[version 1; peer review: awaiting peer review]
No competing interests were disclosed.
The fruit of
Ripe fruits collected in Morondava in February 2021 were dried, ground and analysed on a dry matter basis. Total carbohydrates were calculated by difference. Total sugars, reducing sugars, sucrose and starch were determined using appropriate chemical methods. Crude fibre was measured by the Weende method and lignin was estimated by near-infrared spectroscopy (NIRS). Analyses were performed in triplicate.
Total carbohydrates represented 87.80 ± 1.44% dry matter, including 53.59 ± 3.09% residual carbohydrate fraction. Total sugars were low (3.91 ± 0.00%), with no detectable reducing sugars. Sucrose reached 3.71 ± 0.00%, starch 13.56 ± 0.27%, crude fibre 34.21 ± 1.66%, and estimated lignin 18.08 ± 0.65%.
The fruit exhibits a low content of soluble sugars and a high proportion of structural constituents, suggesting potential nutritional interest for glycaemic control, which remains to be confirmed by biological and clinical studies.
Carbohydrates are a major source of dietary energy, yet their metabolic effects differ according to their chemical structure, degree of polymerisation, and digestibility (
The nutritional interest of carbohydrate-rich foods depends, however, not only on their total carbohydrate content, but also on the nature of the available sugars and their association with fibre fractions within the food matrix. Foods characterised by a low proportion of rapidly available sugars and a high proportion of cell wall components, such as cellulose and lignin, may be of particular interest in nutritional approaches aimed at attenuating the postprandial glycaemic response (
Among still poorly documented plant resources, traditionally consumed palm fruits constitute a group of interest. Although several species are valued in local food systems, their detailed carbohydrate and fibre profiles often remain insufficiently characterised (
The present study was therefore undertaken to characterise the main carbohydrate and fibre fractions of the whole fruit of
Ripe fruits of
Botanical identification was established on the basis of diagnostic morphological characteristics and comparison with authenticated reference material. A voucher specimen (NZ 01) was prepared and deposited at the National Centre for Pharmaceutical Application and Research (CNARP), Antananarivo, Madagascar.
After collection, the fruits were transported to the laboratory for processing. Foreign matter and external debris were removed, and the plant material intended for analysis was rinsed with clean water. Samples were then shade-dried at ambient temperature (25 to 30 °C) for 30 days until constant weight was obtained, in order to reduce moisture content while limiting thermal degradation. The dried material was subsequently cut into small pieces, mechanically ground, and sieved to obtain a fine and homogeneous powder suitable for subsequent biochemical analyses.
The powder obtained from the whole fruits was thoroughly homogenised, accurately weighed, and divided into aliquots for the different analyses. Each aliquot was stored in a hermetically sealed container and labelled with the species name, plant material type, collection date, and sampling site to ensure traceability. Samples were maintained under controlled laboratory conditions, protected from light and moisture, until analysis.
Unless otherwise stated, the chemical reagents used in this study were of analytical grade and were obtained mainly from Sigma-Aldrich, Prolabo, Merck, and Labosi. The amounts, concentrations, and operating conditions are provided in the relevant methodological sections. Owing to the partial unavailability of procurement records, some catalogue numbers could not be documented retrospectively in full.
Total carbohydrate content was calculated by difference from proximate composition data. Because protein, lipid, and ash contents were expressed on a dry matter basis, total carbohydrates were estimated on the same basis as the residual fraction obtained after subtraction of these constituents from 100%. This value corresponds to total carbohydrates calculated by difference and includes the fibrous fraction. In accordance with international food composition frameworks, the interpretation of carbohydrates calculated by difference depends on the analytical definition of fibre used. In order to obtain an operational indicator of the carbohydrate fraction not accounted for by crude fibre, the crude fibre value was subtracted from total carbohydrates. However, because crude fibre does not measure all dietary fibre and is not equivalent to total dietary fibre, this value was interpreted only as an approximate estimate of the carbohydrate fraction not included in crude fibre, and not as a formal measure of available carbohydrates (
The following equations were applied on a dry matter basis:
The determinations of the other proximate constituents, namely moisture, total proteins, total lipids and ash, were performed independently according to standard analytical procedures for food composition.
