Introduction

The food processing industry continues to grow, and consumer demand for ready to eat foods, particularly bakery products, has increased, with baked goods increasingly formulated using composite flours to meet evolving preferences for convenient and nutritious products [1–3]. The use of composite flour has become an attractive option for bakery producers, driven largely by economic factors such as reducing reliance on imported raw materials like wheat flour, and research has shown that composite flour blends can improve nutritional and functional attributes of bakery products, including bread [4, 5]. Consequently, composite flour production represents a practical alternative while also encouraging diversification of flours derived from local agricultural resources to enhance nutritional quality and product variety in bakery applications [6–8].

The utilisation of local agricultural sources such as fruits, vegetables, herbs, and tuber crops has been increasingly promoted by small and medium enterprises in the development of innovative food products, as composite flour research continues to emphasise functional and nutritional enrichment from diverse raw materials. Previous studies have highlighted the potential of composite flour derived from various underutilised crops, including legumes, tubers, and pseudocereals, for enhancing the quality of baked goods such as bread through improved nutrient profiles and functional properties [9–12]. Composite flour development not only emphasises product quality but also nutritional value that can benefit consumer health, with research demonstrating that blending locally sourced flours can increase dietary fibre, protein, and bioactive compounds in bakery products [13–15]. This has contributed to the diversification of bakery products in the market, particularly bread, as producers explore composite formulations that respond to both health trends and sustainability goals [16].

Bread is one of the most widely consumed bakery products, as modern consumers increasingly prefer foods that are convenient, portable, and quick to consume, and it remains a staple carbohydrate source in global diets [17]. Bread is typically produced from wheat flour, sugar, salt, yeast, and water, with wheat flour’s gluten proteins imparting the structural and textural qualities characteristic of leavened bread [18]. To ensure the commercial viability of composite-flour-based bread, systematic scientific studies are required to determine optimal processing methods and formulation strategies for producing high-quality bread, and recent research has focused on formulating functional bread blends using alternative flours from legumes, tubers, and other sources to improve nutritional profiles while maintaining sensory acceptability [19–23]. This has led many researchers to investigate the use of composite flour in bread production. Previous studies suggest that wheat flour substitution up to approximately 10–20% can produce acceptable bread quality without significantly compromising sensory attributes [24]. However, higher substitution levels often negatively affect bread quality due to gluten dilution and weakened dough structure, leading to changes in loaf volume and texture that challenge acceptability [25].

Tarap (Artocarpus odoratissimus), a fruit native to Borneo, remains underutilised despite emerging evidence of its nutritional and compositional potential. Recent research highlights the fruit’s value as a source of carbohydrates, minerals, phytochemicals, and other bioactive compounds that could support food product development and nutritional applications [26, 27]. Converting tarap into flour provides an opportunity to develop value-added bakery products while promoting the use of indigenous crops. Therefore, this study aims to evaluate the effects of incorporating tarap composite flour in bread production on rheological, physical, and sensory properties. A consumer acceptance test was also conducted to assess product quality and potential market acceptance.

Materials and methods

Results

Farinograph measurements showed that dough consistency increased as the proportion of TF increased, with values rising from 464.16 FU (Farinograph Unit) in TCF0 to 522.25 FU in TCF4 (Table 3). Significant differences (p < 0.05) were observed at higher substitution levels. Water absorption decreased progressively from 58.70% to 55.30%, with significant reductions at higher TF incorporation. Dough development time declined from 6.68 min to 4.51 min, and dough stability decreased from 8.90 min to 4.54 min, both showing significant differences among formulations. Taken together, these farinograph results indicate clear changes in dough mixing behaviour as TF substitution increased.

Consistent with the farinograph findings, TPA of the dough showed changes with increasing TF incorporation. Dough hardness increased progressively from 1.42 N in TCF0 to 2.02 N in TCF4 (Table 4). However, based on the statistical analysis, TCF0 and TCF1 were not significantly different (p > 0.05), while TCF2, TCF3, and TCF4 exhibited significantly higher hardness values (p < 0.05) compared to the lower substitution levels. For adhesiveness, values increased from 40.80 N to 60.02 N with increasing TF substitution. TCF0, TCF1, and TCF2 did not differ significantly from each other (p > 0.05), whereas TCF4 showed significantly higher adhesiveness (p < 0.05). TCF3 displayed intermediate behaviour and was not significantly different from either the lower or higher substitution groups. These results indicate that significant changes in dough texture properties become more pronounced at higher TF substitution levels (> 10% for hardness and > 20% for adhesiveness), which may subsequently influence the physical characteristics of the baked bread.

