readings for each sample. The total color difference, tech Inc., Norwood, MA), or plated undiluted by hand, onto TSA. DE, was calculated using the following equation: The plates were incubated overnight at 37 °C and the colonies were qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ÀÁÀÁÀÁ counted for measurement of bacterial population sizes. The mean 2 2 2 DE ¼ Lf L0 þ af a0 þ bf b0 ð1Þ value of the population size of Pediococcus sp. on the almonds was calculated. Each experiment was done in triplicate. The subscripts f and 0 denote roasted and raw almonds, respectively. 2.5. Microbial inactivation kinetics Since there is no standard value for color of roasted almonds, to quantitatively determine the degree of roasting of commercial pro- The following equation was used to describe the survival curve: duces, commercial samples marketed as light, medium and dark roasted almonds were obtained from a commercial producer and N t ¼ 10 D ð5Þ used as the references to assess the color characteristics, DE, com- N0 puted with the Eq. (1). The light, medium and dark roasted samples where N is the surviving Pediococcus sp. populations at a given time had DE values of 5.73, 11.5 and 21.4, respectively. (CFU/g), N is the initial Pediococcus sp. population (CFU/g), D is dec- The color change of browning reactions is generally zero-order 0 imal reduction time (min) at a given roasting temperature and t is or first-order (Demir et al., 2002; Lopez et al., 1997; Krishnamurthy the roasting time (min). et al., 2008; Özdemir and Devres, 2000; Vikram et al., 2005). Total The D-value is dependent on temperature which is directly cor- color change, DE, was chosen as a representative of degree of roast- related with the thermal resistance constant, z, and can be calcu- ing in this study. A general reaction rate equation for a browning lated as follows (Singh and Heldman, 2001): reaction in terms of total color change can be formulated as follows: T T dDE z ¼ 2 1 ð6Þ ¼ kDEn ð2Þ log D log D dt T1 T2 where t is the roasting time (min); k is the rate constant for total where DT1 and DT2 are decimal reduction times at roasting temper- color change; and n is the order of reaction rate. Since there is no atures at T1 and T2. definable first-order reaction rate for total color change due to 2.6. Sensory quality evaluation DE0 being as zero, the zero-order model, given in Eq. (3) was used to describe the color data. Because medium roasted almonds are major products in the al- n ¼ 0 zero-order : DE ¼ k t ð3Þ mond industry, the medium roasted almonds produced by different The effect of temperature on non-enzymatic browning reaction methods at different temperatures were evaluated for sensory quality is usually described by the Arrhenius equation (Özdemir and in this study. The sensory attributes were evaluated by a panel of 90 Devres, 2000; Chen and Ramaswamy, 2002): untrained panelists. The panelists were asked to rate the samples for flavor, texture, appearance and overall quality on a 9-point hedonic Ea scale anchored at the endpoints from ‘‘non-preferred’’ to ‘‘preferred’’. k ¼ k0 expð Þð4Þ RT The almonds with HA roasting at 130 °C were used as a reference; each where k is the reaction rate constant of non-enzymatic color degra- attribute of the reference sample was given five points. dation, k0 is frequency constant, Ea is the activation energy (kJ/mol), R is the universal gas constant (8.314 kJ/mol K) and T is the absolute 2.7. Statistical analysis temperature (K). The results were expressed as mean ± standard deviation (SD) 2.4. Microbial inactivation for independent experiments performed in triplicate, except the sensory analysis wherein 90 independent observations were ob- The non-pathogenic bacterium Enterococcus faecium strain NRRL tained, and the data were analyzed by Analysis of variance (ANOVA) 6 B-2354 (labeled as Pediococcus sp.) was obtained from the National