Walnuts (Juglans regia L.) and almonds (Prunus dulcis (Miller) D.A. Webb P. amygdalus Batsch) are crops of increasing interest for the food industry. Worldwide productions are about 3.400.000 and 1.934.817 tn, respectively (FAO, 2015). The kernel is the edible part of these nuts. They are considered important snacks and confectionary foods, with important nutritional value arising primarily from their high lipid content.

Both walnut and almond kernels contain high levels of oil (52 – 70 and 48 – 67%, respectively) (Martínez et al., 2008; Martínez et al., 2013; Martínez and Maestri, 2015). Walnut oil (WO) is mainly composed of polyunsaturated fatty acid (linoleic and linolenic acids together represent 65 - 75% of the total fatty acid (FA) content). In contrast, almond oil (AO) mainly contains oleic acid (a monounsaturated fatty acid, 50 – 70% of the total FA content). Detailed compositions of these nut oils, including genetic and geographical variations, can be obtained from studies by Kodad et al., 2008, Martínez et al., 2010, Maestri et al., 2015 and Martínez and Maestri, 2015, among others.

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 et al., 2001, Singh et al., 2002; Zengh, 2003). Furthermore, Owolarafe et al. (2002) compared both types of mechanical presses with regards to yield, chemical quality and costs for palm oil extraction. No differences in chemical quality were observed and the screw-press type was more efficient and economical.

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 et al., 2001; Martínez et al., 2008; Savoire et al., 2013; Martínez and Maestri, 2015; Martínez et al., 2017). Seed moisture content appears to be a key process variable, as reported in several research projects with various oilseeds (Singh and Bargale, 1990; 2000; Hamm and Hamilton, 2000; Wiesenborn et al., 2001; Singh et al., 2002; Martínez et al., 2008; 2012; 2013; 2017).

Using screw-pressing at pilot plant scale, the highest oil recoveries from both walnut and almond kernels were obtained at similar moisture contents (Martínez et al., 2008; 2013). In each case, the oil extraction performance was also dependent on the pressing temperature and other processing conditions. On the basis of these studies, the present work was aimed to scale-up the oil extraction processes at industrial scale. Theoretical models were run against experimental data in order to optimize extraction conditions that gave both higher oil recovery and better oil quality.

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 (Table 1), based on results obtained from pilot scale press extraction (Martínez et al., 2008).

Figure 1 shows the most relevant effects on oil recovery. The two factors analyzed (SMC and RD) showed p-values lower than the level of significance (p ≤ 0.05). OY and FC values were between 44.8 – 80.4% and 10.6 - 15.1%, respectively (Table 1). Increasing SMC from 3 to 8% (WB) significantly increased OY. Similar trends were obtained at pilot scale extraction conditions (Martínez et al., 2008). By increasing SMC from 2.5 to 7.5% (WB) these authors observed an increase in walnut OY from 61.0 to 83.5% (at 25 °C), and from 64.7 to 89.3% (at 50 °C). The positive effect of increasing water content on OY may be explained through an increasing expansion and breaking of cell structures which expedites oil release. Moreover, water has a barrel lubricant effect so it increases the plasticity of seed material and contributes to press feeding. In the present study, the highest OY (80.4%) was obtained at 8% SMC and 10 mm RD. This treatment showed one of the lowest FC (11.5%), which is considered a positive trait since it facilitates subsequent operations of oil clarification. At 8% SMC, an increase in RD produced a significant reduction in OY (Fig. 2). This was due to insufficient compression of seed material during pressing, causing a lesser amount of oil to be diverted to the barrel openings.

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 (Table 2). Considering that the combination of factor levels which suggested a maximum in OY (80.4%) coincides with one extreme of the factors discussed, a new experimental design was carried out in order to find the extraction conditions that maximize OY (Y₁). The new 3² Factorial design tested the effect of the incoming factors, SMC and RD, in 13 experiments (5 central points). The experimental design and the response variables considered are shown in Table 3.

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 (Table 2). The combination of factor levels that suggests maximum OY and minimum FC coincides with the treatment number 2 (7.75% SMC (WB) and 10 mm RD) (Fig 3). The predicted value (81.5%) and the experimental value (84.5 ± 2.3%) showed an error of 3.7%, thus suggesting a good prediction level of the model.

Even though an OY value of 84.5% is lower than that obtained at pilot scale extraction conditions (89.3%, Martínez et al., 2008), it is a promising value since it provides around 515 g oil per kg of milled and hydrated material. When the extraction yield (oil recovery, OY) is evaluated as a function of the two process variables (SMC and RD), a maximum is observed at a moisture contents near 7.50% (WB) which coincides with results from pilot scale conditions (Martínez et al., 2008) (Fig 4). The maximum OR is obtained with relatively small reduction (10 mm) which provides a good relationship between the fluidity and compression of the material without generating clogging.

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 et al., 2006; Martinez et al., 2010).

On the basis of the results obtained under pilot scale extraction conditions (Martínez et al., 2013), a preliminary exploratory experimental study (data not shown) using the industrial screw-press in which the behavior of the almond with a moisture content of 8%, 10% and 12% (w.b.) and a restriction die of 10 mm was evaluated. The results showed that, under those process conditions, the clogging of the material inside the press occurred, so an experimental design of 13 treatments (5 central points) was conducted for almond oil extraction, using the factors and levels described in Table 4.

Figure 5 shows the most relevant effects on oil recovery. The two factors analyzed (SMC and RD) showed p-values lower than the level of significance (p ≤ 0.05). OY and FC values were between 49.2 – 70.2 % and 8.6 – 11.0%, respectively (Table 4). The treatment at 10% SMC (WB) - 12 mm RD gave the highest OY (Figure 6) and the lowest FC (9.9%). Chemical parameters and oxidative stability were not significantly affected by the different extraction conditions tested. PV, FFAC, K₂₃₂, K₂₇₀ and OSI values were lower than 0.09 meq O₂/kg oil, 0.02 g oleic acid/g oil, 1.72, 0.05 and 12.9 hours, respectively.

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 (Table 2). The combination of factor levels which suggested a maximum on OY and a minimum in FC within the experimental values tested was 9.42% SMC (WB) and RD 12 mm. The result demonstrates that the regression equation allowed an accurate prediction of OY. The predicted value (70.1%) and experimental value (71.9 ± 3.5%) presented an error of 2.6%, thus suggesting a good prediction level for the model. The oil obtained at this condition showed a OSI value (13.01 ± 0.85 h) and fatty acid composition (palmitic acid, 6.65 ± 1.23%; palmitoleic acid, 0.43 ± 0.04%; stearic acid, 0.72 ± 0.06%; oleic acid, 71.90 ± 2.45%; linoleic acid, 20.30 ± 1.32% and linolenic acid, 0.06 ± 0.00%) in accordance with data reported elsewhere (Martinez et al., 2013).

The evaluation of the real performance of the predictive models obtained for both walnut and almond oil extraction was done according to Desobgo et al., (2015). According to these authors, a perfect agreement between observed and predicted responses is related to Bf and Af values of 1 and AAD of 0. The correlation coefficients associated with AAD, Bf, and Af values allowed validation of the models, as shown in Table 2.

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.