Journal of Stored Products Research 59 (2014) 152e157
Contents lists avai
Journal of Stored Products Research
journal homepage: www.elsevier .com/locate/ jspr
Properties of walnut influenced by short time microwave treatmentfor disinfestation of insect infestation
Ipsita Das a, *, Narendra G. Shah b, Girish Kumar a
a Department of Electrical Engineering, Indian Institute of Technology, Bombay, Powai, Mumbai 400076, Indiab Centre for Technology Alternatives for Rural Areas, Indian Institute of Technology, Bombay, Powai, Mumbai 400076, India
a r t i c l e i n f o
Article history:Accepted 12 July 2014Available online
Insect infestation is the major cause of losses in many stored products such as grains, seeds, cereals andnuts. The use of chemicals to control insect infestation has widely been adopted but now is a cause ofconcern because of hazardous side effects. Alternative quarantine methods such as ionizing radiation,controlled atmosphere, conventional hot air treatment and dielectric heating have been suggested.Recent studies have indicated that microwave treatment is a potential means of replacing other tech-niques because of selective heating, absence of pollution in the environment, the achieving of equivalentor better quality retention and energy minimization. However improper application of microwaves cancause irreversible changes in the quality of the final product. The main objective of this research is toevaluate the effect of microwave power level and exposure time on the quality of walnuts. The qualityparameters studied were water activity, colour change, temperature rise, peroxide value (PV) and freefatty acid levels (FFA). It was found that microwave power level and exposure time significantly affectedcolour change and temperature rise. Untreated walnut kernels exhibited significantly higher (p < 0.05)peroxide and FFA values than treated walnut kernels. Microwave treatment to target temperatures of 50e55 �C (which is unfavourable for insect survival) made the PV of walnut drop down to between 1.35 and1.42 meqO2/Kg (from the initial value of 2.89 ± 0.048) and FFA value to 0.63e0.69% (from 1.08 ± 0.037).
The walnut is a highly appreciated nut because of its uniqueorganoleptic characteristics (Lopez et al., 1995) and good sources ofdietary fibre, various vitamins and minerals USDA, (2011). Walnutis being used as ingredient in a variety of bakery products such asbreads, muffins, cakes, biscuits and confectionery as well as fla-vouring agents in beverages and ice-cream. Walnut kernels have alipid content of 65% (Vanhanen and Savage, 2006) of which 7% issaturated, 20% is monounsaturated and 73% is polyunsaturatedfatty acids (Crews et al., 2005), although values do vary betweencultivars (Zwarts et al., 1999). Compared with most other nuts,which contain mostly monounsaturated fatty acids (MUFA), wal-nuts are highly enriched in omega-6 and omega-3 polyunsaturatedfatty acids (PUFA), which are essential dietary fatty acids (Amaralet al., 2003).
India had an annual walnut production of approximately 285thousand MT in 2011e12, and has exported 5300 MT of walnuts.
ail.com (I. Das).
Major export destinations (2012e13) of walnut from India areUnited Kingdom, Netherlands, Germany, United States, China,Australia and Taiwan.
(http://www.apeda.gov.in/apedawebsite/SubHead_Products/Walnuts.htm). Almost all countries have imposed a zero toleranceof insects on imported food products. One of the main problems inproduction, storage, marketing and exporting of dried fruits/nuts isthe loss caused by insect infestation. It is estimated that more than50% of the crop is lost annually due to pests and diseases in nuts(Haribabu et al., 1983). Some estimates put the loss of food grainsbecause of infestation at 40% of world production valued at billionsof dollars. Nuts suffer serious damage and loss because of insectinfestation during long term storage. Also, Nagaraja, (1998) re-ported that any bruises in the nuts make the fats liable to becomerancid, and thereby integrity of the kernels is lost.
The nut industry relies heavily on fumigation with methyl bro-mide (MeBr) and hydrogen phosphine for postharvest insect con-trol (Carpenter et al., 2000). Owing to the regulatory actions againstthe continued use of MeBr, widespread resistance to hydrogenphosphine and public concern over residues in treated products,there has been great interest in developing technically effective andenvironmentally sound quarantine methods, especially thermal
Table 1Experimental details (process parameters and their levels).