Table 5 presents the physical characteristics of tarap composite bread, including loaf weight, volume, specific volume, and density. Bread weight increased from 144.3 g in the control to 148.8 g at 20% substitution, with significant differences appearing at the highest level. In contrast, bread volume decreased progressively from 662 cm³ to 517 cm³, with significant reductions as substitution increased. Specific volume declined from 4.58 to 3.47 cm³/g, while density increased from 0.21 to 0.28 g/cm³, although density differences were not statistically significant. Alongside these structural changes, noticeable differences were also observed in bread appearance.

Crust colour measurements showed that lightness (L*) decreased from 61.09 to 48.13 with increasing TF levels, with significant reductions beginning at 10% substitution. Redness (a*) increased from 0.92 to 4.23, and yellowness (b*) increased from 11.07 to 16.68, both showing significant differences across substitution levels (Table 6). The browning index increased from 20.65 to 48.20, with significant differences among samples. In parallel with colour changes, mechanical texture of the bread crumb was also affected. Bread hardness increased with higher TF incorporation, ranging from 12.56 N to 22.35 N, with significant differences observed from 10% substitution onward (Figure 1). Beyond physical and textural attributes, TF substitution also influenced the nutritional composition of the bread.

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Figure 1. Bread hardness of composite flour samples. Values are presented as mean (n = 3) ± standard deviation. Bars with different letters indicate significant differences (p < 0.05). TCF: tarap composite flour.

Proximate composition and total dietary fibre of composite tarap bread (%) are summarised in Table 7. Moisture content decreased from 40.32% to 33.22%, with significant differences between the highest and lowest substitution levels. Protein content declined slightly from 10.13% to 9.76%, and fat content decreased from 1.14% to 0.92%. Ash content increased from 1.21% to 1.44%, and crude fibre increased from 10.42% to 10.81%. Carbohydrate content increased from 36.78% to 43.85%, showing significant differences. Total dietary fibre increased from 10.52% to 11.91%, although differences were not statistically significant. These compositional changes were accompanied by differences in sensory perception.

Sensory scores differed significantly among samples (Table 8) and generally declined as the level of TF increased. The control (TCF0) and bread with 5% TF (TCF1) recorded the highest ratings for texture (6.22–6.29), taste (6.67–6.88), colour (6.22–6.74), and overall acceptability (6.33–6.48), with no significant differences between them (p > 0.05). Bread containing 5% TF achieved the highest overall acceptability, with most respondents rating it between “moderately liked” and “liked.” From TCF2 onwards, significant reductions were observed in most attributes: texture decreased from 5.89 (TCF2) to 3.67 (TCF4), taste from 5.59 to 3.70, colour from 4.20 to 3.13, and overall acceptability from 5.70 to 3.48. Bread containing 20% TF (TCF4) consistently received the lowest scores for all attributes. Perceived hardness also differed significantly among samples, showing a continuous decline from the highest score in TCF0 (6.50) to the lowest in TCF4 (2.11).

To further validate these findings, consumer acceptance was evaluated under real-market conditions (Figure 2). Evaluation of the overall acceptability of composite tarap bread (Figure 2A) showed that 68% of respondents liked the bread, 13% were uncertain in their evaluation, and 19% did not like the bread produced. Furthermore, 57% of respondents indicated that they would purchase composite tarap bread if marketed, while 23% were unsure and 20% stated that they would not purchase the bread (Figure 2B). Overall, consumer acceptance was considered moderate.

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Figure 2. Consumer evaluation of composite tarap bread (n = 100). (A) Distribution of consumer liking toward composite tarap bread. (B) Consumer purchase intention for composite tarap bread if introduced to the market.

