Biomedical Sciences, Biomed Biopharm Res., 2022; 19(2):314-336
doi: 10.19277/bbr.19.2.296; PDF version here [+] ; Portuguese html version [PT]
Grape Pomace Flour: from winemaking by-product to sustainable alternative for health benefits
Raphaela Cassol Piccoli 1, Paula Pereira 2,3,4, Marisa Nicolai 2, Maria Lídia Palma 2, Francieli Moro Stefanello 5, Rejane Tavares Giacomelli 1,2,5*
1Postgraduation Program in Food and Nutrition, College of Nutrition, Federal University of Pelotas, 96010-610, Pelotas-RS, Brazil; 2Center for Research in Biosciences & Health Technologies (CBIOS), Universidade Lusófona, 1749-024 Lisboa, Portugal; 3Center for Natural Resources and Environment (CERENA), Instituto Superior Técnico (IST), Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal; 4EPCV-ULHT-Universidade Lusófona 1749-024 Lisboa, Portugal; 5Center for Chemical, Pharmaceutical and Food Science (CCQFA), Federal University of Pelotas, Campus Universitário S/N, 96160-000, Pelotas-RS, Brazil
Grape pomace (GP) is a by-product of wine industry that despite its substantial bioactive compounds content is vastly discarded during winemaking process. The present review aimed to summarize recent evidence on the biological, metabolic, nutritional and sensory properties of grape pomace flour (GPF) supplemented foods administrated to different models. In this sense, the search was carried out in the electronic databases “PubMed”, “Google Scholar” and “SCOPUS” and comprised studies that used grape pomace as its totality for the flour productions. GPF demonstrated a high dietary fiber and polyphenolic content that caused notable changes in organoleptic characteristics such as the color and texture of fortified foods and metabolic features. In some pre-clinical and clinical studies, an increase in antioxidant and anti-glycemic profile and a decrease in blood pressure have been observed, suggesting GPF as a possible health promoting agent when used as a food fortifier.
Keywords: grape pomace flour, fortified foods, phenolic compound, dietary fiber.
Received: 15/11/2022; Accepted: 31/12/2022
Grapes (Vitis vinifera) are one of the most extensively cultivated crops all around the world. According to the International Organization of Vine and Wine (1), the world’s production of fresh grapes in 2019 was approximately 85 million tons. Over 50% of the harvest of this product is used in wine production. After juice extraction, about 25% of the processed grape is essentially a solid waste called wine pomace or grape pomace (GP) (2,3). This residue results from the pressing of fresh grapes, fermented or not, during the winemaking process, and is composed of pressed grapes, peels, seeds, small pieces of stalks, and yeast cells (3-5).
Although the GP presents different uses, such as fertilization, it is still the main raw material for the production of alcohol, pomace spirit, and alcoholic beverages and used in animal feed (3). A large amount of this by-product is deliberately discarded annually by the industry, resulting in serious problems to the environment, such as soil and water pollution. On the other hand, GP contains significant amounts of dietary fiber and phytochemicals such as flavonoids, (for example catechins and anthocyanins), stilbenes and phenolic acids, which remain in the pomace after the winemaking process (6,7).
Considering the environmental context, the most sustainable alternative of the reuse of GP seems to be in the form of grape pomace flour (GPF), which is obtained after bagasse drying and milling (5). Recently, the effects of GPF been studied at the metabolic level and in its use as a fortifying ingredient in foods, with a view to improving sensory properties, increasing nutritional value and as a potential health-promoting agent (7).
Thus, the aim of this review is to summarize recent evidence on the biological, metabolic, nutritional, and sensory properties of GPF-supplemented foods and administration to different models. Given the relevant environmental issue, we summarized the evidence that evaluated the totality of this by-product dried and milled due to the practicality and low cost of this process.
Materials and Methods
Data sources and search strategy
The present review was carried out from the search for scientific articles indexed in the electronic databases PubMed, Google Scholar, and SCOPUS using the English descriptors “grape pomace flour” and “grape pomace.” The articles used in this review were from 2010 to 2022. The bibliographic references of the selected articles were also evaluated to identify other potentially relevant studies.