Factors Levels Variables
Microwave power (W) 240, 360, 480 �Temperature rise�Colour change�Peroxide value�Free fatty acid value
Exposure time (sec) 30, 60, 90, 120, 180, 240
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ones. An important key to develop successful thermal treatments isto balance needs for a complete kill of insects with a minimalthermal impact on product quality. A common difficulty in usingconventional hot air disinfestation methods is the slow heatingrate, non uniform temperature distribution, and possible heatdamage to heatesensitive commodities (Hansen, 1992; Tang et al.,2000; Wang et al., 2001). The irradiation process is an efficientmethod of disinfestation but users are reluctant to accept theprocess due to safety aspects associated with the irradiation andalso it is not available everywhere. Controlled atmospheres requirelonger treatment times and are not suitable for dealing rapidly withhigh levels of infestation. Recent studies have indicated that mi-crowave (MW) treatment is a potential means of replacing othertechniques as it provides several advantages, namely shortening oftreatment time, equivalent or better quality retention, energyminimization, saving in floor space and killing of micro-organisms.Microwave radiation with good penetrability can kill pests existinginside or outside grain kernels. However, in India there is currentlylittle emphasis on use of microwave heating technology for disin-festation applications.
Improper application of microwave energy can cause irrevers-ible changes in the quality of dried products. Microwave heattreatment of whole-kernels needs to be properly controlledbecause it does not only contribute to change in flavour and aromabut also to the colour of the nuts. Colour is an important qualityindicator of the heating process. The effect of heating conditions onchanges in colour were reported by several workers in their studieson peanuts (Cammerer and Kroh, 2009), hazelnuts (€Ozdemir et al.,2001; €Ozdemir and Devres, 2000), sesame seeds (Kahyaoglu andKaya, 2006) and macadamias (Wall and Gentry, 2007). Otherstudies have reported the effect of heating on nutritional compo-sition (Kashani and Valadon, 1983, 1984; Buranasompob et al.,2007; Kita and Figiel, 2007) and storage stability of pistachiosnuts (Raei et al., 2009; Nikzadeh and Sedaghat, 2008). The highlevels of polyunsaturated fatty acids make walnuts prone tooxidative and hydrolytic rancidity which is linked to the appear-ance of unpleasant odours and flavours (Watkins, 2005). Peroxidesare the product of oxidation of unsaturated fatty acids. Oxidativerancidity is enhanced by the presence of oxygen, increased tem-peratures and storage times. Hydrolytic rancidity, on the otherhand, results from enzymatic hydrolysis of triacylglycerols and therelease of free fatty acids (FFA). Free fatty acids contribute off-fla-vours in walnuts. Therefore, the peroxide value (PV) and free fattyacid content (FFA) of walnuts has also been selected as an indicatorof walnut quality.
In this paper, the effect of microwave (MW) power levels andexposure time onmoisture content, surface temperature rise, wateractivity, colour change, and composition (PV and FFA) of walnutkernel were studied and analysed using response surfacemethodology.
2. Methods
2.1. Materials and experimental set up
A domestic microwave oven (LG, Intellowave 3850w2G031A)with maximum output of 900 W at 2450 MHz was used for theexperiments. An outlet was provided on the left upper side of themicrowave oven to allow the removal of water vapour. The ovenwas fitted with a glass turntable (30 cm diameter) and had a facilityto adjust the microwave output power by 20% decrements and thetime of processing.
