Shelf-life evaluation of precooked millet couscous (Pennisetum glaucum) during ambient storage, in Senegal

Millet couscous occupies a significant place in the Senegalese diet and contributes to the valorization and promotion of local cereals. However, information regarding shelf life, physicochemical characteristics, microbiological safety, and sensory quality remains limited. Nevertheless, these properties are often overlooked, can play a key role in the promotion and economical and nutritional competitiveness of precooked millet couscous. This study aimed to determine, through scientific knowledge, the shelf life of precooked millet couscous packaged in polyethylene bags under ambient storage conditions in order to establish its minimum durability date (MDD). Precooked couscous was produced in three samples, packaged at 500g per bag and then stored at laboratory ambient temperature (23 °C–30 °C) with relative humidity varying between 46% and 51% during storage. Samples of precooked millet couscous were taken regularly from T0 to T20 months to assess microbiological quality, pH, moisture content (humidity), water activity and sensory properties during storage. Data were analyzed using analysis of variance (ANOVA), and mean comparisons were performed using Fisher’s Least Significant Difference (LSD) test at a significance level of p < 0.05. Finding showed pathogenic microorganism including, Salmonella, E. coli, sulfite-reducing Clostridium, Bacillus cereus, and Staphylococcus aureus were not detected in precooked couscous millet during the entire storage period. However, aerobic mesophilic flora at 30 °C, yeasts and molds remained below the microbiological limits established by NS Ecostand 062–2017 standard for couscous millet, for up to 20 months of storage. Furthermore, throughout the storage period, couscous maintained its organoleptic properties and pH, which ranged from 6.66 to 6.84, despite a gradual increase in humidity and water activity. Based on the microbiological, physicochemical, and sensory stability of the product, the shelf life was estimated to be at least 20 months under ambient conditions (95% confidence interval: ≥ 20 months, limited by the duration of the study). These findings demonstrate that precooked millet couscous packaged in polyethylene bags maintains its quality and safety during long-term storage and provide scientific evidence to support shelf-life labeling in accordance with Codex Alimentarius CXS 1-1985, thereby enhancing the competitiveness of locally processed millet products in national and regional markets.

Pearl Millet [ Pennisetum glaucum (L) R. Br ], a cereal widely grown in the arid and semi-arid tropical regions of Africa (Kadri and al., 2019), is an essential food source in Senegal. Millet production is estimated at 675,910 tons for the 2024/2025 agricultural season (ANSD, 2025). As an important component of a rapidly expanding value chain, millet is processed into a wide range of traditional products. It is one of the cereals that occupy an essential place in the Senegal’s agricultural economy, generally used for human consumption after being processed into flour and semolina for the preparation of couscous, porridge, rolled products, etc. (Barro, 2021). It is an important means of conservation and economic development. As a result, several researches have been focused on processing these cereals to develop competitive local products of higher quality that meet consumer expectations in terms of taste and ease of preparation (Bundisri and al., 2025).

Pre-cooked millet couscous packaged in bags has become a common product on the Senegalese market. Its convenience, speed of preparation, and suitability to urban lifestyles make it an attractive alternative to traditional preparations, particularly in large cities where the demand for ready-to-cook food is constantly increasing. (De Groote and al., 2021). However, it is essential to improve the presentation of packaged processed local cereals by using attractive labels that highlight all the product’s advantages in accordance with manufacturing and storage standards (FAO/WHO, 2020). Some local food products do not carry regulatory information on the use-by date and shelf life. However, this information is crucial for their competitiveness and access to the mass market (Davezac and Taisne, 2019), in accordance with the Codex Alimentarius (CXS 1, 1985). The lack of credible information on pre-packaged local products is an obstacle to consumer confidence, mainly in urban areas (Davezac and Taisne, 2019). These consumers are looking for a wide variety of cereal products demanding that they have a long shelf life (Cortesi, 2022).

