Exploring the utilization of oat by-products in the circular bioeconomy: The potential for the development of extruded snacks enriched with β-glucan

doi:10.21203/rs.3.rs-7836700/v1

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Exploring the utilization of oat by-products in the circular bioeconomy: The potential for the development of extruded snacks enriched with β-glucan

https://doi.org/10.21203/rs.3.rs-7836700/v1

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The increasing prevalence of oat-based milk substitutes in the market has led to the generation of a substantial volume of β-glucan-enriched oat by-products (OBP) during production. However, the high moisture content of OBP poses challenges, as it is prone to decay and deterioration, making it difficult to recycle and reuse. This study aimed to pre-treat OBP using various drying methods and incorporate different ratios of brown rice with OBP in the extrusion process. The goal was to examine the effects of these variables on the physicochemical properties, textural characteristics, and digestibility of extruded snacks. Results showed that dried OBP was successfully incorporated into brown rice-based extruded snack formulations at levels of 5–15%. The radial expansion ratio, bulk density, and fracturability exhibited a negative correlation with the incorporation percentage, whereas hardness showed a positive correlation (p < 0.05). Additionally, the β-glucan and resistant starch contents of the extruded snacks increased significantly with higher OBP incorporation. The glycemic index of the extruded snacks decreased significantly, from 83.51 to 75.35, as OBP levels increased.

Therefore, this study highlights the importance of carefully selecting incorporation ratios and employing appropriate drying pre-treatments to achieve the desired sensory characteristics in extruded snacks.

Dietary fiber

Extrusion

Plant-based

Sustainability

Recently, there has been a notable surge in health consciousness and sustainable bioeconomy circle background, where the nutritional profile of extruded snacks has been enhanced by incorporating additional nutrients, bioactive elements, and by-products from food industries (grains’ bran, pomace, oilseed cake, and brewers spent must) sourced beyond cereals (Cheng et al., 2024; Mohammed et al., 2024; Sumargo et al., 2016; Ying et al., 2017). Simultaneously, there is a pressing need to address the high levels of salt, fat, and sugar found in numerous snacks available in the market, with the aim of improving their overall nutritional profile (Grasso, 2020). These advancements promote healthier dietary choices and serve as pivotal strategies in addressing the widespread prevalence of malnutrition and realizing zero hunger (Kalahal et al., 2024). Therefore, whether nutritional, economical, earth-friendly, or a combination of these, a specific and quantifiable added value should be provided to evaluate the practical contribution of by-products used rather than the repetitive substituting ingredients in similar recipes for pure publication work (Grasso, 2020). However, the broader food business chain has been long and extensive, involving the agri-food industries (Auestad & Fulgoni, 2015). The social responsibility approach of the established food industry is dedicated to providing products that are safe for consumption, consistent in quality, and offered at reasonable prices (Lucarini et al., 2020). However, this approach can also limit the industry's capacity to allocate resources to research, development, and innovation compared to other sectors with greater profitability. Consequently, this situation has contributed to the prevailing perception of the food industry as traditional (Mansilla-Obando et al., 2024). In addition, it pertains to the notion that the reutilization of developed items and associated surplus resources contribute to a more profound comprehension of the underlying principles of the circular bioeconomy (Do et al., 2022). Remarkably, the recycling of by-products exhibits significant variation between laboratory-scale and scaled-up closed-loop production regarding processed capacity, quantities, and environmental impact (C.-H. Lin et al., 2023). It is imperative to refrain from embracing unrealistic one-size-fits-all approaches when utilizing these by-products and to consider their edibility and palatability (Cheng et al., 2023; Kruijssen et al., 2020; C.-H. Lin et al., 2023). Therefore, apart from advocating for a healthy diet, these approaches can contribute progressively to promoting food security and sustainability towards achieving the United Nations' Sustainable Development Goals (SDGs) (Cooney et al., 2023; Y. D. Lin et al., 2023).

Oat (Avena sativa L.) is one of the globally essential cereal crops, with an annual global production of about 25.13 million metric tonnes (M. Shahbandeh, 2024), and it is extensively used in the food industry. Interestingly, the cultivation has been stable over the last 50 years, with profitable oat crops increasing yield and production value from the demand increased due to health benefits driven by reduced total cholesterol and low-density lipoprotein cholesterol, reduced risks of heart disease, and satiety (Ma et al., 2021; Mahadevan et al., 2016). Moreover, the escalating commercial availability of oats in beverages, healthy foods, cosmetics, and pharmaceuticals indicates a growing trend (Ma et al., 2021). Specifically, its macronutrients of 7–20% high-quality plant protein, dietary fiber (40% water-insoluble and 80% water-soluble), and high digestibility (90–94%), substantial amounts of β-glucan (4–8%) have been known to benefit from reducing the risks of all those chronic and metabolic diseases mentioned above (Ek et al., 2020; Jokinen et al., 2023; Yang et al., 2023). Moreover, incorporating this property into food recipes can be an effective supplement to compensate for the lack of fiber consumption in regular diets (Wu et al., 2024).

