Brewer’s rice was obtained from a local rice mill in Sabah, Malaysia (Sazarice Sdn. Bhd.). Ripe tarap fruits were purchased from the Kundasang local market in Ranau, Sabah. Tarap fruits were selected based on appropriate ripeness and the absence of visible physical defects. Upon arrival at the laboratory, the fruits were washed thoroughly with distilled water, and the edible pulp was manually separated. The pulp was then blended with water to obtain a uniform slurry for cereal formulation.

Five cereal formulations were prepared by varying the proportion of tarap pulp and brewer’s rice while maintaining a constant water content, as shown in Table 1. The cereal formulation was adapted from Okoye et al. [13], with modifications to the proportions of tarap pulp and brewer’s rice in order to produce five formulations containing 0%, 5%, 10%, 15%, and 20% tarap pulp.

The required amounts of brewer’s rice and tarap pulp were weighed according to the formulation and blended with water to produce a homogeneous slurry. The slurry was then subjected to drum drying to produce thin cereal flakes.

All formulations were processed using a twin-drum dryer (Model AM-2530/10, Agridon Sdn. Bhd., Malaysia) at a fixed drum speed of 3 rpm as conducted by Yang et. al. [14]. The slurry was manually fed and spread onto the drum surface under the same operating setup for all formulations. The dried sheet was removed using a stainless-steel scraper, cooled to room temperature, and ground into powder. Although the dryer operated under stable boiler and preset equipment conditions, specific parameters such as drum surface temperature, steam pressure, drum gap, feed rate, and feed total solids were not recorded during the experimental runs. To ensure consistency, all samples were processed using the same drum dryer, drum speed, feeding approach, and scraping conditions.

Proximate composition analysis of the cereal samples was carried out to determine moisture content, ash, protein, and fat contents using standard methods of the Association of Official Analytical Chemists (AOAC) [15]. All analyses were performed in triplicate. Moisture content was determined by oven drying the samples at 105°C to constant weight was achieved, while ash content was measured by incineration in a muffle furnace at 550°C. Protein content was determined using the Kjeldahl method, and crude fat was analysed using Soxhlet extraction. Total carbohydrate content was calculated by the difference, where the combined percentages of moisture, ash, protein, and fat were subtracted from 100 [16, 17]. Total dietary fibre was determined using the AOAC enzymatic–gravimetric method (AOAC 985.29) with a commercial assay kit (K-TDFR-200A, Megazyme).

Sensory evaluation was conducted using 75 untrained panelists to assess consumer acceptability of the developed cereal products by Ling et al. [18]. An affective test using a 9-point hedonic scale (1 = dislike extremely, 9 = like extremely) was employed to evaluate colour, taste, texture, and overall acceptability.

The cereal samples were coded with random three-digit numbers and presented to the panelists in random order. Panelists were asked to evaluate each sample and record their preference scores based on the specified attributes. Drinking water was provided to cleanse the palate between sample evaluations. The sensory evaluation protocol involving human participants was conducted in accordance with the Declaration of Helsinki and was reviewed and approved by the Medical Research Ethics Committee, Faculty of Medicine and Health Sciences, Universiti Malaysia Sabah (Approval code: JKEtika 5/25 (68)). Participation was voluntary and informed consent was obtained from all panelists prior to the evaluation. The formulation with the highest overall acceptability score and favourable sensory performance across the evaluated attributes was selected for further storage study.

The storage stability of the cereal was evaluated through a 28-day real-time shelf-life study under ambient conditions. The control and selected formulation were packed in aluminium pouches and stored at 25 ± 2°C. Analyses were performed on days 0, 7, 14, 21, and 28 to monitor changes in moisture content, water activity (a_w), and colour. Measurements were performed in triplicate unless otherwise stated, and results were expressed as mean ± standard deviation where applicable. Moisture content was measured by oven drying, and a_w was determined using a water activity meter. For colour stability, L*, a*, and b* values were measured in triplicate, and the mean values were used to calculate the total colour difference, ΔE*ab, at each storage interval. The overall colour change during storage was expressed as total colour stability (ΔE*ab) as presented in Equation (1).