Shelf-life determination is a major issue for cereal-based foods because it guarantees food safety, preserves physicochemical and sensory quality, and supports compliance with food-labeling regulations. Some authors (Sidibé, 2017), have carried out research to determine the shelf life (Houssou et al., 2016) of food products made from processed local cereals. International research has demonstrated that the shelf life of dehydrated cereal products depends primarily on moisture content, water activity, packaging materials, storage temperature, and processing methods. Despite its popularity, there are gaps in information and reliable data on the shelf life and microbiological, organoleptic, and physicochemical stability of millet-based couscous. Recent reviews have highlighted that appropriate thermal and non-thermal processing technologies, combined with suitable packaging systems, significantly improve the microbiological stability and extend the shelf life of millet-based products by reducing microbial growth, lipid oxidation, and enzymatic deterioration (Ravi and Rana, 2024; Naraharasetti et al., 2025).

Likewise, studies on the storage stability of millet flour have shown that controlling moisture migration and selecting appropriate packaging materials are essential to maintain product quality during long-term storage (Sruthi and Rao, 2021). Furthermore, Padmaja et al. (2023) demonstrated that thermal treatment combined with laminated modified-atmosphere packaging significantly extended the shelf life of pearl millet flour by improving microbiological stability during storage. Although several international studies have investigated the storage stability of processed cereal products, including millet flour, pasta, dehydrated foods, and wheat-based couscous, little information is available on precooked millet couscous produced under African processing conditions. Most published studies have focused on flour stability or cereal products manufactured under temperate industrial conditions, whereas the effects of long-term storage under tropical ambient conditions have received little attention.

To our knowledge, no published study has comprehensively evaluated the microbiological, physicochemical, and sensory stability of precooked millet couscous stored under ambient conditions representative of Senegal. Consequently, the lack of reliable scientific data to establish an evidence-based minimum durability date (MDD), of scientific protocols and models to establish the shelf life of millet couscous marketed in Senegal can be identified as shortcomings. This represents important knowledge gap that limits compliance with Codex Alimentarius (CXS 1-1985) requirements and constrains the commercialization of locally processed millet products.

Based on this knowledge gap, this study hypothesized that precooked millet couscous packaged in polyethylene bags would maintain microbiological safety, physicochemical stability, and sensory acceptability during prolonged storage under ambient conditions representative of Senegal. The present study was conducted to determine the shelf life of precooked millet couscous, packaged in polyethylene bags and stored under ambient conditions representative of Senegal. By monitoring microbiological quality, physicochemical characteristics, and sensory properties throughout a 20-month storage period, this work provides scientifically validated data for establishing the product’s minimum durability date in accordance with Codex Alimentarius (CXS 1-1985). The findings are expected to support local processors in improving product labeling, strengthening consumer confidence, and enhancing the competitiveness of millet-based products in national and regional markets. Knowledge about the shelf life of precooked millet couscous packaged in suitable packaging would not only help to improve the presentation of the product, but would also reassure Senegalese consumers about its safety. This study hypthesized that precooked millet couscous packaged in polyethylene bags would maintain microbiological safety, physicochemical stability, and sensorty acceptability during prolonged ambient storage.

Plant, technological, and laboratory materials were used to achieve the objective of this study. The plant material used consisted of millet grains (P. glaucum) purchased at the Castors market in Dakar. The methodology adopted consisted first of all of experimenting with millet couscous technology, performing microbiological quality control, conducting real-time aging tests for up to 20 months of storage, and monitoring the evolution of physicochemical factors that may influence shelf life and organoleptic properties at the beginning and end of storage.

Process for making precooked millet couscous

The precooked couscous was prepared from flour obtained by dry milling hulled millet grains using a SISMAR mobile hammer mill equipped with a 0.5 mm mesh sieve. After moistening and rolling, the flour granules were pre-cooked with steam for 20 min using an aluminum couscous maker. After being broken up and sifted using a 2 mm mesh sieve, the couscous underwent a second pre-cooking with steam for 5 min. It was then sifted again and dried for 3 h in an OURAGAN 7 oven (SINGER, France) set at 65 °C. Once cooled, the pre-cooked millet couscous was packaged in polyethylene bags at a rate of 500 g per bag. The bags were stored in secondary cardboard packaging and kept in a laboratory space at an average room temperature of 27 °C with relative humidity ringing 46%–51%.

