Introduction
With emerging biorefineries, industries are extensively exploring pathways to valorize lignocellulosic biomass in various alternative products (Mikkonen
2020). Hemicelluloses are abundantly available, constituting 20–35% of wood biomass (Sjöström
1993). Technologies already exist to efficiently separate wood hemicelluloses, lignin and cellulose (Von Schoultz
2015). These processes contribute to more economic use of natural resources, and respond to the strategic aim for resource-wise circular economy (European Commission
2019). To make the industrial recovery of hemicelluloses worth investment, development of value-added applications is needed.
Hemicelluloses are non-cellulosic polysaccharides that occur in the cell walls of all terrestrial plants. Abundant industrial sources for wood hemicelluloses include thermomechanical pulping process water (Willför et al.
2003), saw meal (Kilpeläinen et al.
2014), or pre-hydrolysate of dissolving pulp, which is often burnt at low efficiency (Saadatmand et al.
2013). Softwoods contain 20–25 wt% hemicelluloses, mainly galactoglucomannans (GGM), while hardwoods contain 25–35 wt%, the majority of which are glucuronoxylans (GX). In the Nordic countries, pine (
Pinus sylvestris), spruce (
Picea abies), and birch (
Betula sp.) are the most important industrial wood species (Luke-Natural Resources Institute Finland
2019). Wood hemicelluloses are not currently isolated for industrial use, but they show great potential as novel food hydrocolloids (Mikkonen et al.
2016a,
b; Valoppi et al.
2019b).
The food industries are actively seeking new natural ingredients and additives that fulfill multiple requirements (McClements et al.
2017). Successful food compounds need to be not only safe and economical, but also have desirable functional and sensory properties. Wood hemicelluloses have unique properties compared to previously known hydrocolloids. Spruce GGM and birch GX showed excellent emulsification and stabilization capacity in rapeseed oil-in-water emulsions, being more efficient stabilizers than widely used gum Arabic (GA) (Mikkonen et al.
2016a,
b). GGM and GX did not form high viscosity in aqueous solutions due to their intermediate molar mass of ca. 7000–10000 g/mol (Mikkonen et al.
2016a), but they showed capacity to reduce the surface tension of water (Mikkonen et al.
2019). GGM adsorbed at the oil–water interface and stabilized it (Bhattarai et al.
2019). Furthermore, due to the presence of lignin-derived phenolic residues that act as antioxidants, GGM and GX efficiently protected emulsified lipids against oxidation (Lehtonen et al.
2018; Lahtinen et al.
2019). The presence of phenolic compounds is expected to affect the sensory properties of hemicelluloses, including color, odor, and taste (Valoppi et al.
2019b). However, the isolation technique and possible purification steps affect the content of phenolic compounds in hemicelluloses (Mikkonen et al.
2019; Valoppi et al.
2019a).
GGM- and GX-rich wood extracts can be recovered without harmful chemicals e.g. by pressurized hot water extraction (PHWE), where only water is used as a solvent (Kilpeläinen et al.
2014). Our recent literature review on the potential use of GGM as a novel food source concluded that safety hazards would be highly unlikely (Pitkänen et al.
2018). On the contrary, positive health effects are expected to arise from the beneficial degradation products generated by human gut microbiota (La Rosa et al.
2019). Tests with a rat model indicated that GGM controls prostate inflammation through gut metabolites and changes in the gut microbiota (Konkol et al.
2016,
2019). GGM and GX were also identified to be suitable for sheep feed (Rinne et al.
2016). However, GGM and GX do not currently have a food grade status as ingredients or additives, and thorough safety evaluation for novel foods is required (Pitkänen et al.
2018). To promote development of these sustainable hydrocolloids, including the novel food evaluation, information on the sensory attributes of GGM and GX is essential, as the sensory quality affects the consumers’ acceptance of new products and can determine the potential uses and applications of a new ingredient (Tuorila
2015). Therefore, sensory profiling is an essential part in developing novel food ingredients.
Systematic sensory profiling of spruce GGM and birch GX or products therefrom has not been performed previously. Sensory profiling of foods is often done by trained sensory panelists using generic descriptive analysis (Lestringant et al.
