Evaluation of compositional changes in pecan nuts during storage and the use of pulsed electric fields as an aid to enhance oil extraction yield

Abstract

Pecan nut kernels have been identified as an important source of mono- and polyunsaturated fatty acids, phytosterols, tocopherols, and phenolic compounds. Likewise, the oil commonly extracted by mechanical pressing of pecan nut kernels, is considered a specialty product due to its phytochemical profile. However, the effect of processing and storage on these bioactive compounds and how to improve the mechanical extraction of pecan nut oil have been scarcely investigated. Pulsed electric fields (PEF) are being applied as a pretreatment to improve oil extraction from different seeds increasing the oil extraction yield (OEY) and enhancing the oil phytochemical profile. Therefore, this dissertation aimed to evaluate i) the effect of drying and storage conditions on phytochemical content and antioxidant capacity (AC) of pecan nuts, and ii) the application of PEF as a pecan nuts pretreatment to improve OEY along with its effect on oil stability and bioactive compounds concentration of the oil and by-product generated usually named cake. Fatty acids, tocopherols, phenolic compounds (PC), and AC of fresh and dry kernels were determined to assess the effect of drying on pecan nuts. PC and AC of kernels and in-shells nuts stored during 240 days at 4 and 25°C were analyzed to study the effect of storage conditions. PEF was evaluated on fresh and dry pecan nuts by comparison of PEF-treated kernels against untreated and soaked kernels. Fresh kernels were pretreated at different levels of specific energy inputs (0.5–17.6 kJ·kg-1) to evaluate OEYTOTAL, oil acidity and AC along with cake PC and AC. Dry kernels were pretreated at 0.8, 7.8, and 15.0 kJ·kg-1 to assess OEYTOTAL and kernels microstructure. Oil stability was investigated by determination of acidity, AC, oil stability index (OSI), phytosterols, tocopherols, and lipoxygenase (LOX) activity. Cake PC and AC were also analyzed. The results showed that the drying process reduced the initial concentration of PC and AC (33.2 and 22.3%, respectively), but it helped to preserve pecan nut kernels for 240 days without microbial deterioration. During storage, a decrement of condensed tannins (CT) concentration (31.5–41.8%) and an increment of AC (188.4–216.4%) on both kernels and in-shell nuts were observed. Regarding the evaluation of PEF, water immersion of fresh kernels to PEF pretreatment decreased the initial oil content (7.3–11.7%). Despite that, the highest OEYTOTAL was achieved in kernels pretreated at 0.5 kJ·kg-1, being 21.4 and 17.6% higher than values of untreated and soaked kernels, respectively. A rise of PC content was observed in cakes generated from kernels pretreated at 0.8 kJ·kg-1 in comparison with cakes from untreated and soaked kernels (9.5–17.8 and 30.1–39.3% for PC and CT, respectively). Despite PC improvement, the PEF pretreatment did not increase the AC of cakes. Dry kernels water immersion also caused a reduction of the initial oil content (9.7%). OEY of untreated kernels was 88.7%, being higher than OEYTOTAL of soaked (76.5–83.0) and PEF-treated kernels (79.8–85.0%). The microstructural analysis of kernels pretreated at 0.8 kJ·kg-1 showed that PEF processing induced oleosomes fusion within the intracellular space of kernels cotyledon tissue. Neither soaking nor PEF processing significantly modified oil acidity, AC, OSI, and LOX activity. In contrast, the application of 0.8 kJ·kg-1 to dry kernels increased cake CT and AC by 43.5 and 21.8%, respectively, compared to untreated kernels while in comparison with control kernels was 27.0 and 24.3% higher, respectively. This dissertation proved that PC and AC of kernels are highly susceptible to drying and storage time, evidencing their impact on the functional properties of pecan nuts. Concerning pretreatment with PEF, it was demonstrated that it is necessary to take into consideration the recovery of the oil retained into the soaking water. Furthermore, the effectiveness of PEF to improve OEYTOTAL depended on whether the pretreatment was applied to fresh or dry pecan nuts. The microstructural analysis showed that the application of PEF modifies kernels oleosomes. Oil extracted from PEF-treated kernels displayed comparable characteristics to pecan nut oil extracted by mechanical and solvent processes. The by-product generated from PEF-treated kernels exhibited an improved CT concentration, making it an attractive ingredient to consider in food formulations.