Walnut and almond kernels are highly nutritious mainly due to their high oil contents. In this study, 32 factorial experimental designs were used to optimize processes for oil extraction by screw-pressing at industrial scale. Experimental designs included seed moisture content (SMC), and restriction die (RD) as the main processing parameters. Theoretical models were scanned against experimental data in order to optimize oil extraction conditions. The response variables analyzed were oil yield (OY), fine solid content (FC) in oil, and oil quality parameters. Fitted models for OY indicated maximum predicted values similar to the highest experimental values. Walnut oil extractions showed a maximum OY (84.5 ± 2.3 %) at 7.21% SMC, and 10 mm RD. For almond kernels, maximum OY (71.9 ± 3.5%) was obtained at 9.42% SMC, and 12 mm RD. Chemical quality parameters from both oils were in the ranges stated in Codex (FAO/WHO) standards for virgin (non-refined) oils.
Walnuts (
Both walnut and almond kernels contain high levels of oil (52 – 70 and 48 – 67%, respectively) (Martínez
Walnut and almond oils are produced at small scale mainly in France, Spain, Argentina, and the USA. They are used as salad dressing and in cosmetic industries as skin cream components. Traditionally, these oils have been extracted by pressing, either using a screw press or a hydraulic press. Generally, oil extraction by screw-pressing has some advantageous over hydraulic-pressing including higher oil yields and the possibility of continuous or semi-continuous processes. This latter feature makes the extraction a promising procedure for its application at industrial scale. They also provide a simple and reliable technology for processing small batches of oilseeds, yielding oils with good chemical quality, which is highly dependent on the process conditions (Wiesenborn
It is important to note, however, that screw-pressing performance strongly depends on the raw material conditioning methods. These consist of a number of unit operations such as cracking, cooking, flaking, drying or moistening (Wiesenborn
Using screw-pressing at pilot plant scale, the highest oil recoveries from both walnut and almond kernels were obtained at similar moisture contents (Martínez
Walnut (
The kernels were milled and sieved through an automatic sieve (EJR 2000 Zonytest®), and particles between 1.43 - 4.76 mm were selected. These sifted materials were conditioned by water-sprinkling according to Singh and Bargale (
Oil extraction was carried out in a single step with a Komet screw press (Model DD85G, IBG Monforts, Mönchengladbach, Germany). The effective and total length and the internal diameter of the press barrel were 6.7 cm, 14.7 cm and 5.60 cm, respectively. The length and diameter of the screw were 19.5 cm and 5.3 cm, respectively. All extractions were performed at 35 – 40 °C, at a screw speed of 20 rpm. The amount of sample pressed in each run was 1.50 kg. The screw press was first run with heating for 15 min without seed material to raise the screw-press barrel temperature to the desired temperature. Then, in order to reach the steady-state operation, 3.00 kg of material was allowed to pass through the press before sampling. Running temperature was constantly monitored with a digital thermometer (TES Thermometer 1307 Type K) inserted into the restriction die. Pressing was carried out by using three different restriction dies (10, 12, 14 mm for walnut; 12, 13, 14 mm for almond). After each run, all press devices were cleaned and dried.
The oil yield (OY) was calculated considering the initial oil content in the incoming material (walnut or almond sifted materials) and the residual oil content in the pressed cake. It was expressed as g extracted oil/g oil present in the incoming material x 100 (g/100 g oil). Oil contents from both sifted materials and pressed cakes were determined by extraction (10 h) using Soxhlet devices, with
The screw-pressed oil samples were centrifuged at 11.000 g for 30 min. The precipitated solids were recovered, washed with
Free fatty acid content (FFAC), peroxide value (PV), and specific extinction coefficients (K232 and K270) were quantified according to standard methods of AOCS (2009).
