The objective of this work was to evaluate the performance of various edible coatings for preserving the quality of walnut kernels (W) during storage. Three edible coatings based on carboxymethyl cellulose (CMC), methyl cellulose (MC) and whey protein isolates (WP) were prepared. Coated and uncoated walnuts (WC) were stored for 210 days at room temperature (23±2 °C). After 210 days, WC presented the highest peroxide value (PV = 3.06 meqO2/kg), conjugated dienes (CD = 3.01) and trienes (CT = 0.31), pentanal, nonanal, hexanal, and decane, 5,6-bis (2,2-dimethylpropylidene) contents. Meanwhile, WMC showed the lowest PV (1.20 meqO2/kg), CD (2.26) and CT (0.17) and the lowest decrease in carotenoid content (0.60 mg/kg). The L* value measured in walnut oil decreased in all samples. MC, CMC and WP coatings showed protection on walnuts against the deterioration process. MC coating displayed the best performance.
The walnut (
Product stability depends on the preservation of quality parameters during storage and is a key issue when determining shelf-life and distribution logistics. Previous studies have demonstrated that the sensory, microbiological and chemical quality of different types of foods and their raw materials can be preserved by applying an edible coating. Edible coatings are a type of biodegradable active packaging material that can extend shelf-life and enhance the functional properties of food products by improving the stability of their lipids (Gayol
Fresh butterfly walnut kernels (Chandler variety) harvested in April, 2016 were provided by Nogales S.R.L, Argentina. The walnuts were stored (warehouse) at room temperature until they were bought in May, 2016. Then, they were stored in the freezer (-20 °C) until the beginning of the experiment.
Carboxymethyl cellulose (CMC) (Parafarm ®) and methyl cellulose (MC) (Parafarm ®) were obtained from SAPORITI S.A.C.I.F.I.A. (Buenos Aires, Argentina), and whey protein isolates (WP) were purchased from Todo Droga (Córdoba, Argentina) to prepare the coatings.
CMC coating: A 0.5% (w/v) solution of CMC was prepared and glycerol was added as a plasticizer (1.9%, w/v) (Grosso
MC coating: A 2% (w/v) solution of MC was prepared and glycerol was added as a plasticizer (1.9%) (Grosso
WP coating: A 11% (w/w) solution of WP was prepared and 11 g glycerol were added to the solution (Grosso
Walnuts were immersed in containers with the corresponding coating solution (CMC, MC, and WP) for 5 minutes. After that, they were removed and put in a strainer for 5 minutes to let the excess solution drip off. Finally, the coated walnuts were placed at room temperature under hood for 24 h to dry off the excess moisture. The final moisture contents (%) in the walnut kernels after the drying process were: 4.05 in WC, 4.52 in WCMC; 5.94 in WMC and 4.24 in WWP. Four treatments were prepared using unshelled walnuts: uncoated walnuts (WC, control sample); and coated walnuts with CMC (WCMC), MC (WMC), and WP (WWP).
The walnuts (WC, WCMC, WMC, and WWP) were stored in 15 x 25 x 5 cm plastic containers (Tupperware, Buenos Aires, Argentina) at room temperature (23±2 °C) under normal atmosphere conditions (20-21% O2; 60-70% RH) for 210 days, which are the usual conditions for storing these products. Each container was loaded with 1 kg of walnut kernels. The samples were analyzed for physical-chemical parameters (peroxide value, carotenoid content, conjugated dienes and trienes and 4 volatile compounds) at 0, 35, 70, 105, 140, 175 and 210 storage days. The experimental design consisted of 4 treatments (WC, WWP, WCMC, and WMC) x 3 replicates of each treatment x 10 chemical variables x 7 periods of time. The physical variable (color) was determined at the beginning and at the end of storage.