Discussion

The rheological behaviour of tarap composite dough reflects the compositional differences introduced by TF, particularly its lower protein content relative to wheat flour. Proteins, especially glutenin and gliadin, play a central role in governing dough viscoelasticity, structure, and mixing tolerance in wheat-based systems, as gluten formation and hydrogen bonding strongly influence the dough’s ability to develop a cohesive and extensible network during mixing [31]. As TF substitution increased, farinograph measurements showed a progressive rise in dough consistency, indicating that protein dilution altered the balance of the dough matrix. With less gluten available to form a continuous viscoelastic network, the dough system likely became less extensible and more resistant to deformation, which is expressed instrumentally as higher apparent consistency. This increase in consistency, together with the observed reductions in water absorption, dough development time, and stability, can be attributed to gluten dilution and reduced protein–water interactions, as non-gluten components from TF compete for water and disrupt the formation of a continuous viscoelastic network.

This behaviour is consistent with the gluten dilution mechanism described earlier, where reduced protein availability limits water–protein interactions and hydration capacity [32]. The shorter dough development times recorded at higher TF levels further suggest that a weaker and less continuous protein network formed, requiring less time to reach peak consistency. In parallel, the decline in dough stability indicates reduced resistance to mechanical mixing, consistent with weakened gluten networks observed in substituted dough systems [33].

The observed changes in water absorption capacity of the tarap composite dough reflect the influence of flour composition on hydration behaviour, a result that aligns with current understanding of dough rheology. This behaviour reflects altered water distribution in the dough system, as non-gluten components compete with proteins and starch for available water. In addition, the lower protein content of TF compared to wheat flour further limits water-binding capacity, contributing to the overall decline in water absorption observed with increasing substitution levels. For example, Zhao et al. [34] reported that the addition of dietary fibre to wheat flour significantly modified water absorption and elasticity in dough, likely because fibre competes with gluten for water and disrupts normal hydration patterns in composite dough systems.

Similarly, studies on legume- and root-based composite flours have demonstrated that increased fibre and non-gluten fractions alter water binding and mixing characteristics, leading to lowered water absorption and changes in rheological behaviour relative to 100% wheat dough [22, 35]. The concurrent reductions in dough development time and dough stability further support the interpretation that compositional shifts specifically lower protein and modified water interactions weaken the dough’s capacity to form and sustain a strong gluten network during mixing.

Shorter dough development times and reduced dough stability at higher TF inclusion indicate the formation of a weaker and less cohesive gluten network that reaches its optimum more quickly but with lower structural strength. With fewer gluten-forming proteins available, the dough requires less time to develop yet becomes less tolerant to continued mixing and breaks down more easily under mechanical work. Because dough stability is a common indicator of gluten strength and mixing tolerance, its decline with increasing TF reflects reduced structural resilience and greater susceptibility to overmixing. Similar reductions in dough stability and development properties have been reported when wheat flour is partially substituted with non-wheat flours, reflecting gluten dilution and compositional effects on the protein network [32]. Such responses are typical in composite doughs where non-gluten flours introduce starches and fibre that compete for water and physically interrupt gluten continuity, limiting hydration and cross-linking of gluten strands. Together, gluten dilution and altered water distribution yield doughs that are less elastic and more fragile, a pattern widely observed in studies on wheat–non-wheat composite flour systems [36].

TPA further supports the influence of protein dilution and altered hydration on the structure of composite dough. In this study, TPA results showed that dough hardness and adhesiveness increased progressively with higher TF incorporation. This pattern aligns with established evidence that reducing the proportion of gluten-forming proteins, which are hydrophilic components that help create a cohesive and elastic network, generally results in dough that is firmer and less extensible [37]. Research on composite flours (e.g., wheat blended with legume or root crop flours) has similarly reported increases in dough firmness and changes in viscoelastic behaviour when gluten dilution occurs, linking greater hardness to the reduction of gluten network continuity and shifts in water distribution within the dough matrix [35, 38, 39]. The increased hardness observed may also be associated with reduced gluten elasticity and the presence of fibre-rich components, which promote a more compact and less extensible dough structure.

The increase in adhesiveness at higher TF levels also aligns with the known effects of altered water distribution in composite systems. With reduced water-holding capacity due to lower gluten content and increased non-gluten components (such as fibre and starch), more free water remains in the dough, contributing to stickier behavior during handling and increasing measured adhesiveness. Similar responses have been observed in studies replacing portions of wheat flour with non-gluten alternatives, where increased dough stickiness was attributed to limited protein hydration and the presence of free water in the system [40, 41].