Reducing sugars were determined by the Luff–Schoorl method, a classical iodometric titration method based on the reduction of copper (II) in alkaline medium by sugars bearing a free carbonyl group. After heating with the Luff–Schoorl reagent, the amount of residual unreduced copper was determined by iodometric titration with sodium thiosulfate, and reducing sugar content was obtained using the method’s conversion tables and expressed as glucose equivalents. Total sugars were determined after acid hydrolysis of non-reducing sugars according to the same titrimetric principle. Sucrose content was calculated from the difference between total sugars after inversion and reducing sugars before inversion, using the conventional factor of 0.95 (
A 2.5 g sample was introduced into a 250 mL volumetric flask and treated with 200 mL of 40% (v/v) ethanol. The mixture was stirred for 1 h in order to extract soluble sugars. Clarification was then carried out by successive addition of 5 mL of Carrez I solution (zinc acetate and glacial acetic acid) and 5 mL of Carrez II solution (potassium ferrocyanide), with stirring for approximately 30 s after addition of Carrez I and 1 min after addition of Carrez II. The volume was then brought to the mark with 40% ethanol, the mixture was homogenised and filtered. Two hundred millilitres of the filtrate were taken and evaporated to approximately half volume in order to remove most of the ethanol. The residue was then transferred quantitatively into a 200 mL volumetric flask using hot water, cooled, brought to volume with distilled water, homogenised and filtered if necessary. The resulting solution was used for the determination of reducing sugars and, after inversion, total sugars.
For the assay of reducing sugars, an aliquot of the prepared solution not exceeding 25 mL and containing less than 60 mg of reducing sugars expressed as glucose was taken. If necessary, the volume was adjusted to 25 mL with distilled water. The determination was then carried out according to the Luff–Schoorl method.
For the determination of total sugars, 50 mL of the test solution were pipetted and transferred into a 100 mL volumetric flask. A few drops of methyl orange were added, and 4 N hydrochloric acid was introduced gradually, with continuous stirring, until a distinct red endpoint was reached. Subsequently, 15 mL of 0.1 N hydrochloric acid were added. The flask was placed in a boiling water bath for 30 min in order to perform inversion. After rapid cooling to approximately 20 °C, 15 mL of 0.1 N sodium hydroxide were added to neutralise the medium. The volume was made up to 100 mL with distilled water and homogenised. An aliquot not exceeding 25 mL and containing less than 60 mg of reducing sugars expressed as glucose was then taken; if necessary, it was adjusted to 25 mL with distilled water before assay by the Luff–Schoorl method.
The assay was carried out by introducing into a 300 mL Erlenmeyer flask 25 mL of Luff–Schoorl reagent and 25 mL of the clarified sugar solution originating either from the reducing sugar determination step or from the total sugar determination after inversion. Two pumice granules and 1 mL of 3-methylbutan-1-ol were added to regularise boiling and limit foaming. The mixture was brought to the boil in approximately 2 min, then maintained at boiling for 10 min under a reflux condenser. After immediate cooling under cold water and standing for approximately 5 min, 10 mL of 30% potassium iodide solution followed by 25 mL of 3 N sulfuric acid were added carefully. The liberated iodine was then titrated with 0.1 N sodium thiosulfate until a pale yellow colour was obtained; after addition of starch indicator, titration was continued to the endpoint.
A blank test was carried out under the same titration conditions, replacing the sample solution with 25 mL of distilled water, without a boiling step.
Reducing sugar and total sugar contents were determined from the difference between the volume of sodium thiosulfate used for the blank (Vb) and that used for the test sample (Ve), according to the following relationship:
The ΔV value was then converted, using the Luff–Schoorl conversion table, into an equivalent glucose mass expressed in milligrams. In accordance with the method, the results for reducing sugars and total sugars after inversion are first expressed as glucose.
Reducing sugar content, expressed as a percentage of the sample, was calculated according to the following general relationship:
Total sugars after inversion, expressed as a percentage of the sample, were calculated according to the following relationship:
Where C is the glucose-equivalent mass (mg) corresponding to reducing sugars read from the Luff–Schoorl table, C′ is the glucose-equivalent mass (mg) corresponding to total sugars after inversion, M is the sample mass (g), V0 is the total volume of the extract, V1 is the volume taken for inversion, V2 is the final volume after inversion, Va is the volume of solution subjected to assay, and D is any additional dilution factor.
Sucrose content was calculated as follows:
Where 0.95 is the conventional conversion factor used to express as sucrose the increase in reducing sugars observed after inversion.