These analysis were performed on walnut oil. The oil was obtained by cold pressing 20 g of walnuts, using a 20-ton press (HE-DU, Hermes I. Dupraz S.R.L., Córdoba, Argentina). Peroxide value was measured according to AOAC (2010). Conjugated dienes and trienes were measured at 232 nm and 268 nm, respectively (COI, 2001). Carotenoid contents were analyzed at 470 nm following the procedures described by Mosquera
The volatile compound analysis of the walnut samples was performed in walnut kernels by headspace solid phase microextraction fiber (HS-SPME) according to Martin et al. (
Color measurements were taken on the oil samples on a white background with a Minolta colorimeter (Minolta CM-508d, Tokyo, Japan). The CieLab parameters (L*, a* and b*) for each sample were measured at least in five random positions. The measurements were performed at 3 different periods of time: 0, 105, 210 days.
Three replicates of the experiment were made. The data were analyzed using InfoStat software, version 2016p. A two-way analysis of variance (factors: ‘treatment’ and ‘time’) and LSD-Fisher was used to detect significant differences among treatments (ANOVA, α = 0.05). Pearson coefficients were calculated to establish correlations among dependent variables. A principal component analysis (PCA) was performed on the correlation matrix of normalized data. The objective of the PCA analysis was to explore associations among treatments and variables. A Cluster Analysis (CA) was carried out to obtain groups of walnut treatments with similar characteristics. Sample similarities were calculated using the Euclidean distance, and groups of walnut treatments with similar characteristics were obtained using the unweighted pair-group method (UPGMA).
The changes in the PV and CD and CT of WC, WCMC, WMC and WWP are illustrated in
(a) Peroxide value (meq O2/kg), (b) Conjugated trienes (E1%, K268), (c) Conjugated dienes (E1%, K232), (d) Carotenoid content (mg/kg) in walnut samples coated with carboximethyl cellulose (WCMC), methyl cellulose (WMC) and whey protein (WWP); and walnuts without coating (WC) analyzed during 210 days of storage (n = 3; α = 0.05).
The CD is related to the formation of hydroperoxides, conjugated dienes and carboxylic compounds, while CT reflects the concentrations of secondary oxidation products formed from the initial compounds detected at 232 nm (Ancin Azpilicueta and Martínez Remírez,
(a) Hexanal, (b) Pentanal, (c) Nonanal, (d) Decane, 5,6-bis (2,2-dimethylpropylidene) measured in electronic counts in walnut samples coated with carboximethyl cellulose (WCMC), methyl cellulose (WMC) and whey protein (WWP), and walnuts without coating (WC) analyzed during 210 days of storage (n = 3; α = 0.05).
Carotenoids are compounds that play an important role in the oxidative stability of walnuts because they can act as antioxidants (Özrenk
In the present study, different volatile compounds were detected in walnut kernels by GC/MS analysis.
Volatile compounds (electronic counts 106) per gram of walnuts coated with carboximethyl cellulose (WCMC), methyl cellulose (WMC) and whey protein (WWP), and walnuts without coating (WC) analyzed in fresh product (storage day 0).