followed by post hoc Duncan’s multiple range tests (p 0.05) when Center for Agricultural Utilization Research (NCAUR), US Depart- appropriate (SAS software version 9.2, SAS Institute, USA). All tests ment of Agriculture and was used as a surrogate of SE PT 30 in this were randomized to obtain independent observations. study. This strain was identified and validated by the Almond Board of California (ABC) Technical Expert Review Panel as appropriate for 3. Results and discussion quality control in almond thermal processes (Anon, 2007b). Pediococcus sp. was cultured to stationary phase in tryptic soy 3.1. Temperature of almonds during dry-roasting broth (TSB) on a rotary shaker at 35 °C, subcultured on tryptic soy agar (TSA), and then inoculated onto almonds according to The roasting process was conceptually divided into two parts: the procedure reported by the ABC (Anon, 2007c). The inoculated an initial heating period, where the almond’s surface temperature almonds were stored at 4 °C in a plastic container. Prior to roasting, rose to the target roasting temperatures, and the holding period, 276 J. Yang et al. / Journal of Food Engineering 101 (2010) 273–280 wherein the almonds were kept at the respective roasting results in dissipation of energy directly into heat and also the total temperatures. heat flux provided by hot air is lower than that provided by infra- During the initial heating period of HA roasting, the average red heat (Ginzburg, 1969). Datta and Ni (2002) reported that even temperature difference between the almond surface and center at temperatures higher than those used herein, hot air provided an was 6 ± 2.6 °C, where the temperature differences between surface initial heat flux of approximately 2760 W/m2 which, unlike IR, de- and center had parabolic nature with heating time at all roasting creases gradually as the material heats up and is still lower than temperatures. The initial heating period took 680, 850 and 890 s the heat flux provided by IR in this study. to reach the target roasting temperatures of 130, 140 and 150 °C, We observed that during the holding period of IR roasting, there respectively (Fig. 2). It should be noted that during the HA holding was a 5–7 °C difference between the almond surface and center period, the temperature difference between the kernel surface and temperatures. However, as mentioned above, the temperature dif- center was negligible. ferences during holding period of HA roasting was negligible. The initial heating times to reach the target roasting tempera- Therefore, the initial heating period with IR heating was followed tures of 130, 140 and 150 °C during IR roasting were 104, 124 with HA holding period, namely SIRHA roasting, to increase the and 155 s, respectively. Contrary to the logarithmic heating charac- uniformity of temperature. Furthermore, to accelerate the initial teristics of hot air, within the temperature range of this study, the heating period, equipment generating higher IR intensity was used. temperature increase during IR heating was almost linear, which Reaching the intended roasting temperatures of 130, 140 and resulted in more than 80% time reduction for roasting (Fig. 3). This 150 °C required only 39, 44 and 53 s, respectively (Fig. 4). This pro- is due to the ability of IR radiation to penetrate the food materials vided up to 94% time reduction during the initial heating period
150
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130
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70 Surface Temperature (150 ) 60 Center Temperature (150 ) Temperature (°CTemperature ) 50 Surface Temperature (140 ) 40 Center Temperature (140 ) Surface Temperature (130 ) 30 Center Temperature (130 ) 20
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0 0 100 200 300 400 500 600 700 800 Hot air roasting time (sec)
Fig. 2. Temperature profiles of almonds during HA roasting.