Inclusion and exclusion criteria
The inclusion criteria were: 1. Studies that investigated metabolic, biochemistry, sensorial or physicochemical aspects of administration, or formulation of products with GPF; 2. GP used as its totality (not isolating a single part of the pomace); 3. Publication as a full text in the English language. All articles that did not meet the inclusion criteria were excluded.
The selection of articles included in this review followed the sequence: 1. Launching of descriptors in the databases; 2. Selection by titles; 3. Reading the abstracts of the articles as selected by title (pre-selection); 4. A complete reading of articles pre-selected by review of the abstract; 5. Inclusion of articles with relevant data; 6. Articles used in this review were from 2010 to 2022. The initial search yielded 32 articles. After reviewing titles and retaining only articles that met the review criteria, 13 articles remained for review. Among those, two were selected from SCOPUS, two from Google Scholar and nine from PubMed.
Results and Discussion
Tables 1, 2 and 3 presents the main characteristics and results of the studies included in the present review and are divided according to the study design, as follows: preclinical studies, observational studies and sensory evaluation of foods, respectively. The studies were published between 2014 and 2021. Four of the studies were conducted in Chile, three in Brazil, two in Portugal, one in Tunisia, one in Thailand, one in the United States and one in Spain.
In 2015, Hernández-Salinas et al. (8) performed a study to investigate whether GPF supplementation for sixteen weeks may prevent the disturbances caused by a high fructose diet-fed animal model of metabolic syndrome (MS) on glucose metabolism and oxidative stress in rats. The animals were fed with a control diet, control diet plus 20% of GPF, control diet plus 50% of a fructose diet or a control diet plus 50% fructose diet plus 20% of GPF. The authors then measured blood glucose, insulin and triglycerides, arterial blood pressure, and body weight. The homeostasis model assessment (HOMA) index was also calculated, a glucose tolerance test was performed, and oxidative stress index was measured in the plasma and kidney. At the end of the experimental protocol the authors could observe that GPF supplementation was able to prevent the increase of thiobarbituric acid reactive substances (TBARS) levels in plasma and renal tissue caused by the administration of a high fructose diet. Furthermore, it also prevented the increase of HOMA index, fasting blood glucose, which ranged from 104.3 ± 2.8 in the untreated fructose group to 91. ± 2.84 mg/dL in the fructose group that received GPF and plasma insulin, which ranged from 912 .2 ± 1.3 to 8.9 ± 0.7 μU/mL compared with the untreated intervention group. Although arterial blood pressure and body weight were evaluated, none of the groups presented significant differences in those parameters.
According to the authors, high fructose concentrations stimulate uric acid production at the time of its metabolization. In extracellular environment, uric acid performs an antioxidant activity, but inside the cell environment, it may induce an oxidative cascade, mediated by NADPH oxidase. This mechanism is dose-dependent and can lead to an oxidative stress (OS) state (8). In addition, according to the literature, a hyperglycemic ambient induces the installation of inflammatory pathways through the stimulation of proinflammatory cytokines release such as tumor necrosis factor-alpha (TNF-α), interleukin-6 (IL-6) and IL-1β. OS and inflammation are closely related to the development of chronic diseases. In this sense, previous studies in the literature have indicated that dietary polyphenols may suppress system inflammation, and consequently OS, through the modulation of the activities of key transcription factors that regulate cellular responses to OS and inflammation such as proinflammatory cytokines release, the regulation of inflammation-related signaling pathways and proinflammatory enzymes such as COX-S, MAPK and protein-c kinase (9).
Charradi et. al. (2017) (10) reported the potential anti-lipotoxic effect of grape seed and skin flour (GSSF) in the brain of either female or male obese rats induced by a high-fat diet (HFD), with an emphasis on the protection against lipotoxicity-induced oxidative stress and inflammation. The animals’ diet was mixed with 5% GSSF for eight weeks. At the end of the experimental period, the authors observed that this supplementation resulted in protection against the increase of LDL-cholesterol accumulation in plasma and brain, lipoperoxidation, protein carbonylation and a decrease of antioxidant activity caused by the HFD. GSSF also cancels the disturbances caused by the HFD in transition metals and associated enzymes, intracellular mediators and associated enzymes, peripheral adipokines and lipid brain deposition. The gender-dependence of the effects of HFD is known, more significantly altering brain lipotoxicity in male than female rats, and the lower probability of females developing inflammatory-related brain diseases is suggested to be linked with the anti-inflammatory role of estrogens. However, the GSSF supplementation efficiently protected both sexes.