Recently, there has been a growing consumer trend toward plant-based protein beverages as a viable alternative to animal-based dairy products, with oat-based milk substitutes emerging as a prominent choice (Jokinen et al., 2021; McCarron et al., 2024; Silventoinen-Veijalainen et al., 2024). Notably, these plant-based products or animal-based substitutes have been recognized as sustainable for the environment, as these foods reduce negative environmental impacts, namely by contributing to the promotion of the aforementioned circular bioeconomy with the SDGs (Auestad & Fulgoni, 2015; Mansilla-Obando et al., 2024). Specifically, the starch pasting and hydrolysis in oats affect the sensory quality of the end product, whereas the richness of dietary fiber and the insoluble particles derived from insoluble proteins cause a negative impact on the beverage's properties (palatability and smoothness) (Sethi et al., 2016; Silventoinen-Veijalainen et al., 2024). It can also bring a sandy mouthfeel and cause sedimentation during the storage of the end products (McCarron et al., 2024; Sharma et al., 2024; Silventoinen-Veijalainen et al., 2024). However, in the commercially available manufacturing process, relatively larger particles have been eliminated during beverage production to improve the beverages' palatability, and the remaining liquid is then homogenized (McCarron et al., 2024; Sethi et al., 2016; Sharma et al., 2024). Regrettably, the necessary compromises, specifically filtration of the OBP enriched β-glucan, to meet consumer acceptability have led to significant waste in cost and circular bioeconomy concerns. Therefore, considering the above issues, this study proposed to develop OBP into a palatable extruded snack using extrusion processing technology; while increasing the added value of OBP, we also expect to improve the digestive properties of starch to make it a functional snack.

Materials

The brown rice (1st grade of Chinese National Standard 2424 (CNS, 2019)) was purchased from Golden Rice Castle Co., Ltd. (Chiayi, Taiwan). AGV Products Co. (Chiayi, Taiwan) was provided as the source of oat raw material for this study, the OBP (oats’ that originated in Australia and were harvested in 2022). Unless otherwise stated, all chemicals were purchased directly from Sigma-Aldrich® (Merck KGaA, Darmstadt, Germany) and used without further processing.

Extrusion processing parameters

The grains cake biscuit machine (Mibo-C1, YUAN CHUANG Food Machinery Co., Ltd., New Taipei City, Taiwan) was utilized in this study. The specific conditions were: mold hole diameter of 3 mm, screw speed of 50 HZ, feed rate at 16 rpm, sleeve temperature of 80°C, and cutter speed of 2 rpm. The above conditions were obtained through preliminary experiments.

Oat residues drying and extrusion treatment prior to processing

The raw material OBP in this study was dried by FD and CD, respectively, due to the high moisture content of OBP, both of which were controlled to less than 5%. The dried OBPs were packed in aluminum zipper bags with desiccant and stored in a desiccator. Subsequently, the raw material formulations of extrusion feed were mixed with brown rice in different ratios of OBP (5, 7.5, 10, 12.5, and 15%), respectively. Detailed grouping and composition as shown in Fig. 1.

Determination of radial expansion ratio (ER)

ER was determined following the method described by Cheng et al. (2024). Each group of random sampling (n = 20). Afterward, the diameter of the samples was measured using an electronic digital vernier caliper. The samples were rotated 90° and measured again while averaged to obtain the expanded diameter of the sample (mm). Finally, the ER (mm/mm) was calculated by dividing the average expansion diameter of each group by the hole diameter (3 mm) of the extruder.

Textural profiling analysis (TPA)

The TPA of the sample was determined utilizing the methodology described by Huang et al. (2023) and Lee et al. (2024), with some modifications. This study used a texture analyser (TA-XT2, Stable Micro Systems, Godalming, UK) with a P/50 probe (n = 20), and the specific parameters included compression mode; pre-test speed was 1 mm/sec; test speed was 2 mm/sec; post-test speed was 10 mm/sec; distance was 2 mm; and trigger force was 5 g. The hardness (N) and fracturability values were determined for the samples. The unit of fracturability was defined as the number of positive peaks within the sample.

Determination of bulk density (BD)

The sample’s BD was determined using the method described by Lin et al. (2023) with minor modifications. Glass beads of known volume (290 mL) and a measuring cylinder (500 mL) were prepared, followed by sampling (5 g of each group) and weighted. The glass beads were initially placed at the bottom of the cylinder. Next, one layer of glass beads and one layer of samples were stacked on each other. The highest part of the cylinder was covered in glass beads, and the space inside was filled with glass beads. The measurement of the height of the cylinder was recorded, resulting in the calculation of BD by dividing the weight by the volume.