2019). In descriptive analysis, sensory attributes important for the studied samples are first defined qualitatively, and then the intensities of the attributes are measured quantitatively (Lawless and Heymann
2010). Results of a descriptive analysis are often visualized as a spider chart (sensory profile).
The aim of the present work was to systematically define the sensory profile of spruce GGM and birch GX in water and in complex yogurt-based food products. The hypothesis was that GGM- and GX-rich wood extracts may introduce unique sensory properties and that the sensory profile of GGM and GX is influenced by the food matrix (yogurt). This study reveals the characteristics of GGM and GX as novel wood-based food hydrocolloids, and supports future product development that may utilize these sustainable ingredients.
Discussion
Sensory profile of wood hemicellulose-rich water solutions
The sensory profile of wood hemicelluloses was evaluated for the first time to help define their applicability as food ingredients or additives. Wood hemicellulose-rich water solutions had typically brownish color and wood-like odor and taste, which are likely caused by phenolic compounds that are co-extracted with hemicelluloses. The spray-dried wood extracts (sGGM and sGX) were rated as more opaque and brown, more intense in wood odor and overall odor, more astringent and bitter, and more intense in wood taste and overall aftertaste than the corresponding ethanol precipitated wood extracts (eGGM and eGX). This implies that these sensory characteristics are strongly contributed by components that were lost in the ethanol precipitation but remained after spray drying of the wood extracts. The composition of sGGM, sGX, eGGM and eGX was previously compared (Mikkonen et al.
2019). The sGGM and sGX showed clearly higher contents of lignin-derived phenolic compounds, as well as lignans and other extractives, than the eGGM and eGX (Mikkonen et al.
2019). In general, phenolic compounds are known for their bitter taste and astringency (Drewnowski
2001; Soares et al.
2018). The opaque appearance of the water solutions can be explained by incomplete solubility of hemicelluloses in an aqueous system. A recent study by Bhattarai et al. (
2020) showed that eGGM and sGGM contain two and three size classes of colloidal particles, respectively.
Processing method (ethanol precipitation vs. spray drying) had a stronger influence on the sensory profiles of the water solutions than the origin of the hemicelluloses (spruce vs. birch wood). Spruce-derived sGGM had a rather similar sensory profile as birch-derived sGX, although sGX was evaluated as slightly more bitter and astringent than sGGM. Likewise, spruce-derived eGGM had a similar sensory profile as birch-derived eGX. However, eGGM was rated as more intense in brown color, overall odor, and wood odor than eGX. Among the hemicellulose samples, the sensory profile of eGX had the least woody notes, and closely resembled that of the commercial GA.
Model product prototypes
Sensory profiles of strawberry yogurt with hemicellulose solutions followed a similar pattern to the sensory profiles of the solutions themselves. However, the yogurt characteristics partially masked the woody properties of the hemicelluloses, compared to the water solutions. Since the panel did not consider bitterness and astringency as important sensory attributes for the evaluation of the yogurt models, these attributes were not included in the sensory profiling of the yogurts. The yogurt matrix may have reduced the bitterness and astringency that were notable in the sGGM and sGX water solutions.
Again, the processing method had a stronger effect on the sensory properties than the origin of the hemicelluloses. Yogurts with spray-dried hemicelluloses (sGGM and sGX) had similar sensory profiles, which both showed much stronger woody characteristics (brown color and wood odor, taste, and aftertaste) than either of the samples with ethanol-precipitated hemicelluloses (eGGM and eGX). In contrast, the strawberry characteristics of the yogurt base (pink color and strawberry odor and taste) were perceived as more intense in eGGM and eGX than in sGGM and sGX. Thus, the woody characteristics of sGGM and sGX may have partially masked the strawberry characteristics of the yogurt.
When hemicelluloses were added to the yogurt model as an emulsion, the pattern of sensory profiles was mainly similar to those of yogurts with hemicelluloses added as a solution. Again, sGGM and sGX had obvious woody characteristics, while eGX largely retained the strawberry notes of the yogurt. However, in the yogurt with added emulsions, the sensory profile of eGGM was in between those of sGGM/sGX and eGX. Compared to yogurts with hemicellulose solutions, the yogurts with hemicellulose emulsions had higher fat content due to the rapeseed oil. Hemicelluloses may be located partially at the oil-droplet interface and partially in the continuous phase of emulsions (Bhattarai et al.