Starch content was determined using the polarimetric method. This method is based on measuring the optical rotation generated by optically active carbohydrates released after acid hydrolysis of starch. Two polarimetric determinations were carried out: the first measured the total optical rotation of all soluble optically active compounds after hydrolysis, whereas the second quantified the optical rotation of substances soluble in 40% (v/v) ethanol and not attributable to starch. The difference between these two measurements, corrected using the specific optical rotation of starch, was used to estimate the starch content of the sample (
A 2.5 g sample was treated with 25 mL of 3.6 N hydrochloric acid and heated in a boiling water bath for 15 min in order to hydrolyse starch and solubilise the hydrolysis products. After hydrolysis, the solution was clarified by successive addition of 5 mL of Carrez I reagent and 5 mL of Carrez II reagent, with stirring. Filtration was then performed. The filtrate was subsequently collected and brought to volume. The optical rotation of the clarified solution was measured using a polarimeter fitted with a 200 mm tube at a controlled temperature of 20 °C. The value obtained, denoted P, represents the total optical rotation of the soluble optically active substances present in the sample.
A 5 g sample was extracted with 80 mL of 40% (v/v) ethanol in order to solubilise optically active substances soluble in this medium. The extract was filtered, and 50 mL of the filtrate were taken and acidified with 10 mL of 3.6 N hydrochloric acid. The solution was heated in a boiling water bath for 15 min, then clarified by successive addition of 5 mL of Carrez I reagent and 5 mL of Carrez II reagent, with stirring. After mixing, filtration was carried out. The filtrate was then collected and brought to volume. The optical rotation of the solution was measured under the same conditions as for the first determination. The value obtained, denoted P′, represents the contribution of substances soluble in 40% ethanol.
Starch content was calculated according to the following equation:
Where 2000 is the conversion factor applied under the conditions of the method, P is the total optical rotation, P′ is the optical rotation of substances soluble in 40% (v/v) ethanol, and [α]D20° is the specific optical rotation of starch, taken as 184°.
Crude fibre content was determined by the Weende method, a classical gravimetric method used in proximate analysis of plant materials. In this approach, the sample is subjected to successive acid and alkaline hydrolyses in order to eliminate soluble constituents, notably sugars, starch, proteins and part of the hemicellulosic fraction. The resulting insoluble residue, consisting mainly of cellulose, lignin and other associated insoluble structural compounds, is then dried, weighed, incinerated and weighed again. Crude fibre content is calculated from the loss in mass observed during incineration of the dried residue (
A 3 g test portion was mixed with 200 mL of 0.26 N sulfuric acid solution. The suspension was heated under reflux for 30 min in order to solubilise acid-labile constituents. After hydrolysis, the mixture was vacuum-filtered through a fritted glass filter of porosity No. 1, previously prepared with a thin layer of sand. The residue was washed with hot distilled water until complete elimination of residual acid.
The insoluble residue obtained after acid hydrolysis was quantitatively transferred to a clean vessel and treated with 200 mL of 0.23 N potassium hydroxide solution. A few drops of antifoaming agent were added, then the mixture was rapidly brought to the boil and maintained under reflux for 30 min. After alkaline digestion, the suspension was vacuum-filtered through the same fritted glass filter. The residue was washed several times with hot distilled water until neutral pH was obtained, then rinsed with 25 to 30 mL of acetone to facilitate drying.
The residue was transferred into a dried and weighed crucible. The assembly was placed in an oven at 103 °C until constant mass. After cooling in a desiccator, the crucible was weighed and the obtained mass was denoted X1. The crucible was then placed in a muffle furnace at 550 °C for 3 h in order to incinerate the residual organic matter. After cooling in a desiccator, it was weighed again and the obtained mass was denoted X2.
Crude fibre content was calculated according to the following equation:
Where X1 represents the mass (g) of the crucible and residue after oven-drying at 103 °C to constant mass, X2 represents the mass (g) of the crucible and residue after ashing at 550 °C, and m represents the mass (g) of the initial sample, excluding the crucible, before any treatment.
Lignin content was estimated by near-infrared spectroscopy (NIRS), a rapid and non-destructive analytical technique based on absorption of near-infrared radiation by vibrational overtones and molecular combination bands, mainly associated with C–H, O–H and N–H functional groups. In plant matrices, NIRS can be used to predict certain structural constituents when spectral data are interpreted using chemometric calibration models established from reference analytical methods. In the present study, lignin values were therefore obtained as NIRS-predicted estimates and not by direct chemical determination (
A finely ground and homogeneous sample was prepared in order to improve reproducibility and stability of acquisition. Spectral measurements were carried out using a near-infrared spectrometer operating over a wavelength range of 1100 to 2500 nm. The recorded spectra were processed using the instrument’s internal chemometric algorithms and interpreted on the basis of a pre-existing calibration database established from reference analytical procedures, including AOAC-based methods (
In order to provide an integrated description of the balance between carbohydrate fractions and structural fibres, several ratios related to carbohydrate composition and likely to assist in interpreting the potential effect of fibre on glycaemic regulation were considered (
The energy contribution of the carbohydrate fraction was considered descriptively on a dry matter basis. However, total carbohydrates determined by difference include the fibrous fraction; consequently, direct application of the general Atwater factor of 4 kcal/g to this value does not strictly correspond to the energy of available carbohydrates (
The calculation was performed as follows:
When an approximation of the non-fibrous carbohydrate fraction was considered, it could only be regarded as indicative, based on the residual carbohydrate fraction obtained by subtracting crude fibre from total carbohydrates. This value was, however, interpreted only as a descriptive index, since crude fibre is not equivalent to total dietary fibre.