Determination of β-glucan

The β-glucan in the samples was determined using the Megazyme β-glucan (Mixed Linkage) assay kit (Neogen Co. Lansing, MI, USA). The sample (100 mg) was weighed into a tube and extracted with 10 mL of 50% ethanol at 80°C for 10 min, then centrifugation (1,000 ×g for 10 min) with a centrifuge (Megafuge 16R, Thermo Fisher Scientific, Waltham, MA, USA) to remove the supernatant, repeating this procedure twice. Then, the residue was added with 50% ethanol 0.2 mL mixed by oscillation, and then 20 mM phosphate buffer solution 4 mL was added and remixed. Next, the solution was heated in a boiling water bath for 1 min, then taken out, oscillated evenly, then placed in a boiling water bath for 2 min, and oscillated evenly again. Equilibrate the tubes in a 50°C water bath, then add 0.2 mL of lichenase solution with shaking to mix, followed by reaction for 60 min in a 50°C water bath. During the reaction, oscillation was required every 15 min for the enzymatic reaction to be fully activated. Afterward, 5 mL of 20 mM sodium acetate buffer solution was added to each tube and shaken evenly, allowing it cooled to 25°C, then centrifuged (1,000 ×g for 10 min). Each sample was divided into 3 tubes (all containing 0.1 mL of the above supernatant), where 2 tubes continued to react with 0.1 mL β-glucosidase, while the remaining tube was added 0.1 mL of 50 mM acetic acid buffer solution as a blank value. Next, all tubes were added with 3 mL of glucose oxidase-peroxidase mixed buffer solution (glucose oxidase > 12,000 Unit (U)/L, peroxidase > 650 U/L, 4-aminoantipyrine (81.3 mg/L)), followed by a 50°C water bath reaction for 20 min. Finally, the absorbance of each sample at 510 nm was determined using a microplate spectrophotometer (BioTek Epoch 2, Agilent Technologies, Inc., Santa Clara, CA, USA). The following equations were used to calculate β-glucan content in the samples.

$$\:\beta\:-glucan\:content(\%,wet\:basis)=\varDelta\:E\times\:F\times\:94\times\:\frac{1}{1000}\times\:\frac{100}{W}\times\:\frac{162}{180}\left(1\right)$$

$$\:\beta\:-glucan\:content(\%,dry\:basis)=\frac{Equations\left(1\right)}{100-Moisture\:content\left(\%\right)}\left(2\right)$$

where,

△E: absorbance value of the reaction solution -absorbance value of the blank solution F: Conversion factor for glucose content (µg) (as 100 µg glucose/100 µg glucose absorbance)

94: Volume correction factor (0.1 mL of analytical solution from 9.4 mL)

1/1000: Conversion value of µg and mg.

W: weight of test sample (mg)

162/180: glucose conversion factor

Determination of resistant starch (RS) and non-RS

The RS and non-RS determination was performed using the Megazyme® RS assay kit (Neogen Co. Lansing, MI, USA) and following the standard operating procedures provided by the manufacturer. The sample (100 mg) was milled to 40 mesh, placed in a 50 mL centrifuge tube, and added 4 mL of pancreatic α-amylase reagent (10 mg/mL AMG). The mixture was mixed and reacted for 16 h at 37°C in a water bath. 4 mL of ethanol (99%) was added and centrifuged (6,000 ×g, 10 min), then the supernatant was collected. The residue was mixed with 8 mL of 50% ethanol, centrifuged at 6,000 ×g for 10 min, and then separated and collected from the supernatant. This step was repeated twice. The above residue was mixed with 2 mL of 2 M potassium hydroxide, stirred in an ice bath for 20 min, and then added into 8 mL of acetic acid buffer solution (1.2 M, pH 3.8). Subsequently, 0.1 mL of the supernatant was incorporated with 3 mL GOPOD reagent, then reacted at 50℃ for 20 min while the absorbance was measured using a microplate spectrophotometer (BioTek Epoch 2, Agilent Technologies, Inc., Santa Clara, CA, USA) at 510 nm. As mentioned above, the standard was 0.1 mL of 100 mg/mL glucose, and the procedures were repeated. Subsequently, the RS content of the sample was ascertained by utilizing the subsequent formula:

$$\:Resistant\:starch\left(RS<10\%;\frac{g}{100gsample}\right)=Absorbance\:value\:of\:the(Sample-Control)\times\:\raisebox{1ex}{$Absorbance\:value\:of100\mu\:gglucose$}\!\left/\:\!\raisebox{-1ex}{$100mg$}\right.\times\:9.27\left(3\right)$$

Regarding the determination of non-RS content, the above-prepared supernatant was repeated in the same manner as RS's, and the following equation calculated the non-RS content in the sample.