(1) ∆ E * a b = ∆ L * 2 + ∆ a * 2 + ∆ b * 2

where ΔL*, Δa*, and Δb* represent the differences between colour coordinates on 0, 7, 14, 21 and 28 days. The ΔE*ab metric is based on the CIE76 colour difference model.

All experiments were performed in triplicate unless otherwise stated, and the results are presented as mean ± standard deviation. Data were analysed using one-way analysis of variance (ANOVA) to determine significant differences among formulations for proximate composition, physicochemical properties, and sensory attributes. Where significant differences were detected, mean comparisons were performed using Tukey’s honestly significant difference (HSD) test at a significance level of p < 0.05. For total dietary fibre, comparisons between the control and the selected formulation were analysed using an independent samples t-test at p < 0.05. Data from the storage study were analysed descriptively and, where applicable, by comparing changes across storage time and between samples. Statistical analysis was performed using IBM SPSS software.

The proximate composition of all five cereal formulations (control, A, B, C, and D), containing 0%, 5%, 10%, 15%, and 20% tarap pulp, respectively, is presented in Table 2. Compared with the control group, each formulation shows significant differences in moisture, protein, and total carbohydrate content. The moisture content of the cereal samples ranged from 0.89% to 9.61%. The control sample exhibited the highest moisture content (9.61 ± 0.12%), while formulation D showed the lowest moisture content (0.89 ± 0.17%).

Ash content varied between 0.18% and 0.65%, with the control formulation having the lowest ash value (0.18 ± 0.03%) and formulation D showing the highest value (0.65 ± 0.04%). Protein content ranged from 7.75% to 8.62%. Although formulations containing tarap pulp showed slightly higher ash and protein values than the control, the magnitude of the differences was small.

Fat content of the cereal formulations ranged from 0.16% to 0.49%. Formulation A recorded the highest fat content (0.49 ± 0.13%), while formulation C showed the lowest value (0.16 ± 0.08%). Total carbohydrate content ranged from 82.11% to 89.77%. The highest carbohydrate content was observed in formulation D (89.77%), whereas the control formulation recorded the lowest value (82.11%).

The physicochemical properties of all five cereal formulations, including water absorption capacity and colour parameters, are presented in Table 3.

Total dietary fibre was determined only for the control and the selected formulation (formulation D), which was chosen for further evaluation based on its highest overall sensory acceptability was presented in Table 4. The total dietary fibre values ranged from 4.66% to 4.85%, and no significant difference (p > 0.05) was observed between the control and formulation D.

The sensory evaluation results of the tarap-based cereal formulations are presented in Table 5. Significant differences (p < 0.05) were observed for most formulations for colour, taste, texture, and overall acceptability. The control sample recorded the lowest scores for all sensory attributes, with overall acceptability of 4.29 ± 1.58. Sensory scores increased with increasing levels of tarap incorporation. Formulation D obtained the highest scores for colour (7.47 ± 1.33), taste (6.93 ± 1.36), texture (7.56 ± 1.14), and overall acceptability (7.32 ± 1.24).

Based on the sensory evaluation results, formulation D (20% tarap pulp) was selected for the storage study and compared with the control formulation. Shelf-life study for the selected formulation D and control was conducted until 28 days after the drum drying process. The moisture content, a_w, and colour stability have been performed as shown in Table 6. Microbial counts were not determined in the present study; therefore, the storage results should be interpreted as preliminary physical stability data rather than a complete safety evaluation.

The proximate composition results indicate that the incorporation of tarap pulp influenced several nutritional components of the cereal formulations. The observed reduction in moisture content with increasing levels of tarap incorporation may be associated with changes in the composition of the cereal matrix during drum drying. The control formulation consisted mainly of brewer’s rice, whose starch matrix tends to retain more moisture during gelatinization. In contrast, the addition of tarap pulp introduces non-starch solids, including dietary fiber and soluble sugars, which may reduce the water retention capacity of the matrix and promote more efficient moisture removal during drum drying [19].

