Chemical evaluation of two tropical wood species for use as grilling planks

Abstract

This research evaluated the possible use of tropical hardwood species (Myrocarpus frondosus and Ocotea porosa) for grilling plank production. Physical, chemical, and organoleptic properties were evaluated and compared with properties of a well-used wood species for grilling planks, western red cedar (Thuja plicata). For chemical analysis, one technique was used: hydrodistillation. Normality tests and analyses of variance (ANOVA) were used for the comparisons between Thuja plicata and tropical hardwoods. The results of the organoleptic, chemical, and statistical analyses demonstrated the possibility of using Myrocarpus frondosus and Ocotea porosa in grilling plank production.

INTRODUCTION

Humans have used wood since antiquity for shelters, agricultural implements, and tools. Its use has increased, as it is used in the furniture and food industries (Lahr et al. 2016; Christoforo et al. 2019; Sthapit 2019). In the food industry, conventional smoking is used to preserve food. This technique enhances the organoleptic process, augmenting the flavor, color, and smell of food (Ledesma et al. 2017).

An alternative means of smoking food is the use of grilling planks. In this process, food is in contact with wood planks, which add to the food complex flavors and moisture. In this case, smoking does not aim to improve the food’s lifetime but improves its taste and smell. A grilling plank works as a smoker, as the porous wood surface absorbs moisture and then, when taken to a barbecue or heated oven, releases it with an aromatic smoke (Zoschke 2016). The use of wood grilling planks originated with the Native Americans of the Pacific coast, who used western red cedar (Thuja plicata) wood planks to prepare foods. These foods, such as salmon and vegetables, are tastier, have a soft flavor, and are not burned. In Europe, wood grilling planks have become widely used in restaurants to enhance flavor and smell (Zoschke 2016; Ledesma et al. 2017).

Brazil has the greatest number of wood species in the world (8715 wood species) and a large vegetal cover, being 52% of Brazilian territory (Beech et al. 2017; IBÁ 2017). This amount of wood species and vegetal cover includes a unique diversity of aromatic hardwood trees, such as the Anacardiaceae, Annonaceae, and Meliaceae families. The families Cupressaceae, Abietaceae, and Betulaceae are aromatic softwoods. Considering the diversity of chemical components present in these families, it is easy to select wood species with potential for grilling plank production (Lorenzi 1998; Márquez 2004).

To expand the use of Brazilian tropical woods as grilling planks and to advance the food industry, this study examined the chemical and physical characteristics of Myrocarpus frondosus Allemão and Ocotea porosa wood species and compared their properties with those of Thuja plicata, one of the first wood species used to produce grilling planks.

EXPERIMENTAL

The wood species Thuja plicata, Myrocarpus frondosus Allemão, and Ocotea porosa were harvested and stored in homogeneous batches at an equilibrium moisture content of 12%, following the method of Brazilian Standard ABNT NBR 7190 (1997). The batches were deposited at the Wood and Timber Structures Laboratory (LaMEM), University of São Paulo, São Carlos, São Paulo, Brazil.

For the analysis of the organoleptic properties of the wood species, sensory analyses were performed to check the smell, color, taste, grain, and texture, following the parameters described in the literature (Zenid 2019). The wood smell, the main characteristic to be analyzed, was evaluated in three conditions: in natura, after immersion in cold water for 60 min, and during use on a heat source (190 °C), to identify smell changes as would occur with their use as grilling planks. Five male LaMEM employees (ages ranging between 30 years and 60 years) qualified the smell of each species using a hedonic scale: bad, poor, fair, good, or excellent.

Evaluation of apparent density (ρ_ap) followed the method of the Brazilian standard ABNT NBR 7190 (1997).

For chemical analysis, wood samples were obtained according to TAPPI T 257 cm-85 (1985) and TAPPI T 264 cm-97 (1997). The wood was crushed with mill equipment (Wiley SL-31, Piracicaba, Brazil) to reach small particles passing a 30 mesh. The total extractives were evaluated according to TAPPI T 204 (1997). After extractives were removed, the samples were washed with distilled water and dried in an oven at 100 °C ± 2 °C for 24 h. The extractive content was calculated by mass difference. The resulting extractive-free wood was used to determine Klason lignin content by a modified Klason method (Gomide and Demuner 1986), by the sum of insoluble and soluble lignins. The holocellulose content was determined by the difference between lignin content, ash content, and extractive-free wood mass (Tjeerdsma and Militz 2005). For each chemical analysis, 12 specimens were used, considering hydrodistillation method.