The shelled walnut kernels (Variety: Kashmir budded, Gradedesignation: Light half) procured from a local market were usedfor experimentation and graded by size to eliminate the variations
with respect to exposed surface area. About 200 g of fresh sampleswere taken for each experiment. A container made of poly-carbonate with provision to spread the samples uniformly wasplaced inside the microwave oven cavity for an even absorption ofmicrowave energy. The experiments followed a factorial design.Table 1 shows the experimental design parameters. The ranges ofexperimental parameters were selected based on preliminary tri-als. The independent variables considered were: microwave powerlevel (240, 360 and 480 W) and exposure time (30, 60, 90, 120,180, 240 s). The response functions (dependent variables) weretemperature rise (Tr), colour change (Cch), water activity (aw),peroxide value (PV), and free fatty acid content (FFA). Each treat-ment (power and exposure time combination) was replicated 3times.
2.2. Quality attributes
A standard hot air oven method (AOAC, 2002) was used todetermine the initial moisture content of walnut kernels. Nutswere first ground and then 2e3 g flour samples were placed inPetri-dishes and kept in a hot air oven at 80 �C for 24 h. Therewere three replicates for each measurement. The experimentswere conducted at microwave power level 3 and six levels ofexposure time respectively. For moisture loss analysis, fresh wal-nuts were exposed to different power levels and exposure times.The weight loss of sample with time for each run was recorded.The moisture content (% wb) was estimated using standardtechniques.
The water activity of samples was determined as a measure ofstorage stability using awater activity meter (Decagon Devices, Inc.,USA, Model No: CX-3TE) which had a dielectric humidity sensor tomeasure the water activity of a sample. Two replicates were madefor each sample and the mean of 3 readings taken for each replicatewas used for analysis.
The colour of the samples was measured with a Hunter LabColour meter (USA, Model No: Colour flex 45/0) and calculated interms of colour difference (Cch) (see Eq. (1)) which indicates thedegree of overall colour change of a sample in comparison to a freshsample, examining colour values of L*, a*, and b*. The less the Cchvalue, the closer it was to the untreated sample.
Cch ¼h�L � L*
�2 þ �a� a*
�2 þ �b� b*
�2i0:5(1)
Untreated walnuts were taken as the optimal sample having L*,a*, and b* values of 42.64, 6.55 and 14.20 respectively.
Surface temperatures of nuts weremeasured using a noncontactinfrared thermometer (DIT 130, range �32 to 380 �C, Germany). Amicrowavable rectangular box was made to hold 100 g of sample.The nut samples were immediately kept in the box after thetreatment and surface temperature was measured. Ambient roomtemperature (29 �C) was used as the initial sample temperature foreach test.
Peroxide value (PV) and free fatty acid content (FFA) of treatedsamples was carried according to AOAC standards, 1998 [965.33,940.28].
Table 2Values of different quality parameters of microwave (MW) pretreated walnut.
I. Das et al. / Journal of Stored Products Research 59 (2014) 152e157154
2.3. Storability study
At the end of each microwave treatment, samples were left onthe sample tray for 2e3 min to allow the heat to redistribute byconduction in nuts, a means of imparting added lethality, beforebeing transferred into air tight containers for the storability study.The treated nuts were analysed after 6 months of storage for theappearance of any infestation or change in composition. The stor-ability of fresh walnut kernels under ambient conditions was alsostudied.
Response surface method (RSM) was used to determine therelative contributions of independent variables (power level andexposure time) on various responses (temperature rise, colourchange and compositional analysis) under study. Since the func-tional relationship between the responses and factors was un-known, the second order polynomial expression (Eq. (2)) was usedto estimate the actual response surfaces. Design expert software(version 9, STAT-EASE, Inc., USA.15.1.1.0) was used for running theRSM and 3-D plot. The coefficients of the polynomial were repre-sented by b0 (constant term), b1 and b2 (linear effects), b11 and b22(quadratic effects), and b12 (interaction effects). The analysis ofvariance (ANOVA) tables were generated and the effect andregression coefficients of individual linear, quadratic and interac-tion terms were determined. The significance of all the terms in thepolynomial was judged statistically by computing the P value at 5%level of significance.