Fatty acid composition was analyzed by gas chromatography according to procedures reported earlier (Maestri
The oxidative stability index (OSI) was determined by the Rancimat (Metrohm, Herisau, Switzerland) method (Cd 12b-92 AOCS, 2009) using 3 g oil aliquots. The air flow rate was set at 20 L/h, and the temperature of the heating block was maintained at 110 ºC. Results corresponded to the break points in the plotted curves and were expressed as induction time (in hours).
Response surface methodology (RSM) was used to model and optimize the conditions of oil extraction for walnut and almonds fruits (Montgomery,
The results were analyzed by a multiple regression method. The experimental results were applied to obtain the regression models. The quality of the model fitness was evaluated by ANOVA. The fit of the model to the experimental data was given by the coefficient of determination, R2, which explains the extent of the variance in a modelled variable that can be explained with the model. Only models with high coefficient of determination were included in this study.
All determinations were performed at least in duplicate, randomly, and replicas of the central point were done to allow estimation of pure error as square sums. For walnut oil extraction, two different designs were carried out in order to define the adequate extraction conditions. For almond oil extraction, a single experimental design was done due to limitations in the physical behavior of the raw material and the screw press used. Under certain moisture contents the press was obstructed.
Statistical analyses were performed using Statgraphic Plus software (v5.1. USA). For model validation, the response variable (OY,
For walnut oil extraction, an experimental design of 13 treatments (5 central points) was conducted in the first instance using the factors and levels previously described (
First industrial scale 32 factorial design. Effect of process variables on walnut oil yield and quality parameters
Assay | Factors |
OY | FC | PV | FFAC | K232 | K270 | OSI | |
---|---|---|---|---|---|---|---|---|---|
X1 | X2 | ||||||||
1 | 5.50 | 14 | 55.6 ± 0.1 | 15.1 ± 0.1 | 0.94 ± 0.23 | 0.12 ± 0.05 | 0.16 ± 0.01 | 1.94 ± 0.02 | 2.16 ± 0.09 |
2 | 3.00 | 14 | 44.8 ± 0.1 | 11.2 ± 0.1 | 1.05 ± 0.22 | 0.07 ± 0.02 | 0.16 ± 0.01 | 1.94 ± 0.01 | 2.12 ± 0.07 |
3 | 8.00 | 12 | 67.5 ± 0.1 | 13.6 ± 0.1 | 0.70 ± 0.08 | 0.06 ± 0.05 | 0.16 ± 0.01 | 1.96 ± 0.02 | 2.21 ± 0.01 |
4 | 5.50 | 10 | 73.3 ± 1.4 | 13.1 ± 1.1 | 0.81 ± 0.21 | 0.06 ± 0.01 | 0.17 ± 0.01 | 2.13 ± 0.10 | 2.17 ± 0.05 |
5 |
5.50 | 12 | 56.3 ± 0.1 | 14.8 ± 0.1 | 0.65 ± 0.01 | 0.07 ± 0.01 | 0.16 ± 0.01 | 1.93 ± 0.01 | 2.26 ± 0.03 |
6 |
5.50 | 12 | 57.3 ± 1.6 | 13.9 ± 1.1 | 0.70 ± 0.22 | 0.08 ± 0.01 | 0.16 ± 0.01 | 1.94 ± 0.01 | 2.26 ± 0.07 |
7 |
5.50 | 12 | 55.1 ± 0.2 | 14.7 ± 0.1 | 0.82 ± 0.24 | 0.12 ± 0.01 | 0.16 ± 0.01 | 1.95 ± 0.01 | 2.27 ± 0.09 |
8 |
5.50 | 12 | 56.8 ± 0.1 | 15.1 ± 0.1 | 0.55 ± 0.16 | 0.14 ± 0.01 | 0.16 ± 0.01 | 1.94 ± 0.01 | 2.29 ± 0.01 |