Analytical results were expressed as mean ± standard deviation (SD) of three repeated determinations. Coefficients of variation (CV, %) and 95% confidence intervals were calculated for descriptive purposes. Data processing and statistical calculations were performed using XLSTAT software, version 2014. Given the limited number of replicates, statistical treatment was restricted to descriptive analysis.
The results relating to the carbohydrate composition, crude fibre and lignin contents, derived carbohydrate-to-fibre ratios, and the estimated theoretical energy contribution of the carbohydrate fraction of
| Analysed components | Mean ± SD (%) | Range | CV (%) | 95% CI |
|---|---|---|---|---|
| Total carbohydrates | 87.80 ± 1.44 | 86.28 to 89.14 | 1.64 | 84.23 to 91.38 |
| Residual carbohydrate fraction | 53.59 ± 3.09 | 50.41 to 56.59 | 5.77 | 45.91 to 61.28 |
| Total sugars | 3.91 ± 0.00 | – | – | – |
| Reducing sugars | ND | – | – | – |
| Sucrose | 3.71 ± 0.00 | – | – | – |
| Starch | 13.56 ± 0.27 | 13.29 to 13.83 | 1.99 | 12.89 to 14.23 |
| Analysed components | Mean ± SD (%) | Range | CV (%) | 95% CI |
|---|---|---|---|---|
| Crude fibre | 34.21 ± 1.66 | 32.55 to 35.87 | 4.85 | 30.09 to 38.33 |
| Lignin | 18.08 ± 0.65 | 17.39 to 18.68 | 3.59 | 16.46 to 19.69 |
The estimated theoretical energy contribution of the total carbohydrate fraction was 351.20 kcal/100 g dry matter, equivalent to 1469.42 kJ/100 g dry matter.
The derived ratios related to carbohydrate composition are presented in
| Ratios | Values |
|---|---|
| Residual carbohydrate fraction/Total carbohydrates | 0.61 |
| Total sugars/Residual carbohydrate fraction | 0.07 |
| Sucrose/Residual carbohydrate fraction | 0.07 |
| Starch/Residual carbohydrate fraction | 0.25 |
The ratios between fibre fractions and carbohydrates are presented in
| Ratios | Values |
|---|---|
| Lignin/Residual carbohydrate fraction | 0.34 |
| Crude fibre/Residual carbohydrate fraction | 0.64 |
| Lignin/Total sugars | 4.62 |
| Crude fibre/Total sugars | 8.75 |
The fruit of
Total carbohydrate content reached 87.80 ± 1.44% DM, a value higher than those reported for other species of the same genus. For comparison, the pulp of
The residual carbohydrate fraction, estimated at 53.59 ± 3.09% DM, indicates that a substantial proportion of total carbohydrates is not accounted for by the crude fibre fraction. This estimate should, however, be interpreted with caution, since the crude fibre method reflects only a limited part of total dietary fibre. Consequently, the resulting value cannot be assimilated to a direct measure of available carbohydrates and may lead to overestimation of the fraction that is actually digestible and absorbable.
Overall, the carbohydrate values obtained for
The combined profile of total carbohydrates and the residual carbohydrate fraction in
The fruit displayed very low levels of soluble sugars, with total sugars reaching 3.91 ± 0.00% DM, no detectable reducing sugars, and sucrose accounting for 3.71 ± 0.00% DM, indicating a very limited contribution of simple sugars and the absence of detectable reducing sugars under the analytical conditions used. This result does not, however, allow the conclusion that free glucose or fructose are absolutely absent.