An experimental study designed by Souza et al. (2019) (11) evaluate whether dietary supplementation with GPF for ten weeks was able to prevent or reduce the hepatic oxidative damage of grass carp (Ctenopharyngodon idella) experimentally infected by Pseudomonas aeruginosa. In this sense, the authors included 150 or 300 mg/kg of GPF in the basal diet of the fish. The authors evaluated the effects of the administration of two different concentrations of GPF, only the 300 mg/kg supplementation demonstrated significant results. In this sense, they observed an increase of the enzymatic and non-enzymatic antioxidant defense systems due to a significant upregulation of hepatic enzymes Superoxide Dismutase (SOD) and Catalase (CAT) activities and on hepatic total antioxidant activity against peroxyl radicals (ACAP) levels, demonstrating a high capacity to scavenge the peroxyl radical. Additionally, the levels of reactive oxygen species (ROS), metabolites of nitric oxide (Nox), and TBARS of the treated animals remained similar to the control groups, while the non-treated showed significantly higher concentrations of those markers, demonstrating that the prophylactic supplementation was able to protect the liver against oxidative damage.
According to the literature, several pieces of evidence have demonstrated that the bioactive compounds of grapes exert hepatoprotective effects, sometimes via synergetic action. Even so, resveratrol, the main component of red grapes is considered the main responsible for the antioxidant actions that can be observed in these crops (12). Among the means of action of resveratrol is the suppression of ROS synthesis by inhibiting enzyme depletion or by chelating trace elements involved in free radical production, scavenging ROS and up-regulating or protecting the antioxidant defense and activating defense pathway factors (11). On the other hand, despite the solid scientific data indicating resveratrol’s health benefits, this polyphenol use still raises doubts due to its low oral bioavailability despite its high bioactivity. Resveratrol’s oral poor bioavailability is attributed to its lower aqueous solubility, membrane permeability, and metabolic stability (13). In this sense, studies have been conducted in order to list administration routes, concentration, and classes of derivatives that provide better bioavailability and, consequently, efficacy. Therefore, although this compound is abundantly present in the chemical composition of GPF and has demonstrated effects as an isolated compound in in vitro protocols (14,15), studies are still needed to elucidate its effects and metabolization from GPF in isolation.
Also in 2019, Alba et al. (16) conducted an experimental design to evaluate the effects of GPF on antioxidant activity, biochemistry variables, components of the immune system and milk production and quality of Lacaune sheep in heat stress. The animals received 0.8 kg/day of a concentrate of 1 or 2% of GPF, twice a day, for two weeks. The authors demonstrated that the concentrate supplementation resulted in an elevated antioxidant response in the sheep serum. In addition, serum levels of urea were reduced (from 38.0 in the control group to 27.6 mg/dL) while serum glucose (from 54.7 to 66.6 mg/dL) and triglyceride (from 20.1 to 36.9 mg/dL) concentrations were higher in animals supplemented with 2% GPF, which could be explained by the increase in the amount of ether extract in the diet containing 2% GPF (16). The ether extract is related to higher percent of energy in the diet.
This study demonstrated that the experimental administration of GPF resulted in greater control of oxidative stress, an increase in productive efficiency (around 18%), and an improvement in sheep health. In this sense, it was observed that the increased serum concentrations further support the upregulation of antioxidant capacity in the milk after the GPF administration, which is related to antioxidants, such as quercetin, resveratrol, and phenols that constitute the chemical composition of the grape. Besides, the authors assume that the increase in productive efficiency after the inclusion of GPF may be a result of the greater control of oxidative stress and decrease of free radicals concentration. In addition, it is hypothesized that as the GPF supplementation exerted protection against milk lipid peroxidation, it could potentially increase the shelf life of milk and its derivative products (16).