Determination of digestibility (In vitro)

The digestibility of the sample, which consisted of both rapidly digested starch (RDS), slowly digestible starch (SDS), and estimated glycemic index (eGI), was assessed using the methodology described by Wang et al. (2023). The mixture, consisting of 1.82 g of porcine pancreatic α-amylase reagent, was then supplemented with 6 mL of distilled water, subjected to stirring for 15 min, and centrifuged at 1,500 ×g for 10 min. Subsequently, 5 mL of supernatant and 62.5 µL of amyloglucosidase solution (AMG; 3,300 U/mL) were mixed uniformly for the preparation. Next, 100 mg of the sample was mixed with pH 5.2, 4 mL of 0.1 M sodium acetate buffer solution, and 1 mL of the amylase mentioned above. The mixture was placed in a water bath for 0, 20, 30, 60, 90, 120, 150, and 180 min (with 120 rpm shaking), followed by a sampling of 0.1 mL for each. Afterward, 0.9 mL of 95% ethanol was added to terminate the reaction. Then, centrifuged at 1,500 ×g for 10 min, 0.1 mL of the supernatant was taken, and 3 mL of glucose oxidase-peroxidase aminoantipyrine (GOPOD) reagent was added. The absorbance value was determined using a microplate spectrophotometer (BioTek Epoch 2, Agilent Technologies, Inc., Santa Clara, CA, USA) at a wavelength of 510 nm. The RDS, SDS and eGI of the sample were calculated using the following equations:

$$\:Rapidly\:digestible\:starch\left(RDS;\%\right)=\left(G20\right.-\left.G0\right)\times\:0.9\left(4\right)$$

$$\:Slowly\:digestible\:starch(SDS;\%)=\left(G120\right.-\left.G20\right)\times\:0.9\left(5\right)$$

$$\:Hydrolysis\:index\left(HI;\%\right)=Area\:below\:hydrolysis\:curve\:of\frac{Sample}{Standard}\times\:100\left(6\right)$$

$$\:Estimate\:of\:the\:glycemic\:index\left(eGI\right)=39.71+0.549\times\:HI\left(7\right)$$

Sensory evaluation

The sensory evaluation methods were based on the descriptions of Huang et al. (2024a) with minor modifications. For this study, 30 panelists were randomly recruited based on consumer type (individuals who were not food science professional-trained personnel) and then evaluated the products based on preference and purchase intention. The evaluation was based on a 9-point Hedonic Scale, which panelists the product's appearance, aroma, off-flavor, fracturability, hardness, stickiness, flavor, and overall preferences. The score includes one point for immensely dislike, five for no comment (not about liking or disliking), and nine for extremely like. Before conducting sensory evaluations, we ensured that participants were given clear instructions regarding informed consent. We informed them of our adherence to established ethical guidelines and obtained their consent to participate in the evaluation. This approach was adopted to ensure that participants were fully informed about the process and could make an informed decision about whether to participate in this sensory evaluation.

Statistical Analysis

Each trial underwent at least three repetitions, and the outcomes were presented as the mean ± standard deviation (SD). This study used the statistical software of XLSTAT (version 2019, Lumivero, Denver, CO, USA) with the one-way variance analysis (ANOVA). Duncan's multiple-range test was used to compare the groups' variability. It was determined that the difference was significant when p < 0.05 was observed.

Effect of different ratios’ oat by-product ratios (OBP) on appearance and physicochemical properties of extruded snacks

This study showed that the appearance of the snacks in each group under the same extrusion processing conditions varied depending on the incorporation of different ratios of OBPs (Fig. 1). Specifically, the 100% brown rice group exhibited the most extended length and maximum cross-sectional area. In contrast, the snacks experienced a reduction in size and flattening of cross-sectional diameters as OBPs incorporation increased. It was hypothesized that the high concentration of dietary fiber in OBP contributed to the snacks’ unappealing appearance. Namely, the high hydrophilicity of the fiber content affects the physical characteristics of the extruded snacks; in particular, ER decreases and thus increases hardness (Kalahal et al., 2024; Nikinmaa et al., 2023a). In addition, the effect of thermal treatment on the extrusion process depends on the different levels of lipids and proteins present (Blandino et al., 2022). Decreased starch pasting is observed in the case of lipids, and less starch swelling is observed in the case of high-protein contents (Dalbhagat et al., 2019). Conversely, it has been reported that modifications in the ratio of these ingredients incorporated as described above can also facilitate the drawbacks (Boakye et al., 2023). Specifically, appropriate fiber incorporation can provide large air pockets and thin cell walls to improve the fracturability and hardness of extruded snacks, while appropriate protein incorporation can inhibit air bubble formation to reduce the fracturability of extruded snacks (Boakye et al., 2023; Korkerd et al., 2016). Another possible explanation for this deficiency might be that the screw speed and temperature are currently suitable for use in the control group, which would necessitate improvement in the future following the determination of a suitable OBP incorporation ratio. However, optimizing each recipe individually is uneconomical and difficult to achieve in practice due to the many analyses required (Nikinmaa et al., 2023a) and the probability of subtle changes that may be undetectable to humans. Thereby, the BD and hardness of the extruded snacks were decreased by modifying the parameters such as die pressure, screw speed, temperatures, specific mechanical energy, torque, feed moisture, and feed speed (Boakye et al., 2023; Dalbhagat et al., 2019; Ying et al., 2017; Zambrano et al., 2024). It is important to note that the unappealing visual presentation and texture of OBP extruded snacks can significantly reduce consumer willingness to purchase them (Kojić et al., 2022; Nikinmaa et al., 2023a). However, it has been suggested that incorporating 10–30% cereal fiber into extruded product recipes as substitutes for flour can provide appropriate hardness and consumer acceptance (Hsieh et al., 1989; Korkerd et al., 2016; Rzedzicki et al., 2000).