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Surface Temperature (150 ) 80 Center Temperature (150 )
Temperature (°C Temperature Surface Temperature (140 ) 60 Center Temperature (140 ) Surface Temperature (130 ) 40 Center Temperature (130 )
20 0 100 200 300 400 500 600 Infrared roasting time (sec)
Fig. 3. Temperature profiles of almonds during IR roasting. J. Yang et al. / Journal of Food Engineering 101 (2010) 273–280 277
Fig. 4. Temperature profiles of almonds during SIRHA roasting. compared to HA heating. Demir et al. (2002) reported that when time increased, L* values decreased and a* values increased thus, the initial heating period is relatively short, the majority of quality resulting in greater DE values. changes occur during the holding period. Therefore, it can be as- During the initial heating period, SIRHA roasting generated the sumed that during SIRHA roasting, the majority of the quality fastest increase in DE values which could be due to the fastest ini- changes occurred during the holding period under hot air. tial heating rate. During the holding period, although the temper- Our findings suggest that IR can be installed in front of the reg- ature profiles of IR and SIRHA were similar, it was observed that ular almond roasting equipment used in the industry to heat the DE values of IR roasted almonds increased faster than SIRHA at almonds to desired roasting temperatures quickly to reduce the all roasting temperatures. The almond industry hypothesized that processing time. the Maillard reaction, which occurs between the carbonyl in a reducing sugar and an amine in proteins or amino acids, causes 3.2. Roasting degrees of almonds browning in almonds (Pearson, 1999). Rosenthal (1992) also found that the principal absorption bands of hydroxyl group of sugars The total color difference, DE, of roasted almonds obtained un- and nitrogen–hydrogen group of proteins are in the far infrared der different processing conditions are shown in Fig 5. The results (FIR) range. Therefore, it is possible that the formation of amadori showed that the roasting temperature and time increased DE value compounds was more extensive during the holding part of IR of almond kernel similar to Özdemir and Devres (2000) and Uysal roasting than that of SIRHA roasting due to the likely interaction et al. (2009) who showed that as the roasting temperature and of FIR with the above-mentioned chemical groups. This may have
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Total color change ( Total IR, 130 IR, 140 8 IR, 150 SIRHA, 130 SIRHA, 140 4 SIRHA, 150
0 0 1020304050607080 Roasting time (min)
Fig. 5. Total color change (DE) under different roasting conditions. 278 J. Yang et al. / Journal of Food Engineering 101 (2010) 273–280 enhanced the extent of the Maillard reaction and increased the 3.4. Effectiveness of pasteurization browning rate. This finding is also in line with the fact that at all roasting temperatures, the curves of HA and SIRHA roasting be- The reductions in Pediococcus population size on almonds ob- came parallel as the roasting continued. tained with various roasting degrees under different conditions Almonds reached to the medium roast level after 34, 18 and are tabulated in Table 2. For all roasting methods, the Pediococcus 13 min during HA roasting at 130, 140 and 150 °C, respectively population size on kernels decreased significantly (p < 0.05) as (Fig. 5). This is consistent with roasting in the industry, where a the degree of roasting increased. Also, within the same degree of medium roasted product is obtained by roasting for approximately roasting, the Pediococcus population size on almonds decreased 55 and 15 min at the lowest (129.4 °C) and at the highest temper- as the temperature increased despite the shorter roasting times re- atures (154.4 °C), respectively. However, for IR roasting, the roast- quired at elevated temperatures. Although not significant ing times were 11, 6 and 4 min and were the shortest among the (p > 0.05), an exception was the slight decrease of Pediococcus pop- three methods for obtaining the same degree of roasting. To com- ulation reduction when the roasting temperature was increased pare with the conventional HA industrial roasting, IR and SIRHA from 140 to 150 °C under IR roasting; this could be due to very roasting provided time reductions up to 68%, 67%, 69%, and 38%, 39%, 62% at 130, 140 and 150 °C, respectively. These time reduc- tions will further reduce the energy usage which is in accord with Hebber et al. (2004) wherein approximately 55% energy reduction was obtained by using IR rather than hot air during drying of pota- to and carrot.
3.3. Reaction rate and Arrhenius dependence
At all roasting temperatures the DE values were fitted to a zero- order model and are presented in Table 1. For all of the roasting methods, the reaction rate obtained from the zero-order model fol- lowed the Arrhenius equation with a correlation coefficient greater than 0.98. The activation energies, Ea, for non-enzymatic browning reactions in foods generally range between 37.6 and 167.2 kJ/mol
(Villota and Hawkes, 2007). The values of Ea computed in our study were 73.58, 52.15 and 67.60 kJ/mol for HA, IR and SIRHA roasting, Fig. 6. Log10-population sizes of Pediococcus during the HA holding period of the respectively. SIRHA roasting process.
Table 1 Zero-order kinetic parameters for total color change of almonds during roasting with different methods.
