3.1. Bioaccessibility of Food Components
Table 2 shows data on the bioaccessible food components released during the digestion process. In the case of coffee fibre, the major components of the bioaccessible fraction of the digest were carbohydrates (11.3%), whereas only trace amounts of sugars such as glucose (0.01%), fructose (0.02%) and mannose (0.03%) were detected. In contrast, a higher content in polysaccharides like galactomannan (2%) was quantitated. TPC represented 1.56% of the bioaccessible food components, and proteins (0.88%) and free amino groups (2.2%) were also present. These results suggest that SCG used as coffee fibre mainly provides non-digestible, complex carbohydrates to the bioaccessible fraction, which is in accordance with the literature [
4,
21].
In the case of SCB, the major components of the bioaccessible fraction of the digest were also carbohydrates (64.8%). Sugars represented 18% of total carbohydrates. Fructose (52.3%) and glucose (47%) were found in the highest amounts, followed by trace amounts of mannose (0.7%). Protein and amino acid contents were lower than 1% (0.41% and 0.80%, respectively) and TPC represented 0.85%. Wheat flour starch seems to be the main contributor to the carbohydrate content of the biscuit (
Table 1). According to the literature, wheat flour is composed of 65% digestible starch [
22], which is hydrolysed by digestive enzymes—salivary and pancreatic α-amylases—to glucose molecules and oligosaccharides [
23]. Sucrose added to the traditional formulation can also be converted into glucose and fructose, mainly by chemical reactions that have occurred during the processing of cereal-based products [
24], and the acid conditions of the stomach [
25], since the intestinal sucrase enzyme was not used in this particular digestion model. These available carbohydrates may turn the traditional biscuit into a food containing high sugar levels, which may consequently cause fast postprandial blood glucose glycaemic responses [
26]. High sugar diets are associated with decreased satiety and increased glucose intolerance, a greater risk of overweight and obesity, and impaired lipid metabolism [
27]. Furthermore, glucose and fructose follow different metabolic pathways after their absorption, resulting in different effects on blood glucose concentrations. Obesity is related to an elevated intake of both sugars. Type 2 diabetes is associated with high glucose diets, while the overconsumption of fructose is associated with non-alcoholic fatty liver diseases and augmented de-novo triglyceride synthesis [
28].
The bioaccesibility of glycaemic sugars in CFB was significantly (
p < 0.05) lower, with reductions of 46.4% ± 8.8% and 35.6% ± 1.1% for glucose and fructose, respectively, compared to the SCB (
Table 2). The stevia sweetener did not significantly contribute to the sugar content of the CFB. Stevia, in particular steviol glycosides, cannot be hydrolysed by digestive enzymes in the small intestine. Stevia is metabolised to steviol by the microbiota of the colon [
29]. However, our digestion conditions did not include the effect of the microbiota on the digestibility of the biscuits and their components. Stevia added to the CFB contained maltodextrin 97% as an additive. Maltodextrin composing stevia seems to have a significantly lower contribution to the bioaccesibility and total glycaemic carbohydrates of the food compared to that observed for sucrose in SCB formulation. Sucrose replacement by stevia played an important role in lowering the bioaccesibility of the glycaemic sugars in foods. Moderate glucose levels found in CFB are a necessary primary energy source for proper cell function in the organism [
30]. On the other hand, galactomannan was present in the CFB unlike the SCB. Coffee fibre included in the CFB is a natural source of galactomannan (
Table 2). Intake of galactomannan is associated with reduced weight gain, adiposity, liver fat and blood glucose levels [
31], making coffee fibre an attractive ingredient for confectioneries. Moreover, FOS incorporated in the CFB as soluble fibre has been shown to reduce post-prandial glycaemic responses [
32]. Marangoni and Poli [
27] also obtained a markedly lower glycaemic index in bread and biscuits by adding a proprietary fibre mixture to their formulations.
Regarding the antioxidant properties of the biscuits, significant differences (
p < 0.05) were found between the CFB and the SCB. Bioaccessibility of antioxidants was estimated as the overall antioxidant capacity of food digests.The digestion of the CFB released a significantly greater amount of antioxidants (15.07 ± 1.45 mg CGA eq./g digest) than in the SCB (10.43 ± 0.90 mg CGA eq./g digest). Most of the antioxidants of the SCB may be ascribed to phenolic compounds (
Table 2); in contrast, the CFB might also contain other non-phenolic antioxidants which may contribute to its overall antioxidant capacity. The bioaccessible fraction of the digested coffee fibre had a high antioxidant character (46.14 ± 3.61 mg CGA eq./g digest). Furthermore, stevia [
33], FOS [
34], and gluten peptides released during the digestion process by chemical and enzymatic hydrolysis [
35] may also exert an antioxidant character.
The high antioxidant properties of the CFB may play an important role in reducing the risk of obesity and diabetes. Consequently, introducing food antioxidants through the diet may be of great interest. High-antioxidant diets have been related to reduced inflammation and increased circulating antioxidants in cross-sectional and randomized intervention studies [
36].
In summary, the bioaccessibility of the analysed nutrients, such as total carbohydrates, proteins, amino acids and phenols, was not significantly different (
p > 0.05) between the two biscuits. However, the carbohydrate profile of CFB was enhanced by replacing sucrose with stevia, FOS and coffee fibre, providing the biscuits with the potential to augment satiety and reduce hyperglycaemia [
26].