The increase in ash content observed in the tarap-containing formulations suggests a greater mineral contribution from the fruit pulp. Tarap has been reported to contain appreciable levels of essential minerals such as potassium, iron, and zinc, which contribute to the overall ash content of food products [20]. Similar trends have been reported in composite cereal products where the incorporation of nutrient-rich ingredients increases the mineral content of the final product [21].

Only minor variation in protein content was observed among formulations, suggesting that tarap incorporation had a limited effect on the protein profile of the final product under the conditions tested. Although rice and rice by-products typically contain moderate protein levels ranging from approximately 7–18% on a dry basis, the addition of fruit components may contribute additional native proteins and nitrogen-containing compounds to the formulation [22]. Similar increases in protein content have been reported in cereal-based composite products formulated with fruit or legume flours [23]. Although some statistically significant differences were observed in ash and protein contents, the magnitude of these changes was relatively small. Therefore, the main contribution of tarap incorporation in this study appears to be related more to product diversification, sensory improvement, and valorization of local raw materials than to major enhancement of proximate nutritional composition.

The fat content showed minor variations among the formulations, which may be attributed to natural lipid components present in the tarap pulp. Fruit tissues may contain small amounts of lipid fractions that can influence the measured fat content of processed products. In addition, heat processing during drum drying may promote interactions between lipids and starch granules, potentially affecting lipid availability within the cereal matrix [24].

The higher total carbohydrate content observed in the tarap-containing formulations may be partly explained by the by-difference method used for carbohydrate calculation. Because carbohydrate was estimated by subtracting moisture, ash, protein, and fat from 100, the lower moisture content of formulations containing tarap pulp contributed to higher calculated carbohydrate values [16]. Furthermore, tarap pulp naturally contains simple sugars such as glucose and fructose that may contribute to the overall carbohydrate content of the cereal product. Although some statistically significant differences were observed among formulations, the magnitude of these changes was relatively small. Thus, the practical significance of tarap incorporation in this study appears to be related more to sensory improvement and utilization of local raw materials than to major enhancement of proximate nutritional composition.

The total dietary fibre content did not differ significantly between the control and formulation D. Although tarap is a fruit and may be expected to contribute fibre, several factors may explain this result. First, the edible pulp used in this study may have contained a relatively modest amount of insoluble fibre compared with other structural fruit fractions such as peel or seed. Second, the level of tarap pulp incorporated into the cereal formulation may not have been high enough to produce a statistically detectable increase in the total dietary fibre content of the final product. In addition, because the cereal matrix was still dominated by brewer’s rice, the overall fibre contribution from tarap pulp may have been diluted within the formulation. Thermal processing during drum drying may also have affected the structure, distribution, or extractability of fibre components, thereby reducing the apparent difference in measured total dietary fibre between samples. Therefore, the absence of a significant difference in total dietary fibre does not necessarily indicate that tarap pulp contributed no fibre, but rather that its contribution was not sufficiently large to be detected under the present formulation and processing conditions. Similar observations have been reported in cereal products where moderate incorporation of fruit ingredients resulted in only marginal changes in dietary fiber content [26].

Sensory evaluation results demonstrated that the addition of tarap pulp positively influenced consumer acceptance of the cereal product. Formulations containing tarap consistently received higher scores for colour, taste, texture, and overall acceptability compared with the control sample. These results indicate that the incorporation of tarap contributes beneficial sensory characteristics to the cereal product.

Tarap pulp contains natural sugars, volatile flavour compounds, and pigments that may enhance the flavour profile and visual appeal of the cereal. The pleasant aroma and sweetness of the fruit are likely to contribute to improved taste perception among panelists. Furthermore, structural changes occurring during drum drying may influence the texture of the cereal flakes. Thermal processing can produce porous structures within the cereal matrix, which contribute to desirable crispness and fracture characteristics in cereal products [28].

Although increasing the level of fruit incorporation may further enhance flavour attributes, excessively high levels of fruit pulp could potentially lead to undesirable sensory effects. High sugar concentrations may promote excessive browning reactions during thermal processing, resulting in darker colour or burnt flavour notes. Therefore, the 20% tarap formulation appears to represent a suitable balance between flavour enhancement, colour development, and desirable texture characteristics.