Y ¼ b0 þ b1Pþ b2Tþ b12 P Tþ b11 P2 þ b22 T2 (2)
3. Results and discussion
3.1. Moisture loss and water activity
The initial moisture content (IMC) of untreated walnut kernelswas found to be 3.7 ± 0.2%. The samples were subjected to differentmicrowave power levels and exposure times as mentioned inTable 1. It was observed that, with increase in exposure time (i.e.from 30 to 240 s), there was a decrease in moisture content from3.70% to 3.01%, 2.61% and 1.18% (wet basis) at 240, 360 and 480 Wrespectively (see Table 2). Figure 1 shows the moisture loss curvefor walnut. As expected, there was a decrease in moisture contentin nuts with increase in power level and treatment time. The wateractivity of fresh and treated walnut for different test conditions isalso listed in Table 2. The water activity was 0.32e0.45 and nosignificant difference was noted among the different treatments.Foods having awater activity between 0.3 and 0.5 are considered tobe shelf-stable dried foods (Raoult-Wack et al., 1991).
3.2. Colour change (Cch)
The values of colour change of treated walnut for differentoperating conditions ranged from 5.39 to 12.91 (Table 2). ANOVA(Tables 3 and 4) indicated linear terms of microwave power leveland exposure time and interaction of power and exposure timesignificantly affected the colour change, the overall effect being todarken the nuts (p� 0.05). The effect of quadratic termswere foundto be non-significant. By neglecting the non significant terms thefollowing equation (Eq. (3)) describes the effect of power level andexposure time on colour change.
Where Cch is the colour change; P is the microwave power level(W); T is the exposure time (sec).
The positive coefficients of the first-order terms of microwavepower level and exposure time (Eq. (3)) indicate that colour changeincreases with increase of these variables. The variation of colourchange with microwave power level and exposure time hasgraphically been presented in the 3-D plot (Fig. 2a). Coefficient ofdetermination R2 for this equation was 0.91 (a value > 0.75 in-dicates a good fit). Several other researchers (Sharma and Prasad,2006, Kumar et al., 2011) have also observed that other foodproducts darken when higher air temperatures and microwavepower levels are being used during microwave drying.
3.3. Temperature rise (Tr)
The measured values of temperature rise for different combi-nation of process parameters are presented in Table 2. The tem-perature varied between 37.6 and 107.3 �C within the combinationof the variables studied. It has been reported previously (White,1995) that temperatures between 50 and 55 �C are lethal for in-sects. The heating time was found to be 30e90 s for walnutsdepending upon the power level. The heating time is defined as thetime taken by nuts to reach the target temperature i.e. 50e55 �C. Asecond order polynomial equation (Eq. (4)) was fitted to theexperimental data and tested for adequacy through ANOVA.
Where Tr is the temperature rise (�C).The temperature of nuts was found to increase linearly with
increase in power level and exposure time. The experimental datafit the second order polynomial equationwell as indicated by a highR2 (coefficient of determination) value of 0.99 and COV value of4.15% (considered best fit if the value is less than 10%) (Kumar et al.,2011). The ANOVA result of the above quadratic regression equationfor temperature rise is described in Tables 3 and 4 The effect of boththe independent variables (linear terms, interaction term andquadratic terms of exposure time) was found to be significant at
Fig. 1. Moisture content with exposure time for various power levels.
I. Das et al. / Journal of Stored Products Research 59 (2014) 152e157 155
p � 0.05. Figure 2b shows the effect of process parameters ontemperature rise.
Table 4ANOVA for different quality attributes of microwave treated walnut.
Source of variation F-value p-value
a) Colour changeModel 36.78 <0.0001*Power (P) 75.39 <0.0001*Exposure time (T) 83.99 <0.0001*P � T 8.68 0.012*P2 0.43 0.524T2 1.26 0.284b) Temperature riseModel 164.57 <0.0001*Power (P) 191.07 <0.0001*
3.4. Compositional analysis of walnut
Walnut contains substantial quantities of polyunsaturated fattyacids, and thus is susceptible to oxidative and hydrolytic ranciditywhich in turn, produces undesirable volatile compounds and off-flavours and limits the shelf life. Chemical analyses for assessingoxidative rancidity and hydrolytic rancidity include PV and FFA. Ithas been reported that when PV and FFA of walnut kernels are lessthan 3.0 meqO2/Kg of oil and 1.0% respectively, it indicatesacceptable quality for walnuts (Buranasompob et al., 2003).