2019), which was miscible with the yogurt matrix.
Results for all three model systems consistently showed that the sensory profile of eGX was the most similar to that of GA, which was used as a commercial comparison sample with a very mild flavor. In yogurt models, no difference was observed in the sensory properties between eGX and GA, except in color. This suggests that eGX is a potential ingredient or additive in applications where woody sensory characteristics are not desired.
There was no correlation between the physical and sensory properties of the studied samples. The amount of emulsion added was so small that it did not cause perceivable sensory differences in texture of the yogurt samples. This finding corresponds to our earlier work, where we found that solutions and emulsions with GGM and GX showed rather low viscosity even at intermediate concentrations (Mikkonen et al.
2016a). However, panelists evaluated the sGGM sample as the thickest, albeit nominally. The droplet size distributions and average oil droplet sizes were similar in all GGM and GX emulsions. Even though the GA-stabilized emulsions had a larger average droplet size, the panelists did not find differences either in oiliness or smoothness while analyzing the samples qualitatively during the first training session.
Outlook on wood hemicelluloses as novel hydrocolloids
Since hemicelluloses occur in all terrestrial plant cell walls, they are part of many common foods such as cereal grains. Various plant-based side streams are an increasingly interesting source of natural hydrocolloids (Ralla et al.
2018). Isolated wood hemicelluloses are not traditional food ingredients or additives; however, similar compounds from other sources such as guar gum and konjac glucomannan are abundantly used, and xylans from grains are part of our common diet. On the other hand, wood-derived vanillin aroma, glycerol esters of wood rosins (E445), xylitol (E967) and steryls/stanols are being used as food ingredients or additives (Pitkänen et al.
2018). The PHWE extraction of wood saw meal yields samples rich in hemicelluloses that also contain varying amounts of co-components, mainly lignin-derived phenolic compounds (Kilpeläinen et al.
2014; Mikkonen et al.
2019). The phenolic compounds are beneficial for the functionality of hemicelluloses as emulsion stabilizers (Lehtonen et al.
2018; Mikkonen et al.
2019), but they also contribute to the sensory profile of hemicellulose-rich extracts. Wood-like flavor present in sGGM and sGX (due to the phenolic co-components) could be exploited in product design for creating new flavors. If such unique flavor is not desired, it can be largely removed by ethanol precipitation, especially from GX, and masked by the other food constituents. Wood hemicelluloses are potential stabilizers, for example in yogurts, beverages, dressings, and desserts (Valoppi et al.
2019b).
Suggested future studies include determining the interaction effects of the wood-like flavors of hemicelluloses with other various ingredients. In addition, hedonic tests need to be performed to determine if products with wood hemicelluloses are acceptable for consumers.
Conclusions
Sensory profiles of wood GGM and GX were characterized as water solutions and in yogurt models. The characteristic wood-like flavor, bitter taste, and astringency were pronounced in spray-dried sGGM and sGX solutions, which was attributed to the presence of lignin-derived phenolic compounds. Ethanol precipitation, which removed a large part of phenolic compounds from the hemicellulose-rich extracts, reduced the woody flavor. Thus, the hemicellulose processing method (spray drying vs. ethanol precipitation) had a more significant effect on the sensory profile than the wood source (spruce or birch). The results support the hypothesis that lignocellulosic co-components in wood extracts introduce sensory attributes, which the plain polysaccharides are lacking. Furthermore, the bitterness and astringency were fully masked when wood hemicellulose solutions or emulsions were mixed with strawberry yogurt. In particular, the ethanol-precipitated eGX showed a similar sensory profile as GA, which was used as a commercial comparison sample with a mild flavor. Wood hemicelluloses showed promising sensory properties as sustainable novel food ingredients or additives. The intensity of woody characteristics of hemicelluloses can be adjusted to meet the needs of various food applications.
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