Table 1
Effect of different ratios of freeze-dried (FD) and complex drying (CD; autoclaving (121°C, 15 min) combined with hot-air drying) oat by-product (OBP) on the radial expansion ratio (ER) and texture properties (hardness and fracturability) properties of extruded snacks
Groups	Brown rice (%)	OBP (%)	Radial expansion ratio (ER; mm/mm)	Hardness (N)	Fracturability
Control	100	0.00	3.25 ± 0.14^a	82.52 ± 8.99^d	28.6 ± 2.91^a
FD1	95.00	5.00	3.08 ± 0.16^a	103.67 ± 11.76^d	25.4 ± 3.54^ab
FD2	92.50	7.50	2.76 ± 0.18^b	113.82 ± 18.53^d	23.6 ± 3.6^ab
FD3	90.00	10.00	2.08 ± 0.11^c	142.78 ± 23.92^cd	19.73 ± 2.6^bcd
FD4	87.50	12.50	1.69 ± 0.11^d	192.72 ± 35.51^bc	15.67 ± 3.13^cde
FD5	85.00	15.00	1.59 ± 0.16^d	220.04 ± 48.13^b	14.2 ± 3.9^de
CD1	95.00	5.00	3.08 ± 0.12^a	96.63 ± 11.00^d	22.6 ± 1.83^b
CD 2	92.50	7.50	2.73 ± 0.2^b	121.51 ± 14.68^d	20.13 ± 3.6^bc
CD 3	90.00	10.00	2.01 ± 0.15^c	221.58 ± 48.13^b	15.2 ± 2.91^cde
CD 4	87.50	12.50	1.69 ± 0.14^d	325.96 ± 57.33^a	11.93 ± 1.4^e
CD 5	85.00	15.00	1.51 ± 0.13^d	327.82 ± 47.14^a	11.67 ± 3.2^e

Each value was expressed as mean ± SD (n = 3).

Lowercase letters with different superscripts in the same column represent a significant difference, p < 0.05.

Radial expansion ratio (ER) and texture (hardness and fracturability)

This study showed that the ERs of snacks with different ratios of OBP extrusion ranged from 1.51 to 3.08 mm/mm (Table 1), where the maximum and minimum ERs were found in the FD-OBP 5% group (3.08 mm/mm) and the CD-OBP 15% group (1.51 mm/mm), respectively. This also implied a significant decrease (p < 0.05) in ER for increasing the ratio of OBP incorporation, but an insignificant difference was observed between the 12.5 and 15% groups. It was hypothesized that a possible reason for the lower limit of the ER was reached with the 12.5% OBP incorporation. Anton et al. (2009) reported that soybean flour’s higher seed coat fiber contents also influenced the extruded product's decreased expansion ratio. The authors have indicated that this is attributed to the fibers causing cell wall rupture of the product, thereby preventing the bubbles from inflating to their maximum potential. Studies have indicated that incorporating high-protein and high-fiber elements into starch-based formulations with extrusion processes reduces the snack's overall porosity and increases its structural density (Korkerd et al., 2016; Onwulata et al., 2001).

In terms of textural properties, this study found that the control group showed the lowest hardness and the highest fracturability (Table 1), while the CD5 group exhibited the highest hardness but the lowest fracturability. Specifically, the hardness of extruded snacks increased according to the ratio of OBP incorporation. In comparison, fracturability shows the opposite trend. It is worth mentioning that there was a significant difference in hardness (p < 0.05), whereas fracturability was not significant between groups with OBP incorporation over 7.5%. These phenomena can be attributed to the textural properties of the extruded snacks correlated with BD (Dalbhagat et al., 2019). Specifically, incorporating proteins or fibers in the formulation leads to the formation of compression pores, which leads to a tighter tissue structure and fracturability (Ayoub et al., 2013; Beck et al., 2018).