Microbiological quality control

Microbiological analyses were performed on samples of hulled millet grains, flour, and millet-based couscous to assess the hygiene of the manufacturing process and the initial microbiological quality of the finished product before storage.

Microbiological analyses were performed under aseptic conditions using a stock suspension prepared with 25 g of precooked couscous samples in 225 mL of sterile buffered peptone water. The suspension was homogenized using a STOMACHER peristaltic device. It was then used to prepare successive decimal dilutions, which were used to inoculate the culture media. The analyses were performed using isolation and identification methods in accordance with international standards to detect different microbial species. Salmonella spp. Followed ISO 6579–1 (2017), while other tests were performed to enumerate aerobic mesophilic flora at 30 °C (ISO 4833–1, 2013), yeasts and molds (ISO 21527–2, 2008), total coliforms (ISO 4832, 2006), Escherichia coli (ISO 16649–2, 2001), Bacillus cereus (ISO 7932, 2004), sulfite-reducing Clostridium (ISO 15213, 2023) and coagulase-positive Staphylococcus aureus (ISO 6888–1, 2021).

Microbiological shelf life of couscous

The samples of precooked millet couscous, stored in their packaging were subjected to quarterly microbiological analysis from the beginning (T 0) up to 20 months (T 20) of storage at the laboratory’s ambient temperature, which fluctuated between 23 °C and 30 °C, in order to assess their degradation over time. At each endpoint, three samples of couscous bags were randomly selected and subjected to a visual inspection. Since microbiological quality control showed the absence of Salmonella and the non-detection of other parameters analyzed in the pre-cooked millet couscous, microbiological shelf life was determined by aging tests of aerobic mesophilic flora at 30 °C and yeasts and molds, considered to be spoilage organisms and which remain present in the finished product.

Analysis of physicochemical factors

Monitoring of changes in physic-chemical factors that may influence the shelf life of precooked millet couscous was carried out at T 0 and T 20 months of storage. The analyses focused on the determination of pH, water activity (Wa), humidity and temperature of the storage environment of the precooked couscous.

pH determination

The pH was measured on 10 g of test samples, homogenized in 90 mL of distilled water. The reading was taken directly using the HANNA HI-991001 portable pH meter, previously calibrated with pH 4 and pH 7 buffer solutions.

Water activity determination

Water activity (Wa) was measured using an Wa-meter (NOVASINA LabStar. The sample cup was half-filled and then inserted into the measurement chamber, which was then closed by lowering the top cover. The water activity result was displayed at the end of the measurement, indicated by an acoustic signal.

Moisture content determination

The moisture content was determined by drying 5 g of the precooked couscous sample in an oven set at 105 °C ± 2 °C until constant weight was reached. Weight loss is calculated as the water content of the sample using the AOAC (2000).

Storage temperature and relative humidity measurement

The temperature and relative humidity of the storage environment was measured directly using an HTC-8A digital thermohydrometer at the time of sampling for microbiological analysis.

Organoleptic tests

Sensory evaluation was conducted to assess the acceptability of the developed products. The study followed the general principles of sensory analysis described in ISO 8589 and ISO 13299. Two sensory tests were carried out, at the T 0 and T 20 months of storage to observe the evolution of the organoleptic properties of precooked millet couscous.

A total of 30 untrained panelists, considered as potential consumers, were recruited. The selection criteria included regular consumption of cereal-based products, availability, and the absence of self-reported sensory impairments. No training in sensory analysis was provided to the panelists in order to reflect real consumer perception conditions. The tests were carried out in a dedicated sensory analysis room under controlled conditions, free from external odors and noise interference, with a temperature maintained at 20 °C–25 °C. Samples were randomly coded using three-digit blinding codes and presented in a randomized order to avoid carry-over and order effects. Each panelist evaluated the samples individually in separate booths to prevent interaction bias. Water and plain unsalted crackers were provided for palate cleansing between samples The panel was asked to assign a score of 1–5 for each assessment criterion: from “not acceptable” to “very acceptable”. Bottled water has been made available for tasters to rinse their mouths between each tasting.