The physical characteristics of bread made from composite flour clearly reflect the structural impact of gluten dilution. In this study, increases in TF substitution were accompanied by progressive decreases in loaf volume and specific volume and an increase in bread density. These trends are consistent with the central role of the gluten network in capturing fermentation gases and forming an elastic matrix that allows dough expansion during baking; when gluten is diluted by non-gluten components, gas retention is compromised and loaf expansion is reduced. Similar reductions in specific volume and overall loaf size have been documented in recent composite flour studies where wheat flour was partially substituted with alternative ingredients, resulting in decreased physical quality due to weakened gluten structure and disrupted gas-holding mechanisms (e.g., lower specific volume and firmer, denser crumb in breads with higher non-wheat proportions) [42]. Dahdah et al. [43] reported that as substitution levels of non-gluten flours increase, loaf volume tends to decline, and the resulting bread exhibits a more compact and dense structure, highlighting the importance of gluten continuity for optimal bread. This reduction in loaf volume and specific volume is directly linked to decreased gas retention capacity, as weakening of the gluten network limits the ability of the dough to trap and retain carbon dioxide during fermentation and baking.

Crust colour development in composite bread appears strongly influenced by formulation-driven compositional shifts, including increased carbohydrate, fibre, and natural pigments from TF. In the present results, higher TF substitution was associated with progressively darker crust colour (lower L* values) and higher redness (a*) and yellowness (b*) values, as well as an increased browning index. These changes align with known nonenzymatic browning mechanisms such as the Maillard reaction between reducing sugars and amino groups during baking, which produces brown melanoidin pigments and is a major contributor to crust colour formation in baked goods [44].

Studies on composite breads have similarly reported that adding non-wheat components with higher sugar and pigment contents leads to darker crust colour and increased browning intensity compared with 100% wheat bread, indicating that flour composition can significantly alter colour development during baking [45]. Additionally, the presence of natural pigments and differences in carbohydrate fractions can contribute to intensified redness and yellowness as substitution increases, supporting the interpretation that both Maillard reaction products and inherent colour compounds in TF influence crust colour progression in composite bread systems.

Bread crumb firmness is strongly influenced by both gluten structure and starch behaviour in baked products. In the present study, crumb hardness increased progressively with higher TF substitution indicating a firmer crumb structure at higher TF levels. This aligns with research showing that starch retrogradation, the process where gelatinised starch molecules, particularly amylopectin, reorganise and recrystallise contributes to increased firmness in baked products as they cool and age, particularly when gluten structure is weakened or diluted by non-wheat components [46].

The dilution of gluten proteins by TF likely reduces the continuity and elasticity of the protein network, which decreases gas retention during baking and leads to smaller gas cells and a denser crumb matrix that is measured as higher hardness. The presence of fibre from TF may further contribute to this effect by interfering with gas cell expansion and promoting the formation of a denser crumb structure. This interpretation is consistent with findings that reduced gluten strength and altered protein–starch interactions can accelerate texture firming in composite or fibre-rich breads [47]. Additionally, studies have shown that retrogradation of amylopectin which occurs as starch molecules reassociate after baking is a primary mechanism behind crumb hardening and increased firmness in starch-based foods, reinforcing the link between starch structural changes and harder crumb texture at higher substitution levels [48]. It should be noted that the water content was kept constant across all formulations and was not adjusted according to the water absorption capacity of the composite flours. As a result, the dough and bread characteristics may not represent fully optimised conditions for each substitution level. Adjusting water levels based on farinograph measurements could potentially improve dough handling and bread quality, particularly at higher TF substitution levels.