9 |
5.50 | 12 | 55.5 ± 0.1 | 14.2 ± 0.1 | 0.50 ± 0.14 | 0.10 ± 0.01 | 0.16 ± 0.01 | 1.97 ± 0.01 | 2.19 ± 0.10 |
10 | 8.00 | 10 | 80.4 ± 0.9 | 11.5 ± 1.4 | 0.92 ± 0.17 | 0.07 ± 0.01 | 0.19 ± 0.01 | 2.23 ± 0.10 | 2.23 ± 0.02 |
11 | 8.00 | 14 | 63.0 ± 0.1 | 14.2 ± 0.1 | 0.93 ± 0.08 | 0.17 ± 0.01 | 0.15 ± 0.01 | 1.98 ± 0.01 | 2.17 ± 0.05 |
12 | 3.00 | 10 | 45.7 ± 1.0 | 12.6 ± 1.1 | 1.60 ± 0.29 | 0.06 ± 0.01 | 0.18 ± 0.01 | 2.09 ± 0.02 | 2.20 ± 0.08 |
13 | 3.00 | 12 | 44.9 ± 1.0 | 10.6 ± 0.1 | 0.76 ± 0.15 | 0.07 ± 0.01 | 0.17 ± 0.01 | 2.01 ± 0.03 | 2.28 ± 0.09 |
X1, seed moisture content (g/100 g seed, WB); X2, restriction die (mm); OY, oil yield (g/100 g oil); FC, fine solid content in oil (g solids/100 g extract); PV, peroxide value (meq/kg oil); FFAC, free fatty acid content (g oleic acid/g oil); OSI, oxidative stability index (hours). Values are expressed as arithmetic mean ± standard deviation (n=2).
Central points.
Main significant effects on oil yield in walnut oil extraction (First design).
Effects of seed moisture content and restriction die on walnut oil yield (First design).
All walnut oils obtained at the various extraction conditions had PV ranging between 0.55 and 1.60 meq O2/kg oil, and FFAC between 0.06 and 0.17 (g oleic acid/g oil). Specific extinction coefficient (K232 and K270) values were within the ranges 1.93 - 2.23 and 0.16 - 0.19, respectively. The oxidative stability was between 2.12 and 2.29 (hours). These data show that the different extraction treatments had minimal effects on chemical quality parameters and oxidative stability.
A quadratic polynomial was fitted to model the oil recovery response. The determination coefficient of the model was able to explain 95.9% of the data variability. The SMC had a positive linear effect on OY; on the contrary, RD had a negative linear effect. On the other hand, both the SMC and the cross effect SMC x RD showed negative quadratic effects (
Values of regression coefficients calculated for walnut and almond oil yield
Independent variable | Regression coefficient | Standard error | Significance level (p) |
---|---|---|---|
Constant | 186.897 | ||
X1 | 20.0525 | 2.3012 | 0.0000 |
X2 | - 30.1321 | 2.3012 | 0.0012 |
X12 | - 0.4657 | 3.3917 | 0.1300 |
X22 | 1.3195 | 3.3917 | 0.0170 |
X1*X2 | - 0.8244 | 2.8183 | 0.0222 |
R2 | 95.9 | ||
Constant | 430,864 | ||
X1 | 4,0643 | 1.6876 | 0.0490 |
X2 | - 61,5551 | 1.6876 | 0.0002 |
X12 | - 0,9305 | 2.4874 | 0.0068 |
X22 | 2,1376 | 2.4874 | 0.0002 |
X1*X2 | 0,9362 | 2.0669 | 0.0047 |
R2 | 94.5 | ||
AAD | 0.034 | ||
Bf | 0.966 | ||
Af | 1.035 | ||
Constant | -577.904 | 1.0633 | |
X1 | 58.6122 | 2.0909 | 0.0124 |
X2 | 62.4494 | 2.0909 | 0.0009 |
X12 | - 3.2982 | 3.0817 | 0.0696 |
X22 | - 2.7387 | 3.0817 | 0.1187 |
X1*X2 | 0.2970 | 2.5608 | 0.8232 |
R2 | 88.6 | ||
AAD | 0.039 | ||
Bf | 0.976 | ||
Af | 1.025 |
X1: seed moisture (g/100 g seed, WB). X2: restriction die (mm). AAD: absolute average deviation. Bf: bias factor. Af: accuracy factor.