In 2019, Rivera et al.(17) led an experimental study to investigate the impact of dietary supplementation of GPF during one or two weeks in a model of lethal ischemic heart disease. In this sense, male and female SR-B1 KO/ApoER61h/h were administrated 20% high fat, high cholesterol, and cholic acid-containing atherogenic diet mixture to the basal diet, 20% red wine pomace flour (RGPF) plus the experimental diet or 10% of oat fiber plus the atherogenic diet. After the experimental protocol, the authors could observe that RGPF supplementation showed a significant improvement in animal lifespan in comparison to the groups that did not receive the flour. When analyzing the plasma antioxidant capacity of the RGPF, the authors could observe that it decreased the levels of dihydrorhodamine (DHR) oxidation to baseline, while the experimental diet exerted an increase of more than 100% at this parameter. Finally, the authors evaluated heart disturbances caused by the administration of an atherogenic diet and the potential effects of the RGPF supplementation. In this sense, the RGPF group showed reduced formation of atherosclerotic plaques and Oil Red O-stained areas at the end of the experiment and significant restoration of systolic heart function at day 14 to normal levels. The findings elucidated by Rivera et al. (17) showed that in addition to increasing mice lifespan, RGPF consumption was associated with a significant reduction in atherosclerotic lesions and ischemic heart disease and exhibited an antioxidant effect. These results indicate that the effect of RGPF on a diversity of targets is not only related to the abundant presence of fiber since the oat fiber-fed group did not demonstrate the same effects as the RGPF, but is likely related to other specific components within the RGPF flour.
Recently, in 2021, Harikrishnan et al.(18) evaluated the effects of dietary inclusion of GPF for eight weeks on growth, antioxidant and anti-inflammatory profile, innate-adaptive immunity, and immune genes expression in Labeo rohita against Flavobacterium columnaris. The animals were separated into four different groups which received 0 (control group), 100, 200, or 300 mg/kg of the GPF supplemented on the basal diet. After the 60 days experimental protocol, the authors could observe that the 200 mg/kg GPF inclusion diet exhibited the most significant results on growth rate, antioxidant status, and immune defense mechanisms than other concentrations of the flour. The authors could observe a significant increase in parameters such as SOD (from 4.76 ± 0.25 in the infected control group to 6.63±0.36 and 6.33±0.31 U/mg-1 in the groups supplemented with 220 and 300mg/kg, respectively), glutathione peroxidase (GPx) (from 25.14 ± 2.15 to 41.60±2.40 and 34.14± 1.67 U/mg-1 in the groups supplemented with 200 and 300mg/kg, respectively), glutathione (GSH) (from 4.11 ± 0.25 to 8.23 ± 0.44 and 6.75 ± 0.3 mg/g -1 of protein) and phagocytic (PC) activity, respiratory burst (RB), alternative complement pathway (ACP), lysozyme (Lyz), total immunoglobulin M (IgM), toll-like receptor-22 (TLR22) and hepcidin mRNA expression on the groups supplemented with 200 or 300mg/kg of GPF. According to the present work, GPF was capable of improving the antioxidant enzymatic and non-enzymatic pathway throws the increase of enzymes accompanied by a balance between synthesis and exclusion of ROS.