For chemical composition analysis by gas chromatography, one technique was used: hydrodistillation. For the extraction of essential oil from wood species, hydrodistillation was performed, using a graduated Clevenger apparatus protected by aluminum foil. Twenty grams of dry wood of each species was used, ground to 40 mesh weight, with 500 mL of distilled water. All chemical analyses were performed using gas chromatography-mass spectrometry (GC-MS) (Shimadzu GC-2010AF Plus with TQ8040 sequential mass spectrometer detector, Kyoto, Japan), with a capillary column (Rtx-5ms, 30 m × 0.25 mm × 0.25 µm, Restek, Bellefonte, USA).

After qualitative exploratory analysis by GC-MS, the extraction method to be used to obtain a broader chemical profile of the volatile compounds was defined. Extraction was performed with 250 mg of sample moistened with 200 µL of ultra-pure water for 30 min under constant agitation at 80 °C. Then, 1000 µL of the volatile compounds were analyzed by GC-MS (Spectrometer TQ8040 detector), with heating from 40 °C to 120 °C at a rate of 15 °C/min and from 125 °C to 160 °C at a rate of 1 °C/min, remaining at 160 °C for 2 min. The aforementioned capillary columns were used.

To investigate differences in the wood species based on the experimental results for density, contact angle, and percentages of components (extractives, lignin, and ash), an analysis of variance (ANOVA) was performed, with a 5% significance level. Equivalence of averages was the null hypothesis, and non-equivalence (of at least two) was the alternative hypothesis. A p-value equal to or greater than the level of significance (0.05) implies acceptance of the null hypothesis, which is refuted otherwise. To validate the ANOVA, the set of values should be evaluated if present a normal distribution, and the homogeneity of the variances of the groups was investigated, with the aid of the Anderson-Darling (AD) test, considered at the 5% significance level.

RESULTS AND DISCUSSION

Evaluation of Organoleptic Properties of Wood Species

Tables 1 and 2 present the organoleptic properties of the Brazilian tropical hardwoods and their smells in different conditions of grilling planks.

Table 1. Organoleptic Properties of Brazilian Tropical Hardwoods

Table 2. Smell in Different Conditions

Physical and Chemical Properties of Wood Species

Table 3 presents the mean values of physical and chemical properties of the wood species and their coefficients of variation (CV).

As shown in Table 3, the apparent densities of Myrocarpus frondosus and Ocotea porosa were higher than that of Thuja plicata. Both Myrocarpus frondosus and Ocotea porosa woods presented contact angles less than 90° and less than the reference value of Thuja plicata (68.54°), indicating that the surfaces were more wettable than the surface of the reference wood (Thuja plicata) (Wu and Baghdachi 2015). It is also important to consider natural characteristics of the wood, such as chemical composition and surface roughness (Adamson and Gast 1997).

Table 3. Physical and Chemical Properties of Wood Species

Ocotea porosahad the greatest extractive content, indicating more effects on flavor on dishes after wood use as grilling plank. Myrocarpus frondosus presented a higher value compared to Thuja plicata, affecting food flavor. For information of the chemical compositions of the wood species, the lignin, ash, and holocellulose contents were determined. Thuja plicata wood had a holocellulose value of 58.12%, close to that of Myrocarpus frondosus. For Ocotea porosa, the elevated lignin content led to minor holocellulose content (less than 50%). It is important to highlight that ash and extractive contents may vary due to edaphoclimatic factors (Zau et al. 2014).

The result of Anderson-Darling normality test for all analysis presented a p-value greater than 0.05, i.e., the results of tropical hardwoods compared with the reference wood species (Thuja plicata) presented a normal distribution, validating the ANOVA results.