The values of PV and FFA for untreated (Fresh) walnut wererelatively high at the start of our experiment (PV of2.89 ± 0.048 meqO2/Kg of oil, FFA of 1.08 ± 0.037%). After the mi-crowave exposure, the value of PV and FFA varied from 1.13 to 1.51and from 0.3 to 0.81 for the range of conditions studied. Both PVand FFA decreased with increase in power level and exposure time.The effect of microwave power level and exposure time on PV andFFA are illustrated in Fig. 2c, d. ANOVA analyses (Tables 3 and 4)indicated that both microwave power level and exposure time(linear terms) had a significant effect on PV and FFA values(p � 0.05). Microwave treatment to target temperatures of50e55 �C gave a PV of 1.35e1.42 meqO2/Kg of oil and FFA value
Table 3Regression coefficients of polynomial equations developed for different qualityattributes of microwave treated walnut.
0.63e0.69%. The peroxide values and FFA values of microwavetreated walnut kernels increased after 6 months of storage at 25 �C(see Table 5) though the values are within the limits for acceptablequality. However, an increase in PV indicates formation of hydroperoxides and the onset of oxidative rancidity (Van der Merwe,2003). Inactivation of lipase and also removal of moisture athigher exposure time and power levels are responsible for thelower degree of FFA and PV development in walnuts whichincreased shelf life. We could store treated walnuts for 6 months atroom temperature without any infestation and adverse effect onquality. In contrast, fresh walnut samples were found to be heavilyinfested at the end of 1.5 months of storage, with a rancid smell.
Exposure time (T) 626.54 <0.0001*P � T 9.54 0.0094*P2 0.29 0.5990T2 8.63 0.0124*c) Peroxide valueModel 43.79 <0.0001*Power (P) 27.7 0.0002*Exposure time (T) 178.66 <0.0001*P � T 1.69 0.2174P2 0.18 0.6812T2 8.59 0.0126*d) Free fatty acidModel 113.45 <0.0001*Power (P) 46.26 <0.0001*Exposure time (T) 485.05 <0.0001*P � T 0.030 0.866P2 0.57 0.490T2 40.58 <0.0001*
*p-value less than 0.050 indicates model terms are significant.
Fig. 2. 3-D Plots of effect of microwave power and exposure time on (a) colour change, (b) temperature rise, (c) peroxide value and (d) free fatty acid.
Table 5Compositional analysis of walnut after treatment and 6 months of storage.
I. Das et al. / Journal of Stored Products Research 59 (2014) 152e157156
I. Das et al. / Journal of Stored Products Research 59 (2014) 152e157 157
4. Conclusion
The effect of short time microwave heat treatment with powerlevels of 240e480 W and exposure times of 30e240 s on themoisture content, water activity, colour change, temperature rise,and compositional analysis (PV and FFA values) of walnut kernelswas presented using response surface methodology (RSM). In-creases in microwave power level and exposure time caused anincrease in temperature and colour change and decrease in PV andFFA levels. The water activity ranged from 0.32 to 0.45 underdifferent combinations of variables and no significant statisticaldifference was noted among the treatments. Untreated walnutkernels exhibited significantly higher peroxide values and FFAvalues than the treated walnut kernels. The peroxide values andFFA values of microwave-treated walnut kernels increased after 6months of storage at 25 �C though the values were within the limitsfor acceptable quality. Our experiments have shown that heatingwalnuts with microwave energy to temperatures previouslyreported to be lethal to insects has no significant effect on thedevelopment of rancidity and also increases shelf life.
Acknowledgement
The authors acknowledge the Department of Science andTechnology (DST), India (SR/WOS-A/ET-111/2011), for providingfinancial support for this study.
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