1 2 1 2 Roasting methods Temperature (°C) k (min ) R Ea (kJ/mol) k0 (min ) R HA roasting 130 0.318 0.960 73.58 1.13 109 0.986 140 0.601 0.956 150 0.896 0.948 IR roasting 130 1.109 0.989 52.15 6.29 106 0.998 140 1.567 0.917 150 2.315 0.921 SIRHA roasting 130 0.392 0.929 67.60 2.34 108 0.980 140 0.718 0.931 150 1.015 0.810
Table 2 Pediococcus population size reduction on roasted almonds with various roasting degree and under various roasting conditions.
Roasting methods Roasting temperature (°C) Roasting degree Light Medium Dark HA roasting 130 1.58 ± 0.18Aa 3.58 ± 0.70Bab 7.87 ± 0.68Cbc 140 1.89 ± 0.15Aab 4.62 ± 0.33Bcd ZCCab 150 1.96 ± 0.23Aab 5.39 ± 1.12Bed ZCCa IR roasting 130 2.19 ± 0.09Ab 2.94 ± 0.29Ba 3.56 ± 0.06Ce 140 2.91 ± 0.20Acd 3.21 ± 0.26Aab 4.35 ± 0.27Bd 150 2.82 ± 0.29Ac 4.12 ± 0.38Bbc 4.53 ± 0.12Bd SIRHA roasting 130 3.17 ± 0.51Acde 4.10 ± 0.36Bbc 7.45 ± 0.45Cc 140 3.33 ± 0.30Aed 5.82 ± 0.34Be 7.41 ± 0.10Cc 150 3.59 ± 0.17Ae 6.96 ± 0.51Bf ZCCa
The initial average of Pediococcus sp. population was 8.55 ± 0.14-log cfu/g. Values within the same row followed by different uppercase letters are significantly different at p < 0.05. Values within the same column followed by different lowercase letters are significantly different at p < 0.05. ZC: zero-plate count. J. Yang et al. / Journal of Food Engineering 101 (2010) 273–280 279
Table 3 Sensory quality evaluation on medium degree of roasted almonds.
Temperature (°C) Method Appearance Texture Flavor Overall 130 HA 5.77 ± 1.41 5.30 ± 1.15 5.20 ± 1.58 5.37 ± 1.38 IR 5.80 ± 1.79 5.30 ± 1.80 4.63 ± 1.85 5.07 ± 1.80 SIRHA 5.53 ± 1.33 5.30 ± 1.32 5.57 ± 1.77 5.57 ± 1.38 140 HA 5.07 ± 0.58 5.07 ± 1.48 5.10 ± 1.63 5.20 ± 1.45 IR 5.47 ± 1.11 5.30 ± 1.24 4.90 ± 1.60 5.13 ± 1.48 SIRHA 5.17 ± 0.91 5.20 ± 1.06 5.07 ± 1.46 5.13 ± 1.31 150 HA 5.27 ± 1.23 5.37 ± 1.38 5.23 ± 1.59 5.23 ± 1.41 IR 5.03 ± 1.19 5.10 ± 1.24 4.57 ± 1.68 4.80 ± 1.35 SIRHA 5.30 ± 0.92 5.07 ± 0.94 4.97 ± 1.56 5.27 ± 1.20 P-value 0.11 0.98 0.39 0.73
short roasting times to produce light roasted almonds. Table 2 also 130, 140 and 150 °C, respectively, with a z-value of 26.1 °C(Anon, shows that none of the roasting methods met the pasteurization 2007b). requirement of 4-log cfu/g reduction when almonds with a light roasting degree were produced. 3.6. Sensory quality evaluation on medium degree of roasted almonds For medium roasted almonds, all three roasting methods met the pasteurization requirement at the highest roasting tempera- Sensory attributes such as appearance, texture, flavor and over- ture; however, a 4-log pasteurization was achieved at the lowest all acceptance for medium roasted kernel samples were evaluated roasting temperature only with SIRHA roasting. These findings using hedonic scale and are presented in Table 3. Application of are in line with Marks et al. (2007) who observed over 8-log reduc- two-way ANOVA showed that there were no significant differences tion of Pediococcus population sizes at 148.9 °C with a roasting (P > 0.05) for any of the attributes between different roasting time of 15 min. methods and roasting temperatures. This is contrary to the sensory It is interesting to note that at all roasting degrees the smallest findings of Kumar et al. (2009) who observed that higher roasting Pediococcus population size reduction was obtained with IR heat- temperatures (190–200 °C) scored lower than the lower roasting ing. This could be due to rapid surface moisture removal with IR temperatures (160–180 °C) for the same sensory attributes evalu- compared to hot air. It is known that heat resistance of microbial ated in this study. The sensory evaluation of almonds roasted with cells is influenced by many environmental factors, such as water three different methods indicates that IR technology did not impart activity (aw) of the heating medium. Generally, a reduction in aw any undesired flavor, texture or appearance to the almonds that increases the heat resistance of microbial cells (Sumner et al., could be detected by panelists or at an industrial scale by the
