Elsevier

LWT

Volume 86, December 2017, Pages 577-585
LWT

Pentopan mono BG pretreatment of palm kernels modified the aroma of palm kernel oil after kernel roasting

https://doi.org/10.1016/j.lwt.2017.08.055Get rights and content

Highlights

  • A 1.3-fold increment of total soluble sugars was found after Pentopan treatment.

  • Obvious increments of O-heterocyclic compounds were found in treated PKO.

  • Elevation in furfural and 2-furanmethanol was most evident in treated PKO.

  • Treated PKO (medium roast) rendered a more nutty, smoky and caramelic odor.

Abstract

With an interest to enhance the aroma of palm kernel oil (PKO), Pentopan mono BG (xylanase) was applied to alter the carbohydrates in palm kernels (PK) so as to modulate the formation of volatiles during kernel roasting. The result showed that a 1.3-fold increment of total soluble sugars was found in PK after Pentopan treatment, which promoted the generation of O-heterocyclic volatile compounds during kernel roasting. Overall, an increment of 1.5-, 1.4- and 1.3-fold of O-heterocyclic compounds were found in PKO derived from treated PK after light, medium and dark roasting, of which the elevation in furfural and 2-furanmethanol contents was the most obvious. Principal component analysis (PCA) clearly discriminated treated PKO with various kernel roasting degrees from that of control PKO on the basis of all aroma-active compounds; the aroma in PKO from treated, roasted PK was highly correlated with 2-[(methyldithio)methyl]-furan, 5-methyl-2-furancarboxaldehyde, 2-furanmethanol, 2,5-dimethylpyrazine and ethyl pyrazine. Sensory evaluation showed that PKO derived from medium roasted PK imparted more caramelic, nutty and smoky odor notes relative to the untreated PKO. This study suggests Pentopan pretreatment of PK followed by roasting may be a novel way to modulate PKO aroma and potentially widen its application.

Introduction

Palm kernel oil (PKO) is the oil extracted from palm kernels (PK), the seeds of oil palm (Elaeis guineensis Jackqu) (Cornelius, 1977). Commercial PKO is generally well refined and deodorized, with a bland flavor. Due to its specific fatty acid composition which can afford it unique melting characteristics, PKO has a wide range of applications in the food industry as non-aroma related specialty fat (Traitler & Dieffenbacher, 1985). Therefore, it would be of interest to enhance the aroma of PKO, which may expand its application beyond being a fat base.

Roasting has been demonstrated to be effective in enhancing the flavor in PKO and other seed oils due to the occurrence of a series of thermal reactions (Jayalekshmy et al., 1987, Liu et al., 2011, Park et al., 2011, Siegmund and Murkovic, 2004, Zhang et al., 2016). Previous research indicated that PKO extracted from roasted PK presented a favorable and pleasant odor (Jayalekshmy et al., 1987, Zhang et al., 2016). Gas chromatography-olfactometry (GC-O) identified substituted pyrazines, furan and pyran derivatives are the most aroma-active compounds in roasted PKO, most of which are thought to be originated from thermal reactions during the kernel roasting process (Zhang et al., 2016).

The aforementioned thermal reactions, mainly referred as Maillard reaction and caramelization, contribute to the typical nutty, roasted, burnt or caramelic aroma notes of the thermally processed foods (Parker, Elmore, & Methven, 2014). Therefore, the aroma of PKO is expected to be enhanced if those thermal reactions could be promoted. Simple reducing sugars, being the precursors of both Maillard reaction and caramelization, are a critical factor that can directly affect the thermal reactions and the ultimate aroma profile (Laroque et al., 2008).

However, fresh PK contains low levels of simple sugars but a relatively large amount of cellulose, hemicelluloses and other polysaccharides, with hemicelluloses being the largest portion (Lawal, Iyayi, Adeniyi, & Adaramoye, 2010). Our previous study explored the pretreatment of PK with a commercial cellulase (Celluclast 1.5 L) in order to increase the content of simple sugars by hydrolyzing cellulose and we found that the aroma of PK and PKO after kernel roasting was evidently modified positively, owing to more soluble sugars especially glucose being released (Zhang, Zhao, Yang, Zhao, & Liu, 2017).