Second industrial scale 32 factorial design. Effect of process variables on walnut oil yield and quality parameters
Assay | Factors |
OY | FC | PV | FFAC | K232 | K270 | OSI | |
---|---|---|---|---|---|---|---|---|---|
X1 | X2 | ||||||||
1 | 10.00 | 12 | 60.6 ± 0.1 | 13.9 ± 0.1 | 1.04 ± 0.22 | 0.06 ± 0.01 | 0.18 ± 0.01 | 2.12 ± 0.01 | 2.28 ± 0.01 |
2 | 7.75 | 10 | 80.7 ± 1.6 | 10.9 ± 1.0 | 0.88 ± 0.03 | 0.05 ± 0.01 | 0.16 ± 0.02 | 1.97 ± 0.08 | 2.16 ± 0.12 |
3 | 5.50 | 10 | 73.3 ± 1.4 | 13.1 ± 1.1 | 0.81 ± 0.21 | 0.06 ± 0.01 | 0.17 ± 0.01 | 2.13 ± 0.10 | 2.27 ± 0.02 |
4 | 10.0 | 10 | 68.6 ± 0.1 | 12.8 ± 0.1 | 1.14 ± 0.23 | 0.08 ± 0.01 | 0.17 ± 0.01 | 2.11 ± 0.04 | 2.17 ± 0.01 |
5 |
7.75 | 12 | 63.3 ± 3.7 | 15.1 ± 1.3 | 0.85 ± 0.26 | 0.07 ± 0.01 | 0.18 ± 0.02 | 2.04 ± 0.06 | 2.18 ± 0.03 |
6 |
7.75 | 12 | 60.4 ± 0.1 | 16.5 ± 0.1 | 0.75 ± 0.10 | 0.07 ± 0.01 | 0.19 ± 0.02 | 2.09 ± 0.03 | 2.19 ± 0.08 |
7 |
7.75 | 12 | 62.2 ± 2.5 | 14.7 ± 0.9 | 1.09 ± 0.32 | 0.07 ± 0.01 | 0.17 ± 0.01 | 2.07 ± 0.03 | 2.26 ± 0.01 |
8 |
7.75 | 12 | 63.3 ± 1.6 | 15.7 ± 1.3 | 0.81 ± 0.16 | 0.07 ± 0.01 | 0.18 ± 0.02 | 2.04 ± 0.10 | 2.28 ± 0.02 |
9 |
7.75 | 12 | 63.0 ± 1.8 | 14.9 ± 1.3 | 0.81 ± 0.12 | 0.07 ± 0.01 | 0.17 ± 0.01 | 2.03 ± 0.07 | 2.21 ± 0.09 |
10 | 5.50 | 12 | 57.1 ± 0.6 | 13.8 ± 0.3 | 0.58 ± 0.06 | 0.10 ± 0.02 | 0.18 ± 0.01 | 1.88 ± 0.03 | 2.25± 0.01 |
11 | 7.75 | 14 | 61.6 ± 0.3 | 13.7 ± 0.6 | 0.89 ± 0.05 | 0.11 ± 0.01 | 0.17 ± 0.01 | 2.06 ± 0.11 | 2.22 ± 0.03 |
12 | 10.0 | 14 | 67.8 ± 0.1 | 13.1 ± 0.1 | 1.09 ± 0.01 | 0.07 ± 0.01 | 0.17 ± 0.01 | 2.10 ± 0.03 | 2.17 ± 0.04 |
13 | 5.50 | 14 | 54.8 ± 0.1 | 14.7 ± 0.1 | 0.90 ± 0.18 | 0.12 ± 0.02 | 0.15 ± 0.02 | 1.93 ± 0.05 | 2.28 ± 0.02 |
X1, seed moisture content (g/100 g seed, WB); X2, restriction die (mm); OY, oil yield (g/100 g oil); FC, fine solid content in oil (g solids/100 g extract); PV, peroxide value (meq/kg oil); FFAC, free fatty acid content (g oleic acid/g oil); OSI, oxidative stability index (hours). Values are expressed as arithmetic mean ± standard deviation (n=2).