1991). A low aw of the medium had a protective effect against ther- consumers. mal stress in Salmonella typhimurium and enhanced the survival of the pathogen 100-fold (McCann et al., 2009). These findings are 4. Conclusion also in accord with our previous results, which showed that pre- wetting almond kernels before IR exposure provided an additional Among the three roasting methods tested, we conclude that 0.43-log reduction of SE PT 30 N population size despite a lower SIRHA roasting both reduced the processing time and was the only kernel temperature due to evaporative cooling (Brandl et al. roasting method that met the pasteurization requirements at all 2008). This is also in synch with the above-mentioned Maillard roasting temperatures for medium or higher degree roasted al- reaction phenomena, where low moisture contents are favorable monds. The zero-order reaction model described the total color for its occurrence. change data with satisfactory correlation coefficients for all roast- Results in Table 2 also reveal that over 4-log reduction in Pedio- ing methods at all roasting temperatures. Therefore, we suggest coccus population size was achieved at medium and dark roasting that the greatest benefit of IR technology in industrial almond degrees at all roasting temperatures when almonds were initially roasting would be to install the IR heaters in front of the regular al- heated with IR and then held with hot air (SIRHA). It is important mond roasting process in order to heat the almonds to desired to consider that high temperature roasting is generally not favored roasting temperatures rapidly, thus reducing the processing time, by the industry because it may reduce the shelf life of roasted and then continue roasting the kernels with hot air. products. Therefore, our study suggests that SIRHA roasting is superior to IR and HA roasting methods since even at lower roast- Acknowledgments ing temperatures, mandated reductions in bacterial population sizes were achieved, in addition to reduced roasting time. We thank the Almond Board of California for its partial financial support. We also appreciate the technical assistance from Mr. Don- ald Olson. This research was funded by the Almond Board of Cali- 3.5. Thermal inactivation kinetics of Pediococcus sp. fornia and by USDA CRIS projects 5325-4100-060-00D and 5325- 42000-044-00D. The experiments were conducted at the Western Thermal steady-state and isothermal conditions were achieved Regional Research Center of the USDA-ARS and in the Department only during the holding period of SIRHA roasting (Fig. 6). Therefore, of Biological and Agricultural Engineering, University of California, to be able to use Eq. (5) to calculate decimal reduction time, D-va- Davis. lue, the microbial inactivation data for the holding period of SIRHA was used. From the plot of the inactivation data, presented in Fig. 6, References the D-values at 130, 140 and 150 °C were calculated as 8.68, 3.72 and 1.42 min, respectively, with a correlation coefficient greater Anon, 2007c. Guidelines for process validation using Enterococcus faecium NRRL B- than 0.92. The z-value was then calculated as 25.4 °C. The D- and 2354, Almond Board of California. Available from:
Anon, 2007b. Guidelines for validation of dry roasting processes. Almond Board of Lopez, A., Pique, M.T., Boatella, J., Romero, A., Ferran, A., Garcia, J., 1997. Influence of California. Available from: