The plant material used in this study is oil palm (Elaeis guineensis Jacq.) sap, freshly harvested after the palm has been felled.

Palm sap was collected at 7 a.m. from a palm wine producer in the commune of Songon, in the south of Côte d’Ivoire. The first daily production of sap from old, freshly felled palms was considered. A volume of 20 litres of sap was collected in 5 sterile plastic containers of 4 litres each and transported in a cooler containing carboglasses. The sap was kept in the 4-litre containers and stored at laboratory temperature (20˚C - 25˚C). Daily samples of 4 litres were taken to produce palm syrup over a period of five days.

Palm syrup was produced using the method described by Akpro et al. [15] . After filtration, the palm sap was thermally evaporated in a single phase for 60 minutes. To do this, 1 L of sap was heated to 120˚C for 30 min. Without interruption, the temperature was reduced to 100˚C for 10 min, then to 80˚C for 10 min, and finally to 60˚C for 10 min ( Figure 1 ). During heating, the sap was continuously turned to accelerate the syrup and avoid any crust formation. After cooking, the syrup is cooled to room temperature and packaged in plastic bottles.

1) Composition of the Panel

Two sensory analysis tests were carried out on the palm syrups produced. Firstly, a quantitative sensory profile was carried out with a panel of 25 trained tasters to describe the different syrups [16] . Next, a preference ranking test was carried out with 60 naïve tasters (not officially trained). All these tasters were recruited from the Université Nangui ABROGOUA (Abidjan, Côte d’Ivoire) and ranged in age from 19 to 30. They were recruited based on their experience in sensory analysis, their availability, and their willingness to take part in the study. Verbal consent was obtained from the tasters, who also declared that they were not diabetics prior to their inclusion in the analysis. The organoleptic descriptors considered were the sweetness, honeyed flavour, and acidic aftertaste of the syrup samples produced.

2) Quantitative sensory profile test

The quantitative sensory profile was carried out in one session with 25 qualified tasters. Samples of palm syrup were evaluated against reference samples, including pure sucrose, honey, and lemon juice, for sweetness, flavour, and aftertaste, respectively. All the samples (palm syrups, table sugar syrup, honey, and lemon juice) were coded and presented simultaneously to the tasters. The intensity of the descriptors was determined on an unstructured linear scale with an origin (0) and an end (100). A scale is provided for each descriptor (sweetness, honeyed flavour, and acid aftertaste). The taster is then asked to taste the samples and mark vertical lines on the scale to indicate the position of the samples relative to their intensity [17] . Between two tastings, the taster rinsed his mouth with water.

3) Palm syrup classification test

The hedonic ranking test was used to determine consumer preferences. The tasters tasted and ranked the syrup samples according to their preference for the above-mentioned sensory descriptors and their overall appreciation of each sample. All the samples were presented to the tasters at the same time, in a randomized fashion to avoid any bias linked to the order of presentation. Tasters were provided with classification forms for data collection [17] .

The data obtained from the analyses were entered into Excel, and STATISTICA.7 software was used for statistical processing. A one-factor analysis of variance (ANOVA 1) was performed to compare the means of the different variables (descriptors). Before carrying out the ANOVA, the equality of variances was verified using Levene’s test [18] [19] . The homogeneity of variances was respected in all these analyses at the 5% threshold. When a significant difference was observed between the means of the variables, the ANOVA was completed by multiple comparisons (post-ANOVA test) using Duncan’s test at the 0.05% threshold [20] . For the hedonic classification data, the Wilcoxon test for two linked samples was carried out [17] .

The sensory profile of the different palm syrup samples is presented in Figure 2 . The results show a significant difference (p < 0.05) between the characteristics of the syrup obtained from the sap of the day (SP0J) and those obtained from stored sap. The sweetness and honeyed aroma were very intense in the syrup obtained with palm sap from the day of extraction (SP0J), with levels of 37.83% ± 2.61% and 39.56% ± 2.49%, respectively. On the other hand, these descriptors decrease in intensity as the duration of palm sap storage increases, but do not show any significant difference. Regarding the acid aftertaste, the intensities were higher in the syrups made from stored sap, with a maximum of 38.87% for the syrup from day 4 (SP4J).