Central points.
To sum up at this point, it can be concluded that the different treatments employed for walnut oil extractions had minimal effects on the chemical quality parameters analyzed, but OY was largely affected. A quadratic polynomial was fitted to model the oil recovery response. The determination coefficient of the model was able to explain 94.5% of the data variability. The SMC had a positive linear effect, while RD had a negative linear effect on OY. SMC and RD had negative and positive quadratic effects. A positive cross effect was observed between SMC and RD (
Effects of seed moisture content and restriction die on walnut oil yield (Second design).
Even though an OY value of 84.5% is lower than that obtained at pilot scale extraction conditions (89.3%, Martínez
Main significant effects on oil yield in walnut oil extraction (Second design).
Chemical analyses showed PV, FFAC, K232 and K270 values ranging between 0.65 - 1.14 meq O2/kg oil, 0.06 - 0.17 (g oleic acid/g oil), 1.93 - 2.23 and 0.15 - 0.19, respectively; while the oxidative stability ranged from 2.16 to 2.28 (hours). These results were similar to those obtained using the first experimental design. In addition, the oil obtained at the optimum extraction condition showed an OSI value (2.09 ± 0.08 h) and FA percentages (palmitic acid, 7.03 ± 1.30%; palmitoleic acid, 0.06 ± 0.00%; stearic acid, 2.72 ± 0.20%; oleic acid, 19.3 ± 2.01%; linoleic acid, 56.7 ± 1.68% and linolenic acid, 14.2 ± 0.12%) in accordance with those reported elsewhere (Martinez
On the basis of the results obtained under pilot scale extraction conditions (Martínez
Industrial scale 32 factorial design. Effect of process variables on almond oil yield and quality parameters
Assay | Factors |
OY | FC | PV | FFAC | K232 | K270 | OSI | |
---|---|---|---|---|---|---|---|---|---|
X1 | X2 | ||||||||
1 | 11.00 | 14 | 49.2 ± 1.0 | 10.3 ± 0.1 | ND | 0.41 ± 0.02 | 1.41 ± 0.01 | 0.05 ± 0.01 | 11.6 ± 0.02 |
2 | 9.00 | 14 | 55.1 ± 2.1 | 11.0 ± 0.2 | ND | 0.33 ± 0.04 | 1.30 ± 0.02 | 0.05 ± 0.01 | 11.8 ± 0.04 |
3 | 9.00 | 13 | 69.3 ± 1.8 | 9.9 ± 0.1 | 0.04 ± 0.00 | 0.35 ± 0.03 | 1.27 ± 0.02 | 0.05 ± 0.01 | 11.7 ± 0.04 |
4 | 11.00 | 12 | 61.7 ± 1.6 | 9.1 ± 0.1 | 0.05 ± 0.00 | 0.44 ± 0.13 | 1.33 ± 0.01 | 0.04 ± 0.01 | 10.9 ± 0.08 |
5 |
10.00 | 13 | 64.7 ± 1.0 | 9.2 ± 0.6 | 0.04 ± 0.00 | 0.37 ± 0.06 | 1.27 ± 0.06 | 0.05 ± 0.01 | 12.1 ± 0.05 |
6 |
10.00 | 13 | 65.2 ± 1.3 | 9.8 ± 0.4 | 0.02 ± 0.00 | 0.38 ± 0.05 | 1.24 ± 0.06 | 0.05 ± 0.01 | 12.2 ± 0.01 |
7 |
10.00 | 13 | 64.9 ± 1.6 | 9.9 ± 0.6 | 0.03 ± 0.00 | 0.40 ± 0.04 | 1.23 ± 0.05 | 0.05 ± 0.01 | 12.1 ± 0.01 |
8 |
10.00 | 13 | 65.5 ± 2.4 | 9.5 ± 0.6 | 0.03 ± 0.00 | 0.36 ± 0.05 | 1.25 ± 0.04 | 0.05 ± 0.01 | 12.2 ± 0.02 |