Pentopan mono BG, a xylanase (endo-1, 4-) produced by Aspergillus oryzae, with an activity of 2500 fungal xylanase units (FXU)/g (according to product data sheet), also has the ability to hydrolyze polysaccharides from plant tissues. For instance, the efficiency of bioethanol production from crop residues was found increased due to the degradation of cell wall materials induced by Pentopan (Juodeikiene et al., 2011). Similarly, Pentopan was highly effective in increasing the amount of reducing sugars in rice bran extract by 5-fold (Kim & Lim, 2016). When the substrate is cocoa pod husk, Pentopan was also able to release monosugars when used alone or combined other enzymes such as Viscozyme L and Pectinex 5 XL (Alemawor, Dzogbefia, Oddoye, & Oldham, 2009). Several studies on Pentopan mainly focused on releasing ferulic acid, producing oligosaccharides and modifying the rheological properties of dough (Giet et al., 2010, Saulnier et al., 2001, Selinheimo et al., 2006, Vegas et al., 2008). To the best of our knowledge, Pentopan has not been studied to modify PK polysaccharides so as to modulate the flavor of obtained PKO.

In this study, we aimed to examine the possibility of using Pentopan to release simple pentoses from PK pentosans and their subsequent impact on aroma modulation of PK and resultant PKO after different degrees of kernel roasting.

Section snippets

Materials, chemicals and solvents

Fresh oil palm nuts, which were from a hybrid of Dura and Pisifera variety of Elaeis guineensis species cultivated and harvested in North Sumatera Province of Indonesia in 2015, were provided by PT Wilmar Nabati, Indonesia. After manually removing hulls, palm kernels were obtained with a moisture content of 196.7 ± 2.0 mg/g and stored at −20 °C prior to use. Pentopan mono BG, a xylanase (endo-1,4-) with an activity of 2500 FXU/g, was purchased from Novozymes (Bagsvaerd, Denmark). Sugar

Moisture content and lightness of palm kernels with different roast degrees

Different degrees of kernel roasting are expected to produce PKO with different aromas since the intensity of thermal reactions varies. Aiming to gain a comprehensive understanding of the impact of Pentopan pretreatment on the aroma modification of PKO, PK samples with different roast degrees were collected and moisture content and L* value were determined to differentiate the roast levels. Overall, the moisture content declined from the initial 50 mg/g to 10 mg/g after dark roast and the

Conclusion

Pentopan pretreatment brought about significant changes to sugars and volatile profiles in PK and corresponding PKO after different degrees of kernel roasting. A 1.3-fold increment of total soluble sugars was found in PK after enzyme treatment, though no pentose was released. Nevertheless, the increased soluble sugars and the possible oliogsaccharides elevated the generation of O-heterocyclic volatile compounds during kernel roasting. Overall, an increment of 1.5-, 1.4- and 1.3-fold of O

Acknowledgements

The authors would like to thank Jenny Elisabeth and PT Wilmar Nabati Indonesia for providing fresh palm nuts and thank the panelists (Wang Rui, Wang Youfei, Wu Pinpin, Mei Fangyi and Wang Xu from Wilmar (R&D) Center) for their participation in sensory evaluation.

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      However, fresh PK contains low levels of simple sugars, where only 2.4% of its total carbohydrate are simple sugars while 81% are present as non-starch polysaccharides; of which the majority are linear mannans (78%) with low levels of substituted galactose residues, followed by cellulose (12%) and small amounts of glucuronoxylans and arabinoxylans (3% each) (Daud & Jarvis, 1992; Düsterhöft, Posthumus, & Voragen, 1992). Our previous study found that the commercial cellulase (Celluclast 1.5 L) and xylanase (Pentopan mono BG) could modify the soluble sugar profile of PK with more glucose being released by Celluclast and possibly more pento-oligosaccharides being released by Pentopan (Zhang, Zhao, Yang, Zhao, & Liu, 2017; Zhang, Zhao, Zhao, Yang, & Liu, 2017). Correspondingly, the volatile composition of PKO derived from those hydrolyzed PK (after roasting) changed evidently.

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