Figure 3 shows the tasters’ preferences for palm syrups according to the descriptors and their overall assessment. In terms of sweetness, the syrup from the day sap (SP0J) was the most preferred by the tasters with an average rank of 1.42 ± 0.53. The syrups from the first day’s sap (SP1J) and the second day’s sap (SP2J), which did not show any significant difference, came second. Syrups SP3J and SP4J were ranked third and fourth successively with mean ranks of 3.63 ± 0.16 and 4.29 ± 0.34, respectively. Considering the honeyed flavour, the SP0J syrup obtained the lowest average rank (1.68 ± 0.53), ranking it the most preferred. There was no significant difference (p > 0.05) between the mean ranks of the SP1J, SP2J, and SP3J syrups. These syrups occupy the second position of preference. Syrup SP4J had a mean rank of 3.97 ± 0.35. Regarding the acid aftertaste, syrups SP2J, SP3J, and SP4J were the most preferred by the tasters with lower average ranks and no significant difference (2.63 ± 0.16 to 2.87 ± 0.16). Syrups SP1J and SP0J were the least preferred. Taking the whole samples, the tasters ranked SP0J syrup as their first choice (1.29 ± 0.54), SP1J syrup as their second choice (3.00 ± 10), SP2J and SP3J as their third choice (3.24 ± 0.15 to 3.32 ± 0.17), and finally SP4J syrup as their last choice (4.16 ± 0.34).

The sensory evaluation of different syrups made from palm sap provided information on their sensory profiles and the tasters’ preferences. The quantitative sensory profile of the syrups showed significant differences (p < 0.05) between the intensities of the sweet taste, honeyed aroma, and acid aftertaste of syrups made from fresh palm sap (day 0) and stored palm sap (days 1 - 4). The sweetness and honeyed aroma were more intense in the fresh syrup (SP0J), with values of 37.83% ± 2.61% and 39.56% ± 2.49%, respectively. The acid aftertaste is weak (1.29 ± 0.54).

These results can be explained by the high quantity of sugars still available in the sap on the day of extraction, with a low acid content [21] . Indeed, the thermal evaporation applied in the production of palm syrups has led to a concentration of sugar, which enters into Maillard reactions to develop the syrup’s characteristic properties such as its colour, aroma, flavour, and antioxidant power [3] [22] . However, when the sap is preserved, spontaneous fermentation takes place to break down the sugars, producing alcohol, organic acids, and carbon dioxide [23] . The reduction in the quantity of sugars and the increase in the acidity of the stored sap led to a drop in sweetness and honeyed flavour. At the same time, an increase in acid aftertaste was observed. However, no significant difference was observed between the sweetness and aroma intensities of syrups from stored sap over the four days.

The intensity of the acid aftertaste increases with storage time. Consequently, the peak intensity (38.87%) is reached in the syrup from sap stored for four days (SP4J). This increase in acidity is probably due to the formation of organic acids during fermentation.

Ranking the syrups based on tasters’ preferences showed significant differences between the samples. Preferences evolved in the same direction for the descriptors sweetness and honeyed aroma. Syrup SP0J (day 0) is the most appreciated for its sweetness and aroma, while syrup SP4J (day 4) is the least appreciated. This preference for fresh sap syrup corroborates previous results showing a decrease in sugar content over time. However, the absence of any significant difference between the preferences for SP1J, SP2J, and SP3J suggests a more subtle evolution of the honey flavour over time. Regarding the acid aftertaste, the tasters’ preference was for the SP2J, SP3J, and SP4J syrups, which showed a high intensity for this descriptor. This preference could be justified by the tasters’ eating habits and culture [24] . According to these authors, the sensory traces left over the decades could guide certain food preferences. Similarly, Seddiki et al. [25] showed that consumers preferred the more acidic Hmira syrup (68.3%) (pH = 4.43 ± 0.15; free acidity = 13.15% ± 0.39%) to the less acidic Tagazza syrup (pH = 4.57 ± 0.08; free acidity = 12.43 ± 0.15) while the same panel reported that Robb Hmira was less sweet than the Tagazza variety, which was considered very sweet (75%).

The overall ranking of syrups shows a decreasing preference with storage time (SP0J > SP1J > SP2J, SP3J > SP4J). This trend suggests that the organoleptic characteristics observed previously guided the tasters’ overall assessment [26] . Although SP0J syrup is preferred overall, the preference for SP2J, SP3J, and SP4J syrups in terms of aftertaste suggests that some changes during storage may be appreciated. This could open avenues for the development of products with different sensory profiles targeting different consumer segments. These results suggest that to obtain a syrup with optimal and preferable organoleptic characteristics, it would be desirable to produce it from fresh palm sap. The length of time palm sap is stored before syrup production should be minimized or controlled to preserve the desired sensory qualities.