Bulk density

BD is also one of the critical indices of consumer acceptability of extruded snacks (Jain et al., 2022). This study's results indicated that the BD of each group was the minimum in the control group (0.079 g/mL) (Fig. 2A). In contrast, the remaining groups increased as the OBP incorporation ratio increased, whereas 15% of CD- and FD-OBPs groups exhibited the maximum BD but the minimum ER (Section 3.1.1 above). This can be attributed to the decreased density of extruded snacks associated with the increased ER, and this study's findings were consistent with the trends reported by Pitts et al. (2014) and Sharif et al. (2014). In addition, BD was typically affected by being subjected to barrel temperature and screw speed precisely due to ER changes in cell structure, pores, and voids (Jain et al., 2022). Therefore, the above results showed that OBP incorporation ratio, BD, and hardness negatively correlated with ER and fracturability.

β-glucan contents

This study showed that β-glucan content was positively correlated with the ratio of OBP incorporated (Fig. 2B). Specifically, the CD-OBP groups contained slightly more β-glucan than the FD groups, despite no statistically significant differences. In addition, Nikinmaa et al. (2023b) reported that the determined amount of β-glucan that can be extracted from the extrudes of whole-wheat oat-based matrices ranged from 0.60 to 1.31 g/100 g, suggesting that these results were similar to this study. Notably, the extraction rate of β-glucan was related to increased specific mechanical energy (SME) and β-glucan degradation during the extrusion process (Nikinmaa et al., 2023b).

In Vitro digestibility

In this study, the results showed that the incorporation of the CD-OBP groups resulted in significantly decreased RDS levels (p < 0.05) compared to the FD groups (Fig. 2C). It was hypothesized that the autoclaving treatment caused the amylose and amylopectin molecules to migrate, facilitating the amylose chain connections, destroying the original crystalline structure, and forming new ones (Dominguez-Ayala et al., 2023). Specifically, it has been reported that the enhanced mobility and interactions of molecular chains within the crystalline region encompassing starch granules have been found to play a role in stabilizing the hydrogen bonding between the starch chains (An et al., 2024; Dominguez-Ayala et al., 2023). In addition, compared to the control group, RDS decreased significantly, whereas SDS increased significantly (p < 0.05) for all groups, regardless of the type of OBP added (Fig. 2C). However, it has also been reported that the enhanced digestibility of extruded snacks was associated in some cases with starch digestibility due to starch melting and depolymerizing to a slight increase, thus leading to an adverse health risk in terms of glycaemic response (Roye et al., 2021). Moreover, the high temperature, high pressure, and shear force to which the starch is subjected during the extrusion process can damage the granular structure of the starch (Kumari et al., 2024). Additionally, these factors increase the degree of pasting, making it more susceptible to enzymatic hydrolysis while contributing to an increase in digestibility (Brennan et al., 2013; Soler et al., 2020). Another possible explanation may be that the proteins around starch particles in the mixture are exposed to high temperature and pressure, limiting starch granule swelling and starch pasting, starch structure formation as tight stacks (crystallinity) that are resistant to enzymatic hydrolysis (Huang et al., 2024c; Kaur et al., 2023). Remarkably, the tightly structured is appropriate for co-extruded snacks requiring filling (Blandino et al., 2022). Moreover, it has also been reported that dry heating, RDS in RS, creates more small molecular fractions with suitable chain lengths, enhancing better rearrangement within the starch molecules during annealing (Kumari & Sit, 2023). In addition, freeze-drying can impact starch's solubility, leaching, and damaged crystalline formation, leading to increased water absorption (Huang et al., 2024b; Soler et al., 2020).

Regarding the GI values, this study showed that the control group was significantly higher (p < 0.05) than the other groups by extrusion processing (Fig. 2D). Simultaneously, the GI values of the groups decreased as the OBP incorporated ratio increased. According to reports, elevating the ER of extruded snacks may increase starch hydrolysis, consequently leading to a swifter glucose response (Alam et al., 2016; Nikinmaa et al., 2023b). However, OBP is rich in dietary fiber (β-glucan), which might contribute to reduced starch digestibility by amylose-fat inclusion complexes externally wrapping the starch molecules and restricting access to hydrolytic enzymes (Qadir & Wani, 2022). It also contributed to decreased RDS and GI, while SDS and RS remarkably increased and agreed with published previous results (Soler et al., 2020). Despite the current findings in this study indicate that the extruded snacks remain high-GI (≥ 70) foods (Capriles et al., 2021). However, the inclusion of OBP led to a reduction in GI compared to the control group. Consequently, the following action should explore the synergistic use of other methods, further modification of OBP, or adjustment of the extrusion process parameters to enhance RS levels.