Statistical analyses

All data were recorded and organized using Microsoft Excel 2016 (Microsoft Corporation, USA) and analyzed using XLSTAT software (version 2013.4.08, Addinsoft, Paris, France).

Prior to analysis, assumptions for parametric tests were verified. Normality of data distribution was assessed using the Shapiro–Wilk test, while homogeneity of variances was evaluated using Levene’s test. When necessary, data were checked for outliers prior to statistical testing.

A one-way analysis of variance (ANOVA) was performed to evaluate the effect of the experimental factor on physicochemical, microbiological, and sensory variables. The microbiological results were compared to the criteria established in the NS Ecostand 062 standard (2017) and to those taken from the OGPDRA (2017).

When significant differences were observed (p < 0.05), means were separated using Fisher’s Least Significant Difference (LSD) test. All statistical analyses were performed at a 5% significance level (p < 0.05).

Macroscopic analysis of millet couscous

Visual inspection of the millet couscous samples throughout the storage period in their packaging revealed that the product remained in good condition with no visible signs of deterioration (Figure 1). After 20 months of storage in their packaging, none of the samples showed any apparent damage or visible infestations, which demonstrates the effectiveness of the storage conditions and the millet processing process. This finding corroborates with the microbiological results, which did not reveal significant contamination in the couscous samples stored in their packaging for up to 20 months of storage.

Microbiological quality of precooked couscous during storage

The results of microbiological analyses carried out on samples taken during the processing of millet into couscous are presented in Table 1. The absence of Salmonella spp. was noted, and Clostridium sulfito-reducing bacteria, Bacillus cereus, Staphylococcus aureus, and Escherichia coli were not detected in any of the samples. However, aerobic mesophilic flora (AMF) at 30 °C and yeasts and molds were present in all samples taken during the millet couscous production process, but at contamination levels that complied with regulatory criteria.

AMF at 30 °C and yeasts and molds were counted in precooked couscous sample with average loads of 2.2 × 10 2 cfu/g and 9.3 × 10 1 cfu/g, respectively. These results are consistent with and below the minimum acceptable threshold, which is < 10 3 cfu/g for AMF and 10 2 cfu/g for yeasts and molds. This demonstrates compliance with good hygiene practices in the process and effective cleaning of the equipment used for millet processing. Microbiological quality control made it possible to assess the overall level of contamination in couscous. These two germs, which are present in the raw material and remain in the finished product, are identified by Birollo et al (2001) as hygiene indicators in the food manufacturing process and may be part of the original flora of millet. Sanogo et al. (2022) reported that a satisfactory AMF is an indicator of the absence of spoilage and good overall microbiological quality for consumers. The total coliforms that were counted in millet (2.3 × 10 3 cfu/g) and flour (5.6 × 10 2 cfu/g) disappeared in couscous (<10 cfu/g). This leads to the conclusion that coliforms are sensitive to heat treatment (steaming and drying at 65 °C). The presence of these coliforms in processed products can also be justified by their ubiquitous nature. Millet grains are contaminated in the fields, and the original microbial flora can persist during primary processing and end up in the flour. This finding is confirmed by N’goran-AW, who judged that total coliform could be found in the flour during millet processing due to their ubiquitous nature. They can be an indicator of the microbial quality and hygiene of millet grains and flour.

Furthermore, the results (Figure 2) of aging tests relate to changes in the microbial flora present in millet couscous during storage. The results show that AMF and yeasts and molds persisted for up to 20 months of storage but at levels in accordance with the required criteria (<10 3 cfu/g). However, AMF increased slightly after 12 months of storage (2.4 × 10 2 cfu/g), reaching a maximum of 8.6 × 10 2 cfu/g at 20 months. Yeasts and molds followed a similar trend, starting at 15 months of storage (9.2 × 10 1 cfu/g) and reaching 1.5 × 10 2 cfu/g at 20 months.

