Carla Graça a,b* , Anabela Raymundo a and Isabel Sousa a
2. MATERIALS AND METHODS 1 Raw Materials
3.5 Nutritional and Functional Characterization of the GFB
3.5.1 Nutritional composition
The impact of Cc addition to improving the nutritional and minerals profile of GFB was evaluated, and the results obtained are summarized in Table 6.
The increment of protein by Cc incorporation to GFB was noticeable: comparing CB to CcB20% (higher level tested), the protein content varied from 5.34 g /100g to 9.40 g / 100g, respectively, representing an increase of around 80%. Carbohydrate levels varied from 45.9 g/100g to 31.5 g/100g, representing a considerable reduction of 31.4 % for CcB20%, compared to CB.
These results suggest that the incorporation of Cc as baking ingredients in GFB formulations can be an alternative to obtaining bread with a reduced glycemic index, as proven in this work.
In terms of lipids content, a significant increase of 35 % for upper levels of Cc tested, was obtained.
A considerable increase in ash content was also observed, and this result is reflected in terms of major minerals profile. As can be observed from Table 6, a significant increase in phosphorus (50.0
%), magnesium (6.0 %), and calcium (360.3 %) were registered for Cc bread, which can cover more than 15% of recommended daily value as stated in European community (CE) Regulation Nº 1924/2006 based on nutrition and health claims made on foods [62], and in the Regulation (UE) Nº 432/2012 that specify a list of permitted health claims made on foods [63].
In terms of trace elements (Fe, Cu, Mn, and Zn), since their prevalence in buckwheat flour is higher than in curd cheese, their proportions were diluted by successively curd cheese additions.
A balanced gluten-free diet should provide adequate levels of macro-and micronutrients since the diagnosis of celiac patients is often associated with deficiencies in protein levels and mineral components, caused by the atrophy and subsequent damage of small gut villus [64-65, 42].
These nutritional improvements obtained by the addition of Cc to GFB formulation can give an additional contribution to fulfilling the nutritional daily diet requirements in terms of protein and major mineral profile to celiac patients and individuals susceptible to irritable bowel syndrome.
3.5.2 In vitro starch digestion and estimation of glycemic index of the GFB
Curd cheese and control gluten-free bread samples were subjected to in vitro enzymatic hydrolysis to mimic the starch digestibility. The hydrolysis of the starch was estimated by the amount of glucose released over the digestion time (30 to 180 min).
To evaluate the influence of the curd cheese enrichment on the rate of starch hydrolysis and further correlate its impact on the glycemic response of derived bread, the different starch fractions, rapidly digestible (RDS), slowly digestible (SDS) and resistant starch (RS), were estimated [39, 40].
Emerging Challenges in Agriculture and Food Science Vol. 5 Assessing the Technological and Nutritional Functionality of Curd Cheese in the Overall Gluten-Free Bread Quality
Table 6. Nutritional value (g/100g), including mineral profile (mg/100g), for control bread (CB) and Cc breads (CcB5% up to CcB20%) *
g/100g CB CcB10% CcB20%
Proteins 5.34 ± 0.21a 7.60 ± 0.12b 9.40 ± 0.10c Lipids 4.30 ± 0.29a 5.30 ± 0.61b 5.80 ± 0.20c Ash 1.40 ± 0.03a 2.03 ± 0.11b 2.90 ± 0.07c Carbohydrates 45.90 ± 0.95a 39.60 ± 1.50b 31.50 ± 1.20c Kcal 245.80 ± 1.80a 236.20 ± 1.10a 213.40 ± 2.10b
Minerals (mg/100g) 15% RDV* (mg/100g) **
Na (g/100g) 0.41 ± 0.03a 0.35 ± 0.01a 0.36 ± 0.02b - K 412.0 ± 8.02a 315.35 ± 5.92b 290.35 ± 10.40b 300.0 P 131.80 ± 5.40a 177.00 ± 0.90a 197.40 ± 2.00b 120.0 Mg 50.24 ± 1.02a 50.60 ± 0.56a 53.20 ± 0.82b 45.0 Ca 35.93 ± 2.06a 151.00 ± 0.81b 165.38 ± 2.60c 120.0
Fe 6.94 ± 0.12a 3.68 ± 0.67b 3.50 ± 0.05c 2.1
Cu 0.23 ± 0.01a 0.10 ± 0.02b 0.07 ± 0.03c 0.2
Mn 0.51 ± 0.01a 0.40 ± 0.02a 0.32 ± 0.03b 0.3
Zn 1.12 ± 0.01a 0.70 ± 0.08b 0.65 ± 0.01b 2.3
* Different superscripts (a, b, c) indicate significant statistical differences between CB and CcB: CcB5% up to CcB20%, (at p ≤ 0.05, Tukey test);
** Recommended daily values (RDV) establish by Reg. (CE), Nº 1924/2006 and Reg. (EU) Nº 432/2012.
The RDS is the starch fraction associated to have a strong impact on the increase of postprandial blood glucose level, therefore, increasing the glycemic index [66, 10].