Tables 4 to 9 display the results of the ANOVA, comparing the tropical wood species in apparent density, contact angle, and contents of extractives, soluble lignin, insoluble lignin, and ash content.

Table 4. Relation between Apparent Densities

Table 5. Relation between Mean Values of Contact Angle for Wettability

Table 6. Relation between Extractive Contents

Table 7. Relation between Soluble Lignin Contents

Table 8. Relation between Insoluble Lignin Contents

Table 9. Relation between Ash Contents

As shown in Table 4, there was a significant difference between the apparent densities of Myrocarpus frondosus, Ocotea porosa, and Thuja plicata. For values of mean contact angle (Table 5), both wood species presented values close to the Thuja plicata value. For extractive content, insoluble lignin, and soluble lignin values (Tables 6 to 8), Myrocarpus frondosus and Ocotea porosa values were significantly different from the reference values (Thuja plicata). The ash content of Myrocarpus frondosus was significantly different from that of Thuja plicata, which did not differ significantly from that of Ocotea porosa. The GC-MS results are presented in Tables 10, 11, and 12.

Table 10. Compounds Detected in Myrocarpus frondosus Essential Oils Extracted by Hydrodistillation

(a) Kovats indexes (KI) on capillary column; (b) compounds listed in evolution order in capillary column; I.R. (infrared spectra interpretation);

Table 11. Compounds Detected in Ocotea porosa Essential Oils Extracted by Hydrodistillation

Table 12. Compounds Detected in Thuja plicata Essential Oils Extracted by Hydrodistillation, Soxhlet, and Headspace

(a) Kovats indexes (KI) on capillary column; (b) compounds listed in evolution order in capillary column; I.R. (infrared spectra interpretation of chemical compound);

As shown in Table 10, from the Myrocarpus frondosus extraction by hydrodistillation, it was possible to identify four compounds.

The results of the GC-MS tests presented in Table 11 showed that the species Ocotea porosa contained compounds identified in all the extraction techniques used. From hydrodistillation for Ocotea porosa, 23 compounds were identified, the majority of them, in decreasing order: guaiol (100%), ledol (24.69%), and τ-cadinol (50.92%).

The results of the GC-MS tests presented in Table 12 showed that, through hydrodistillation for Myrocarpus frondosus, 20 compounds were identified, the most of them being, in decreasing order: myrtenol (100%), α-terpineol (49.76%), and terpinen-4-ol (26.24%).

The percentages presented represent chemical content of each compound. The higher content stands out for higher percentage (100%) and so decreasing for other contents.

Nerolidol was present in all wood species. Nerolidol is allowed by the United States Food and Drug Administration (FDA) to be used in the food industry as a food flavoring agent. Then, it supports the possible use of tropical wood species as grilling planks in the food industry, which can improve foods’ preparations (Chan et al. 2016; Ušjak et al. 2017; Azzi et al. 2018).

Also, otherwise Thuja plicata, Myrocarpus frondosus, and Ocotea porosa did not contain, according chemical analysis, toxic volatile chemical compounds such thujone, which is toxic for human being even at low levels (SCE 2002). This absence enables their use as grilling planks, with similar apparent density of a well-known wood specie already used for grilling planks and satisfactory organoleptic properties for use on planks. Also, a lower apparent density for Myrocarpus frondosus and Ocotea porosa contributes a good impregnation process during plank manufacture and lower weight, reducing costs along productive process and transportation.

CONCLUSIONS

1. Chemical analysis supported the possible use of Myrocarpus frondosus and Ocotea porosa tropical hardwoods for production of grilling planks. They have elevated amounts of extractives, which are important for organoleptic properties, such as taste and smell.

2. Apparent density of Myrocarpus frondosus and Ocotea porosa were close to the reference value of Thuja plicata wood specie, being interesting the wood species with lower apparent density.

3. Gas chromatography indicated the presence of nerolidol, used as a flavoring agent in several foods, in all wood species and the absence of the compound thujone, which is toxic for humans. In addition, organoleptic properties were satisfactory supporting the use of Myrocarpus frondosus and Ocotea porosa wood species for grilling plank production.

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