Figure 2. Effects of different ratios of freeze-dried (FD) and complex drying (CD; autoclaving combined with hot-air drying) of oat by-product (OBP) on the (A) bulk density (BD), (B) β-glucan content, (C) rapidly digested starch (RDS) and slowly digestible starch (SDS), (D) resistant starch (RS) content and (GI) value of extruded snacks

. Each value was expressed as mean ± SD (n = 3). Lowercase letters with different superscripts in the same column represent a significant difference, p < 0.05.

Resistant starch (RS) content

This study showed that the RS content of the extruded snacks increased following the incorporation of the CD- and FD-OBP groups (Fig. 2E), particularly in the CD groups, with the increase in RS being significantly higher than in the FD groups (p < 0.05). More specifically, group CD5 exhibited the maximum RS content (19.55 g/100g), while group F4 showed the minimum (9.69 g/100g). This phenomenon can be explained by CD-OBP preparation belonging to the heat moisture treatment (HMT) class, namely, at low moisture (less than 35%) and 84–120°C temperatures (Hoover, 2010; Y.-W. Lin et al., 2023; Soler et al., 2020). This results in changes in the amorphous and crystalline structure of the starch particles, thus increasing the RDS while modifying the physicochemical properties of the starch (Huang et al., 2016; Y.-W. Lin et al., 2023; Soler et al., 2020). However, low-moisture (15 g/100 g) extrusion manufacturing has also been confirmed to increase the RS content in extruded snacks (Brahma et al., 2016; Nikinmaa et al., 2023b). Notably, drying at 90°C and above has been reported to facilitate the formation of long-chain double-helix high-temperature-resistant ordered starch structures and be less favorable to forming amylose-lipid complexes in starch particles (G. Cheng et al., 2024; Nikinmaa et al., 2023a; Thachil et al., 2014). This also implied that the starch showed an increased tolerance to digestive enzymes (G. Cheng et al., 2024). Therefore, it was found that CD-OBP contributed positively to the increase of RS content caused by both high-temperature sterilization and hot air drying.

Sensory evaluation

This study conducted the first stage of sensory evaluation for all the OBP groups with different ratios of extruded snacks by consumer-type sensory evaluation. It was confirmed that the overall preferences of the 5 and 7.5% groups incorporated in this study were significantly better than others (Fig. 3A). However, many panelists provided feedback indicating that the extruded snacks incorporating an OBP ratio over 7.5% were highly rigid in texture, difficult to chew, and suffered from issues such as stickiness and reversion flavor (from oxidized lipids). Therefore, in the second stage, FD- and CD-OBPs were selected as the 5 and 7.5% incorporation ratios compared with the control group for the preference evaluation. The effects of different drying treatments on the appearance, aroma, taste, fracturability, hardness, stickiness, flavor, and overall preferences of the OBPs on the extruded snacks were verified. This study showed that the appearance scores of the control, FD1, and CD1 groups were the same (Fig. 3B), but the scores were significantly different (p < 0.05) compared to FD2 and CD2 groups. Regarding aroma, the control group was the highest while the FD1 and CD1 groups were the second, and there was a significant difference between all groups (p < 0.05). Unfortunately, the same resulted in the off-flavor as well. It was hypothesized to be related to the property of brown rice to oxidize readily. This may be explained by the presence of lipids in the bran layer of brown rice, causing oxidization and negatively affecting sensory characteristics (Gondal et al., 2021; Y.-W. Lin et al., 2023). However, it has been indicated that flavor issues stemming from substandard processing conditions can elevate the lipid oxidation rate of oats, resulting in undesirable attributes such as staleness and moldiness (Nikinmaa et al., 2023a; Zhang et al., 2024). In addition, scores in the fracturability, hardness, stickiness, flavor, and overall preferences groups followed the same trend as above. These results implicate that OBP incorporation over 5% would lead to substantially lower scores for all sensory parameters. Moreover, incorporating OBP prepared with different drying treatments revealed an insignificant difference in the scores of all sensory indicators for extruded snacks. This also implied that the impact on the extruded snacks was the ratio of the incorporated OBP. Therefore, the percentage of OBP that was the most preferred by the panelists in this study was 5%. Agnieszka et al. (2015) reported the best yield for incorporating 20% oat bran to extrude snacks, which performance received the approval of the trained panelists. Similarly, extruded snacks containing 5% citrus pomace fiber have been reported to have the highest level of acceptability in consumer evaluations (Zambrano et al., 2024), similar to this study's results. Therefore, the panelists showed a preference for a 5% OBP.

Estimated extruded snack production costs of oats by-products (OBP)

This study estimated the production cost of OBP extruded snacks, yet the investigation of this study involved the evaluation of the feasibility of production without entering into formal commercialization. Therefore, the subsequent costs of packaging, warehousing, shipment, sales, management, etc., in this study could not be estimated correctly, except for the preliminary calculation of the costs of raw materials and processes. The well-known indicates that projected costs that have not yet been put into practice are susceptible to inaccuracies. The cost estimation findings from this study demonstrate that utilizing CD-OBP for production was the most economical option, as it yields the lowest production cost (Table 2). Therefore, this study verified that the repeated utilization of OBP confirms its ability to yield substantial advantages for the participating companies. Specifically, it enables the maximization of material utilization and contributes to offsetting (or amortization) the expenses associated with procuring oat raw materials.