Volatile compound | Treatment | |||
---|---|---|---|---|
WC |
WCMC |
WMC |
WWP |
|
Cyclopropyl carbinol | 42.22±5.78 a | 49.29±0.73 a | 49.73±3.25 a | 46.40±4.97 a |
Pentanal | 22.62±3.59a | 44.22±4.55b | 56.93±7.73b | 34.27±1.45a |
Butanal | 56.23±4.40 b | 40.83±3.36 a | 44.87±1.83 a | 50.46±3.28 ab |
Nonane | 99.08±9.90 a | 107.54±4.69 a | 109.54±6.74 a | 125.14±9.72 a |
Tridecane | 82.52±5.61 a | 88.78±1.17 a | 85.12±7.33 a | 77.14±1.50 a |
Tetradecane | 1005.19±103.91 a | 1037.93±28.97 a | 1061.90±128.28 a | 958.26±60.88 a |
Pentadecane | 973.34±46.06 a | 1096.18±74.28 a | 919.53±10.06 a | 1035.50±97.56 a |
Hexadecane | 1340.96±2.65 a | 1611.12±83.74 a | 1457.32±111.56 a | 1354.51±120.74 a |
Pentadecane, 2-6-10-14-tetramethyl | 54.70±9.77 a | 50.58±4.75 a | 44.89±2.92 a | 49.27±4.41 a |
Dodecane, 2,6,10-trimethyl- | 362.98±55.88 a | 420.24±26.98 a | 377.04±12.37 a | 384.61±18.52 a |
Decane,5,6-bis(2,2-dimethylpropylidene) | ND | ND | ND | ND |
Benzene, 1-methyl-2-(1-methylethyl)- | 295.43±57.90 a | 292.79±30.65 a | 261.32±20.61 a | 264.78±4.81 a |
Benzothiazole | 39.26±17.74 ab | 75.34±14.48 a | 64.43±6.96 b | 90.05±13.12 b |
Phenol, 2,6-bis(1,1-dimethylethyl)-4-methyl-, methylcarbamate | 124.89 a | 199.30±5.75 b | 138.08±11.43 a | 145.48±11.67 a |
Oxalic acid | 35.77±4.21 a | 3.502±1.75 a | 30.79±2.38 a | 32.21±1.17 a |
Oxalic acid, allyl pentadecyl ester | 106.29±7.49 a | 125.97±6.52 a | 102.30±6.51 a | 112.81±8.59 a |
N-Thio-valero-morpholine | 351.97±33.29 a | 356.50±31.21 a | 277.33±31.18 a | 299.98±18.01 a |
3-Cyclohexen-1-ol, 4-methyl-1-(1-methylethyl)- | 148.04±20.35 a | 135.19±2.55 a | 132.32±13.91 a | 135.63±14.31 a |
Sulfurous acid, cyclohexylmethyl hexyl ester | 743.95±24.09 a | 888.97±69.30 a | 691.59±108.10 a | 775.98±69.26 a |
ND = Not detected
Mean values ± standard deviations followed by different letters in each row indicate significant differences at α = 0.05 (n = 3, LSD Fisher).
Another compound that showed intriguing behavior was decane,5,6-bis (2,2-dimethylpropylidene) (
Walnut color has a direct relationship with pigment and phenolic contents which are contained mainly in the skins of the seeds (Christopoulos and Tsantili,
A PCA was conducted in order to understand the behavior of the different treatments with respect to their ability to preserve the oxidative stability of walnuts during storage. The biplot obtained from the first two principal components (PC) in the PCA is presented in
Bi-plots of principal component analysis. Independent variables: peroxide value (PV), conjugates dienes (CD), conjugated trienes (CT), hexanal, nonanal, pentanal, decane, 5,6-bis (2,2-dimethylpropylidene)-, a*, b*, L* and carotenoid content. Treatments: walnuts coated with carboximethyl cellulose (WCMC), methyl cellulose (WMC) and whey protein (WWP), and walnuts without coating (WC).
These associations were confirmed by the correlation analysis (Pearson coefficients). Pentanal and hexanal showed the strongest correlation of all the volatile compounds analyzed, represented by a coefficient of 0.93. Pentanal and hexanal were also highly correlated with nonanal with coefficients of 0.86 and 0.84, respectively. Decane, 5,6-bis (2,2-dimethylpropylidene) was most strongly correlated with hexanal (0.86). Larrauri
The results from the CA, which consider the dependent variables studied, are presented as a dendogram (
Dendrogram from cluster analysis of walnuts coated with carboximethyl cellulose (WCMC), methyl cellulose (WMC) and whey protein (WWP), and walnuts without coating (WC) considering physical and chemical variables measured during storage.
In general, the use of carboxy methyl cellulose, methyl cellulose and whey protein edible coatings on walnuts helps to prolong the shelf-life of the kernels by preserving their chemical and physical quality properties. The use of methyl cellulose coating lessens the deterioration of walnut kernels. Therefore, the use of edible coatings, especially those containing methyl cellulose, can be used in the food industry as an alternative method to prolong the overall quality and shelf-life of walnuts.
This research was supported by Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) and Secretaría de Ciencia y Tecnología (SECYT-UNC).