9 |
10.00 | 13 | 65.0 ± 1.9 | 8.6 ± 0.6 | 0.05 ± 0.01 | 0.41 ± 0.06 | 1.36 ± 0.06 | 0.04 ± 0.01 | 12.7 ± 0.04 |
10 | 9.00 | 12 | 68.7 ± 1.4 | 9.0 ± 0.1 | ND | 0.47 ± 0.08 | 1.32 ± 0.10 | 0.05 ± 0.01 | 12.1 ± 0.06 |
11 | 10.00 | 14 | 61.5 ± 1.4 | 10.2 ± 0.1 | 0.04 ± 0.00 | 0.39 ± 0.06 | 1.28 ± 0.01 | 0.05 ± 0.01 | 12.1 ± 0.07 |
12 | 10.00 | 12 | 70.2 ± 2.3 | 9.9 ± 0.3 | 0.09 ± 0.01 | 0.37 ± 0.02 | 1.72 ± 0.04 | 0.05 ± 0.01 | 12.9 ± 0.09 |
13 | 11.00 | 13 | 61.2 ± 1.1 | 9.5 ± 0.2 | 0.06 ± 0.01 | 0.39 ± 0.04 | 1.35 ± 0.02 | 0.04 ± 0.01 | 12.4 ± 0.01 |
X1, seed moisture content (g/100 g seed, WB); X2, restriction die (mm); OY, oil yield (g/100 g oil); FC, fine solid content in oil (g solids/100 g extract); PV: peroxide value (meq/kg oil); FFAC: free fatty acid content (g oleic acid/g oil); OSI: oxidative stability index (hours). Values are expressed as arithmetic mean ± standard deviation (n=2). ND: not detected.
Central points
Main significant effects on oil yield in almond oil extraction.
Effects of seed moisture content and restriction die on almond oil yield.
A quadratic polynomial was fitted to model the oil recovery response. The determination coefficient of the model was able to explain 88.6% of the data variability. Both the SMC and RD had a positive linear effect and a quadratic negative effect on OY. Also, a positive cross effect was observed between SMC and RD (
The evaluation of the real performance of the predictive models obtained for both walnut and almond oil extraction was done according to Desobgo
32 Factorial designs were used to optimize walnut and almond oil extractions by means of screw-pressing operations. Experimental designs included seed moisture content (SMC) and restriction die (RD) as the main processing parameters. Theoretical models were scanned against experimental data in order to scale-up the proposed oil extraction process to industrial scale. For both extraction processes, fitted models for oil recovery showed maximum predicted values similar to the highest experimental values, which were reached under the following conditions: 7.75% SMC, 10 mm RD (walnut oil), and 9.42% SMC, 12 mm RD (almond oil). Chemical quality parameters of the oils obtained at these conditions were in the ranges stated in the Codex (FAO/WHO) standards for non-refined oils.
This research was financed with grants from Consejo de Investigaciones Científicas y Técnicas (CONICET), Secretaría de Ciencia y Tecnología de la Universidad Nacional de Córdoba (SeCyT B 203/14),Fund for Scientific Research and Technology (FONCyT - BID PICT 2014-2283) and Secretaría de Políticas Universitarias (SPU-Ministerio de Educación) from Argentina.