From Fig. 7, it can be observed that as the concentration of the curd cheese increases in the bread matrix, the pattern of the starch fractions distribution has changed, compared to control bread: RDS fraction decreased considerably while the SDS take the dominance, followed by the RS fraction.
Fig. 7. Influence of the increasing levels of curd cheese (10% or 20% w/w) on digestible and resistant starch fractions of GFB: RDS - rapidly digestible, SDS- slowly digestible and RS –
resistant starch fractions
Different letters (a–e; A–D) reflect statistically significant differences at p < 0.05 (Tukey test).
Albeit the SDS did not show significant (p>0.05) variations by the increase of curd cheese (10 to 20%, w/w), a remarkable decrease of the RDS is clear, representing a reduction of about 40% and 67%, respectively, compared to control bread. As for RS fraction a considerable increase of around 70%
and 110%, were attained.
a
b
c d
d d
A
B
C
0.0 1.0 2.0 3.0 4.0 5.0
0.0 5.0 10.0 15.0 20.0
CB CcB10% CcB20%
Resistant starch (g/100g)
Digestable starch (g/100g)
RDS SDS RS
Emerging Challenges in Agriculture and Food Science Vol. 5 Assessing the Technological and Nutritional Functionality of Curd Cheese in the Overall Gluten-Free Bread Quality
These results are particularly positive, in a way that since the SDS fraction is slowly digested, it is more desirable than RDS, promoting a gradual increase in glucose and insulin levels [67], therefore, lowering the glycemic response. Since RS is the starch fraction not absorbed during the human digestion, acting as a fibre, and fermented in the large intestine by endogenous bacteria, these results should be considered nutritional gains to the human health diet [68].
The starch gelatinization during the baking step is the main mechanism responsible for the increase of RDS. In that matter, the lesser damaged the starch granules are, the higher the RS fraction [69].
These results should reflect the impact on starch gelatinization performance (C3, Table 2) obtained by the addition of a higher level of curd cheese (20%), in which the dilution effect of the starch, due to the higher level of protein and lipid, by entrapping the granules and hindering the complete gelatinization [70, 71], led to a significant decrease of RDS and a corresponding RS increase [10, 11].
These observations can be supported by the strong linear correlations (R2 > 0.900) found between pasting properties and starch digestibility properties, linking the curd cheese added to the impact observed in RDS, and RS fractions (linear correlations presented in Section 3.6). These results are in line with those reported recently [35], focused on the evaluation of the dairy product enrichment to enhance the glycemic response of wheat bread.
Furthermore, based on the starch hydrolysis index, the glycemic index of the curd cheese and control gluten-free bread samples was estimated. Hydrolysis curves of digested starch vs. hydrolysis time are depicted in Fig. 8.
Fig. 8. Effect of curd cheese addition (10% or 20% w/w) on starch hydrolysis fractions changes occurred during the in vitro starch digestibility by curd cheese incorporation (CcB10% square
symbols, CcB20%, triangular symbols), compared to control bread (CB, round symbols) As can be observed in Fig. 8, the starch digestion of the gluten-free bread samples displays a similar kinetic behaviour characterized by almost linear hydrolysis of glucose between 30 and 60 min, followed by a gradual reduction, as the digestion time proceeded (90–180 min). However, it can be noticed that the increasing addition of curd cheese (10 and 20%) substantially influenced the starch hydrolysis, resulting in indexes (HI) varying from 53% to 42% of starch, compared to control bread (100%).
The estimated glycemic response reflected the impact on starch hydrolysis and the changes of starch fractions obtained (Fig. 7), which has resulted in an intermediate index (GI 55–69), for both curd cheese bread samples (10% and 20%, w/w), representing a reduction of 32% and 40%, respectively.
0 10 20 30 40
0 20 40 60 80 100 120 140 160 180 200
Starch hydrolysis (%)
Time (min)
CB CcB10% CcB20%
Emerging Challenges in Agriculture and Food Science Vol. 5 Assessing the Technological and Nutritional Functionality of Curd Cheese in the Overall Gluten-Free Bread Quality
These registered changes in starch fractions are in line with the decreased starch digestibility, hydrolysis index and glycemic response of the gluten-free bread, which agrees with previous research works [11, 10]. Strong linear correlations (R2 > 0.903) were found between the incorporation of curd cheese, RDS and RS fractions, and the lower glycemic response obtained for the derived gluten-free bread.