The shelf-life study showed that both the control and formulation D remained relatively stable during 28 days of storage under ambient conditions. However, the changes observed in moisture content, a_w, and colour stability suggest some differences in storage behaviour between the two samples. The storage study was designed as a preliminary real-time ambient assessment rather than an accelerated shelf-life study. Future work should incorporate accelerated storage conditions together with microbiological, oxidative, textural and sensory analyses over a longer storage duration to support more robust shelf-life prediction.

Moisture content in both samples increased markedly during the first 7 days of storage, which is consistent with the hygroscopic nature of cereal-based products. Low-moisture foods tend to absorb water from the surrounding environment until equilibrium is approached, especially when stored under ambient conditions [29]. After day 7, the moisture contents of both samples fluctuated rather than showing a continuous increase. This pattern may reflect variations in moisture redistribution within the product matrix and interactions between the cereal components and the surrounding environment during storage. Notably, formulation D consistently showed lower moisture content than the control, suggesting that tarap incorporation may have contributed to a matrix that was less prone to moisture retention during storage.

Although moisture content changed over time, a_w remained low in both samples, with values ranging only from 0.18 to 0.23. This confirms that the water present in the product was largely in a bound state rather than freely available for microbial growth. The low a_w values observed throughout storage suggest limited water availability, which may reduce the likelihood of microbial proliferation, as most bacteria, yeasts, and moulds generally require higher a_w values for growth [30]. However, microbiological safety cannot be confirmed based on a_w and physical analyses alone. Therefore, microbial enumeration and pathogen screening should be conducted in future studies to verify the safety of the product before food application or commercialization.

The colour stability results showed that both samples underwent some degree of colour change during storage, particularly in the early stage. The higher ΔE*ab values observed on day 7 suggest that colour changes were more pronounced during the initial storage period. This may be related to early oxidative or non-enzymatic browning reactions in the cereal matrix. However, the lower ΔE*ab values observed at day 28 indicate that the rate of colour change decreased over time. Formulation D exhibited a much lower ΔE*ab value than the control at the end of storage, suggesting better colour stability. This may be associated with the presence of natural antioxidant compounds in tarap, such as phenolics and flavonoids, which could help reduce oxidative deterioration and preserve product appearance during storage [7].

Overall, although the incorporation of tarap improved sensory acceptability and influenced selected physicochemical properties, some nutritional changes, particularly total dietary fibre and protein content, were relatively modest. Therefore, the findings should be interpreted mainly in terms of product diversification, sensory improvement, and valorization of local raw materials rather than major nutritional enhancement. The present storage study should also be interpreted as a preliminary physical stability assessment. Further microbiological analysis, including total plate count, yeast and mould count, and relevant pathogen screening, is required to confirm product safety. In addition, longer-term storage studies incorporating oxidative stability, texture retention, and sensory quality evaluation should be conducted to provide a more comprehensive assessment of shelf-life performance.

This study showed that tarap can be successfully incorporated into a drum-dried breakfast cereal using brewer’s rice as the main base ingredient. The addition of tarap influenced several important product characteristics, including proximate composition, colour, and sensory quality, while maintaining low moisture content and a_w suitable for ambient storage. Among the formulations tested, formulation D was the most acceptable, indicating that 20% tarap incorporation provided the selected balance of colour, taste, texture, and overall consumer preference. The selected formulation maintained low water activity and relatively stable physical characteristics over 28 days of ambient storage, indicating promising short-term storage stability. Overall, the findings support the potential of tarap and brewer’s rice as promising ingredients for the development of value-added and sensory acceptable ready-to-eat cereal products, particularly from the perspectives of sensory quality and local raw material utilization. Nevertheless, the present study only assessed physicochemical properties, sensory acceptability, and preliminary storage stability. Microbiological safety, longer-term shelf-life performance, oxidative stability, and sensory quality during storage must be further verified before the product can be recommended for future food application or commercialization.