Microbiological analysis revealed that the microbial load remained stable and within acceptable limits. This indicates that storage conditions were appropriate to avoid significant microbial contamination. Due to their sensitivity to heat, steaming and effective drying must have contributed to their disappearance in the couscous, thereby ensuring product stability during storage. Drying is considered by Prescott et al. (2003) to be one of the oldest methods of preserving food against spoilage. These results are consistent with the work of Gandaho et al. (2017), who demonstrated that manufacturing conditions, such as cooking and drying, impact the microbiological quality of the product. This consistency in the analysis results could be attributed to the effectiveness of steam cooking and drying, as well as compliance with good hygiene and manufacturing practices. These factors combined contribute to extending the shelf life of couscous. Sika et al. (2019) reported that the work of Tarhouni et al. (2015) demonstrated that drying and pre-cooking reduced aerobic mesophilic bacteria. Guirrou et al. (2022) also confirmed that low water activity influences food safety. Faiveley (2018) also pointed out that the moisture content of food is linked to its shelf life.

The satisfactory results obtained suggest that couscous can be preserved, as claimed by Wognin (2014). They may also indicate compliance with good hygiene and manufacturing practices or effective cleaning of the equipment used to process millet into couscous. The stability of microbiological quality during storage is important for protecting the health and economic interests of consumers (FAO-WHO, 2020).

Estimation of microbiological shelf life using linear regression

Figure 3 shows the change in aerobic mesophilic flora at 30 °C concentration in precooked couscous during storage. The trend line is determined by an upward-sloping straight line drawn using equation Y = 0.0298x + 2.1659. To estimate the shelf life of couscous, the relationship between the two variables x (shelf life) and Y (microbial concentration (Log 10 (cfu/g) AMF)) is modeled by calculating the linear regression line of the form Y = ax + b. The microbiological criteria required for couscous to be compliant is < 10 3 cfu/g, or Log10 (10 3) = 3.00. This represents the non-compliance threshold from which couscous shows signs of spoilage. To find the predicted value of “x”, the value of Y must be less than 3.00. Assuming Y = 2.90 then the equation of regression curve Y = ax + b for AMF, becomes

The maximum shelf life of couscous can be estimated at 24 months. This corroborates the results of the maximum shelf life determined by aging tests, where the data revealed a microbial concentration measured in Log 10 (cfu/g AMF at 30 °C) of 2.93 after 20 months of storage. These results suggest that the maximum microbial concentration of aerobic mesophilic flora at 30 °C is reached at 24 months, beyond which couscous no longer meets microbiological specifications. Analysis of the results obtained suggests that shelf-life testing is a means of demonstrating that foods will remain safe, palatable, and nutritious until the end of their estimated shelf life (CFIA, 2018).

Changes in the physic-chemical characteristics of couscous during storage

Changes in physic-chemical factors, particularly temperature, pH, humidity content, and water activity, can have a significant impact on the shelf life of precooked millet couscous during storage.

Temperature variation

The temperature of the storage environment was monitored throughout the study. The results (Figure 4), showed a temperature variation between 23 °C and 31 °C. This temperature range is favorable for the growth of aerobic mesophilic bacteria and molds. Touati and al. (2016) reported that the temperature range of 25 °C–35 °C was optimal for the growth of aerobic mesophilic bacteria and most molds. However, microbiological results showed that temperature did not negatively affect the quality of pre-cooked millet couscous during storage.

pH variation

The pH changed precooked millet couscous during storage (Figure 5). The pH values remained fairly constant (pH: 6.53–6.85). This slightly acidic or neutral pH is ideal for limiting the growth of certain pathogens. Alterations are likely to occur in foods with a pH between 4.5 and 9.5 (Touati and al., 2016), and the vast majority of filamentous fungi grow in a pH range of 4.5–8.0 (Botton et al., 1990). The consistency of the pH of couscous during storage indicates that the product remained physically stable, thus contributing to the preservation of its microbiological quality. This suggests that pH, like temperature, is a factor that promotes microbial growth in food products.

Changes in water activity (Wa)

Analysis of water activity provides insight into water exchange between a product and its environment (Guirrou and al., 2022). The results presented in Table 2 showed an increase in water activity from 0.37 to 0.62, indicating a slight absorption of water content during storage. These increases, although small, are compatible with storage conditions and do not compromise the microbiological quality of the product. This increase in water activity (from 0.37 to 0.62) can be explained by the progressive absorption of environmental moisture during storage. Although polyethylene packaging was used, its partial permeability to water vapor may allow moisture exchange over long storage periods. In addition, fluctuations in temperature and relative humidity may enhance these transfer processes. Finally, the hygroscopic nature of millet-based cereal products also contributes to their gradual equilibration with ambient humidity (Bauer et al., 2022).