From a nutritional perspective, the incorporation of TF shifted the bread’s compositional profile toward higher ash, crude fibre, carbohydrate, and total dietary fibre contents, while reducing protein and fat levels. The increase in ash content can be attributed to the higher mineral content of TF compared to wheat flour. Similarly, the rise in crude fibre and total dietary fibre is associated with the naturally higher fibre fraction present in tarap pulp, which contributes additional non-digestible carbohydrates to the formulation. The increase in total carbohydrate content is likely due to the higher carbohydrate composition of TF. In contrast, the observed reduction in protein content is mainly due to the dilution of wheat gluten proteins as TF replaces part of the wheat flour. Likewise, the decrease in fat content may be related to the relatively lower lipid content of TF compared to wheat flour, resulting in an overall reduction in fat proportion in the composite bread. These compositional shifts reflect the inherent nutritional profile of TF and the substitution effect, where wheat flour is progressively replaced by a fibre- and mineral-rich ingredient. This pattern is consistent with published findings that partial substitution of wheat flour with alternative plant-based flours rich in fibre and micronutrients generally increases the mineral and dietary fibre content of bread products. For example, composite breads enriched with millet and sorghum flours showed elevated dietary fibre and mineral levels compared with 100% wheat bread, illustrating that non-wheat inclusions can enhance nutritional properties even as protein and fat proportions change with substitution levels [49].

Similarly, composite bread studies have reported increased ash and dietary fibre contents due to the high mineral and indigestible carbohydrate fractions of substitute flours, supporting the idea that these ingredients can improve bakery product functionality and potential health benefits [4]. Higher total dietary fibre in composite bread aligns with broader evidence indicating that enrichment with high-fibre flours contributes beneficial digestive effects and can help position such products as value-added or functional foods. Although some composite breads may show reduced protein relative to a wheat control due to gluten dilution, this is a well-recognised outcome when substituting wheat with plant-derived flours, and does not necessarily detract from the overall nutritional value when balanced by gains in fibre and minerals [18].

Sensory responses indicate that consumer perception is closely tied to the structural and compositional changes induced by flour substitution. In the present study, increasing TF levels were associated with progressive declines in texture, taste, colour, and overall acceptability, while the lowest substitution level (TCF1) remained comparable to the control. Excessive gluten dilution is known to produce denser crumb structures and darker crusts, which can negatively influence consumer perception. Similar trends have been reported in recent composite-bread studies, where higher levels of non-wheat flour led to lower sensory scores due to changes in crumb firmness and colour, whereas moderate substitution-maintained acceptability [50, 51]. These findings support the idea that limited substitution can preserve bread structure while introducing novel sensory notes. Maintaining this balance between familiarity and innovation is widely recognised as a key factor in successful composite bread development [13].

The observed drop in scores for hardness, texture, and overall acceptability with increasing TF substitution also matches recent studies showing that consumers are sensitive to changes in crumb softness and elasticity, attributes that are directly influenced by gluten network integrity and water distribution in the dough matrix [36]. Conversely, moderate substitution levels appear capable of balancing familiarity with novel sensory characteristics, suggesting that a limited inclusion of TF can enhance nutritional value and introduce unique flavour/aroma without severely compromising acceptability [52].

Consumer testing further illustrates the practical implications of reformulating bread with TF. In the present results, a moderate level of overall acceptance was observed, with a majority of respondents indicating they “like” or “will buy” the composite bread, while smaller proportions were uncertain or not willing to purchase. This pattern reflects the importance of aligning sensory attributes with consumer expectations in product development. Although consumers may be open to novel bakery products, they often prefer familiar texture and flavour profiles similar to conventional wheat bread, as shown by recent sensory research on composite flours where moderate substitution-maintained acceptability but higher inclusion reduced consumer liking. Consumer evaluation remains an essential step in assessing market potential and guiding formulation optimisation, particularly when innovation (e.g., introduction of unique aroma or texture from local ingredients) must be balanced with baseline sensory expectations [23].

Moderate acceptance levels, such as the approximately 57–60% “like” or “would buy” responses seen here, have also been noted in other composite bread products that incorporate alternative flours with distinct sensory profiles, highlighting that familiarity bias toward conventional bread can temper consumer enthusiasm even when nutritional or novel flavour attributes are present. This suggests that while some hesitation is expected, targeted formulation adjustments (e.g., optimising crumb softness or aromatic balance) may enhance consumer appeal without eroding the functional benefits introduced by TF enrichment. Overall, the consumer testing supports the discussion that innovation in bakery product development must carefully integrate sensory quality with market expectations to realise both acceptance and niche potential.