Table 2
The estimated extruded snack production costs for 5% freeze-dried (FD)- oat by-product (OBP) and complex drying (CD; autoclaving combined with hot-air drying)-OBP
Extruded snack process cost	Item	USD $/kg
Material cost	Oats	1.37
	Oat by-product (OBP)	0.21
	Brown rice	3.12
		USD/kg/h
Power consumption	Freeze-drying (FD)	0.41
	Complex drying (CD; autoclaving combined with hot-air drying)	0.07
	Extrusion process	0.15
		USD/kg/h
Finished product cost	5% FD-OBP	15.08
Finished product cost	5% CD-OBP	14.99
Extruded snacks (Brown rice base) market price (USD $/kg): 86.00 Minus fixed costs such as packaging, management, shipment, warehousing, and sales (USD $/kg): 21.50

The findings of this study suggest that integrating OBP into the circular bioeconomy for closed-loop production within the industry presents a feasible solution. Given the paramount importance of ensuring the nutritional safety of food products, it is imperative to address the susceptibility of OBP's high water content to quality degradation, thereby necessitating effective food waste management. In addition, this implies that without considering closed-loop production, it would require drying for a certain period to avoid the proliferation of food-borne pathogens and then producing the extruded snacks under suitable conditions. Therefore, this study combined the formulation of soluble dietary fiber (β-glucan) enriched OBP with brown rice through extrusion processing to enhance the nutritional value of the extruded snack. This involved increasing RS content, reducing digestibility (in vitro), and lowering GI values. This study also determined that an optimal OBP incorporation of 5% and a discernible level of consumer willingness to purchase were identified through consumer-based sensory evaluations. However, further investigation and enhancement are imperative to understand the process better and align with consumers’ practical demands. The implications also imply contributing to increased consumer acceptance of this valuable food matrix and facilitating the development of closed-loop production on a commercial scale. These implications also suggest a potential increase in consumer acceptance of this valuable food matrix and the facilitation of closed-loop production on a commercial scale. However, further exploration of this concept holds promise for promoting sustainable production and enhancing the value of underutilized by-products. Therefore, this study's findings will provide valuable insights into selecting extruded treatments for OBPs to enhance the quality of extruded snacks.

Data availability

Data will be made available on request.

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have influenced the work reported in this paper.

CRediT authorship contribution statement

Chen-Ru Lin: Writing – original draft, Methodology, Formal analysis, Data curation; Ping-Hsiu Huang: Writing – review & editing, Writing – original draft, Data curation; Wen-Chang Chang: Writing – review & editing, Writing – original draft, Project administration, Funding acquisition, Data curation, Conceptualization.

Ethics statement for the use of human and animal subjects

In this study, we did not conduct invasive human trials. Figure 3 of the questionnaire comprise an "Informed Consent" form for participants, along with detailed data collection and usage instructions. Prior to the commencement of the questionnaire, participants were informed that neither the protocol nor the questionnaire provided financial or other forms of compensation for their participation. Additionally, they were assured that their responses would be published anonymously in a scientific paper. Concurrently, participants were informed they had the right to withdraw from this study without facing any consequences. Written consent was obtained from all participants after the experimental protocol was thoroughly explained to them, and they explicitly agreed to the conditions required for participation.

Consent for publication

Not applicable.

Funding

This research work and subsidiary spending were supported by the National Science and Technology Council of the Republic of China (ROC), Taiwan, No. NSTC 112-2221-E-415-001.

Acknowledgments

The authors would like to thank all the individuals who volunteered for this study.

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Exploring the utilization of oat by-products in the circular bioeconomy: The potential for the development of extruded snacks enriched with β-glucan

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Version 1

Abstract

Figures

Introduction

Materials and methods

Materials

Extrusion processing parameters

Oat residues drying and extrusion treatment prior to processing

Determination of radial expansion ratio (ER)

Textural profiling analysis (TPA)

Determination of bulk density (BD)

Determination of β-glucan

Determination of resistant starch (RS) and non-RS

Sensory evaluation

Statistical Analysis

Results and discussion

Radial expansion ratio (ER) and texture (hardness and fracturability)

Bulk density

β-glucan contents

Resistant starch (RS) content

Sensory evaluation

Estimated extruded snack production costs of oats by-products (OBP)

Conclusions

Data availability

Declarations

Declaration of interests

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Ethics statement for the use of human and animal subjects

Consent for publication

Funding

Acknowledgments

References

Supplementary Files

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Version 1