The presence of aerobic mesophilic flora at 30 °C and yeasts and molds in quantities that meet the required criteria, and the absence of Salmonella or microorganisms that were not counted, reflect the good sanitary quality of precooked millet couscous during storage. This suggests that low water activity has a significant impact on the microbiological quality of the product. Consequently, this low water activity appears to have played a key role in preserving the couscous and limiting microbial proliferation during storage. This hypothesis is supported by Guirrou and al (2022), who obtained low water activity results for corn couscous samples, leading them to assume that these samples would be less exposed to microbial contamination, hence their long shelf life. Doukani (2015) shared the same opinion, reporting that water activity directly influences microbial growth. In view of these different interpretations, water activity appears to be an important parameter for determining the quality and shelf life of precooked millet couscous.

Variation of humidity during storage

Moisture analysis is of commercial interest, making it possible to extend the storage time and/or the use-by date (Doukani, 2015). The moisture content, which corresponds to the water content, was determined by drying a 5 g sample of precooked couscous in an oven set at 105 °C ± 2 °C until a constant weight was reached. The weight loss was calculated as the water content of the sample using the AOAC (2000). The results of the analyses of the moisture content of the couscous (Table 2) show variations over time. Initially (C-T0), it was 5.40%, and increased to 8.65% at 20 months (C-T20 months) of storage, while remaining compliant (<11%) with the physic-chemical requirements of millet couscous established in NS Ecostand 062 (2017). These low humidity levels obtained could result from dry grinding of the millet grains, effective drying after the steam cooking operation or good storage conditions. This makes it possible to claim a long shelf life of the couscous without risk of alteration by the microorganisms. However, the increase in the moisture content of the tested couscous during storage, although in accordance with the standard, could be explained either by the quality of the packaging or by the temperature or relative humidity of the storage medium.

Sensory aspect of precooked couscous

Sensory analysis is an important indicator that can be used as a quality control tool by assessing the conformity of the finished product (Ledauphin, 2007). The results of the sensory evaluations (Figure 6) showed that precooked millet couscous retained its organoleptic characteristics during storage. Tasters who gave the same overall rating at the beginning (at T0) and at the end of storage observed no significant change in texture, color, or taste up to T20 months of storage. Texture and color scores were considered acceptable (score 4), indicating that the physical appearance of the product was well preserved. The taste and acidity, rated 3 (low), remained stable, suggesting no sensory or microbial degradation. The sensory stability study conducted between T0 and T20 showed consistency in the perceived quality of the product. All parameters remain within the acceptable range except for acidity, which is rated as low, indicating good stability of the couscous’s organoleptic properties during this period of 20 months. The overall assessment also remains acceptable (rating 4), confirming that quality has been maintained overall.

However, it is necessary to perform sensory evaluation at the same time as microbiological analyses in order to correlate the deterioration in product quality and safety with microbial growth.

The aim of this study was to evaluate the shelf life of dried millet couscous packaged in polyethylene bags through microbiological, physicochemical, and organoleptic analyses during 20 months of storage at ambient laboratory temperature (approximately 27 °C). The results indicate that Salmonella was not detected, nor were E. coli, total coliforms, sulfite-reducing Clostridium, Bacillus cereus, or Staphylococcus aureus enumerated. Only aerobic mesophilic flora at 30 °C, as well as yeasts and molds, were observed from T 0 to T 20, at levels compliant with current Senegalese standards. The physicochemical parameters revealed a stable pH (6.65–6.84) and a slight increase in moisture and water activity, remaining within limits compatible with product stability.

Thus, the stability assessment of millet couscous demonstrated good shelf life under the storage conditions evaluated. The microbiological and physicochemical parameters remained within acceptable limits throughout storage, suggesting that the precooking, drying, and polyethylene bag packaging processes contributed to preserving product quality under the conditions tested.