Available data indicate that
In palm species, sugar accumulation profiles are also influenced by fruit ripening, with simple sugar concentrations generally increasing during maturation, as reported in
Despite these low soluble sugar levels compared with
Consequently, the low simple sugar content measured in
Starch content in
The starch content observed in
The intermediate starch level observed in
The starch fraction quantified in the present study is therefore consistent with the presence of carbohydrate components likely to be digested more slowly than rapidly available simple sugars. Depending on its physicochemical organisation within the matrix, part of this fraction may exhibit limited enzymatic accessibility. Such structural constraints could contribute to slower glucose release during digestion and, consequently, to a more moderate postprandial glycaemic response (
The fruit of
In date palms (
The lignocellulosic values obtained for
The high abundance of insoluble fibre, cellulose and lignin in
The compositional characteristics observed for
Total carbohydrates accounted for 87.80% DM in
Analysis of carbohydrate ratios further clarifies the qualitative balance of the carbohydrate fraction. The proportion of the residual carbohydrate fraction remained moderate relative to total carbohydrates (residual carbohydrate fraction/total carbohydrates = 0.61), whereas the contribution of simple sugars to the residual carbohydrate fraction remained low (total sugars/ residual carbohydrate fraction = 0.07; sucrose/ residual carbohydrate fraction = 0.07). Such ratios suggest a low contribution of rapidly absorbable carbohydrates, generally associated with rapid postprandial glycaemic excursions (
The fibre/carbohydrate ratios further revealed a clear predominance of structural fractions, particularly crude fibre (0.64) and lignin (0.34), relative to the residual carbohydrate fraction. Such a compositional balance could contribute to delaying carbohydrate digestion and reducing enzymatic accessibility within the plant matrix (
When expressed relative to total sugars, the high ratios observed for crude fibre (8.75) and lignin (4.62) further underline the low density of rapidly assimilable sugars within the fruit matrix. Taken together, these quantitative relationships are consistent with a potential attenuation of postprandial glycaemic responses, primarily mediated by the modulatory effects of structural fibres on carbohydrate absorption kinetics, without implying a demonstrated hypoglycaemic effect under in vivo conditions (
Taken together, the calculated ratios provide a coherent quantitative framework for integrated interpretation of the compositional data. They should not, however, be considered direct predictors of clinical outcomes, but rather descriptive compositional indicators reflecting the balance between carbohydrate fractions and structural fibres within the fruit matrix.
This study provides the first detailed compositional data on
From a nutritional perspective, the compositional characteristics of the fruit, as analysed in dried and powdered form, suggest potential applicability in dietary patterns aimed at moderating postprandial glycaemic responses, enhancing satiety, and supporting body weight regulation. Beyond nutritional considerations, the valorisation of
Nevertheless, several limitations of the present study should be acknowledged. First, the findings are based exclusively on compositional analyses and do not include direct assessment of carbohydrate digestibility under either in vitro or in vivo conditions. Second, the specific nature of the starch fraction, rapidly digestible starch, slowly digestible starch, or resistant starch, was not determined, despite its important role in modulating postprandial glycaemic responses. Third, the absence of detailed characterisation of soluble fibre fractions limits interpretation of the potential physiological effects. In addition, because crude fibre quantification captures only a limited portion of total dietary fibre, the calculated content of the residual carbohydrate fraction may overestimate the proportion of carbohydrates that is actually available for digestion. Moreover, the predominance of insoluble and potentially poorly fermentable structural fractions requires further investigation in order to clarify their metabolic relevance. Finally, the use of an NIRS-based prediction for lignin quantification may introduce estimation uncertainty, which underlines the need for validation by reference analytical methods.
These limitations identify several priority directions for future research. Further work should aim to characterise starch fractions in order to refine understanding of carbohydrate hydrolysis kinetics and glucose release profiles. in vitro digestibility assays simulating gastrointestinal conditions would be useful to evaluate the resistance of the lignocellulosic matrix to enzymatic degradation. in vivo studies, initially in animal models and subsequently in humans, will be required to confirm effects on postprandial glycaemia and to define relevant intake levels. Finally, assessment of technological processing approaches, including cooking, milling, drying and granulation, would help identify the nutritionally most relevant forms of the fruit while preserving, as far as possible, its functional properties.
The underlying data are available on Zenodo: Raw and processed analytical data on carbohydrate fractions and dietary profile of
The authors thank all individuals and institutions who contributed to this work. This study was conducted within the framework of a doctoral programme at the Doctoral School of Life and Environmental Sciences, University of Antananarivo, under the supervision of the Biodiversity and Health research team. The authors express their gratitude to the Centre national de recherches sur l’environnement (CNRE), the Foibem-pirenena momba ny Fikarohana momba ny Fampandrosoana ny Ambanivohitra (FOFIFA), and the scientific supervisors who provided technical and methodological support.