Clinical/observational human model studies
In 2015, Urquiaga et al. (19) designed a prospective, randomized controlled parallel-group trial to evaluate the effect of GPF on components of MS in humans. In this sense, male workers who regularly consumed an omnivorous diet and presented at least one component of MS were recruited as volunteers. After the selection of the participants of the study, they were randomly assigned to either intervention or control groups. Both groups were asked to maintain their regular eating and lifestyle habits, with the exception of the intervention group which was also directed to consume 20 g of GPF/day during the 16 weeks of the study. GPF was consumed in bread or biscuits or diluted in water. Participants completed clinical, nutritional, anthropometric and laboratory evaluations at the beginning and end of the study. At the end of the study, the authors could observe that the control group showed a statistically significant increase in body mass index (BMI) (from 27.9 ± 3.5 to 28.3 ± 3.6kg/m² at the end of the experimental period), while no significant differences were observed in anthropometric characteristics of the intervention group. GPF consumption resulted in a significant decrease in systolic (from 127.1 ± 11.5 to 122.8 ± 8.5 mmHg) and diastolic blood pressure (from 79.7 ± 8.3 to 74.4 ± 5.6 mmHg), fasting glucose levels (from 92.7 ± 5.8 to 89.4 ± 7.9 mg/dL) and protein damage measured as carbonyl groups in plasma proteins (from 0.56 ± 0.18 to 0.44 ± 0.19 nmol/mg protein), while γ-tocopherol (from 1.80 ± 0.74 to 2.40 ± 1.36 µM), δ-tocopherol (from 0.70 ± 0.13 to 0.79 ± 0.23 µM) and α-tocopherol (from 31.67 ± 8.58 to 32.48 ± 8.73 µM) increased significantly in GPF supplemented group (19).
More recently, in 2018, Urquiaga et al. (20) designed a three-month longitudinal trial consisting of two treatment periods of four weeks, separated by a third four-week wash-out period. Male workers who regularly consumed an omnivorous diet and presented at least one component of MS and a BMI between 25.0 and 39.9 kg/m2 were recruited as volunteers. Participants were asked to maintain their regular eating habits and lifestyle during the study, except for the daily intake of GPF-burgers containing 7 g of GPF during the first and last four weeks of the experimental protocol. Participants had clinical, nutritional, and anthropometric evaluations at the beginning and end of the study. After the 16 weeks, the authors could observe that GPF-burger consumption resulted in a significant decrease in glycemia and HOMA index value throughout the experimental period and remained low during the washout period. A decreasing trend in plasma insulin levels was observed during the GPF-burger period that remained low during the washout period. According to plasma antioxidants, GPF-burgers consumption period resulted in an increased concentration of vitamin C and decrease of plasma uric acid levels and 2,2-diphenyl-1-picrylhydrazyl (DPPH∙) radical scavenging capacity. In addition, GPF-burgers consumption exerted a significant decrease on advanced oxidation protein products (AOPP) and oxidized low-density lipoprotein (oxLDL) levels. No significant change in anthropometric characteristics of the participants was observed over the intervention period, and there were no variations in the number of individuals with normal weight, overweight or obesity.
It is interesting to mention that both studies evaluated the effect of GPF supplementation in a cohort of individuals with MS. According to the authors, the main objective was to test the hypothesis that the consumption of GPF (rich in dietary fiber and bioactive compounds) could exert beneficial effects on biochemical parameters and markers of oxidative stress in MS, considering that the deregulation of these parameters results in a higher risk for the development of non-communicable chronic diseases, such as cardiovascular diseases and diabetes. In addition, overall food intake was assessed in both articles using a fourteen-item self-reported questionnaire that measured adherence to the Mediterranean diet in Chile, based on eating habits from countries in the Mediterranean region, with modifications that include Chilean habits. The score ranges from 0 (minimum adherence) to 14 points (maximum adherence), and no significant differences in scores were observed between groups.
According to metabolic parameters, the authors could observe a reduction in fasting glucose levels and, as GPF are rich in antioxidant dietary fiber, a known substance that is resistant to digestion by human gastrointestinal enzymes, evidence exists relating its consumption to an improvement on carbohydrate metabolism. However, the mechanism associated with these beneficial effects are not yet known, and it is hypothesized that the dietary fiber acts in synergy with the phenolic content of GPF (20). In addition, observed a statistically significant lowering of blood pressure was observed following GPF intake. Previous data from the literature associate the consumption of grape extracts, which are rich in phenolic acids, flavonoids, anthocyanins, stilbenes, and lipids, with antihypertensive effects and, consequently, cardio protector features (21). Additionally, fiber-associated antioxidant products that are released by colonic fermentation might explain the reduction in blood pressure that was observed following GPF supplementation (19).
According to the results observed by Urquiaga et al. 2015 and 2018 (19, 20), GPF consumption led to increased antioxidant defenses and a reduction in oxidative and protein damage markers, which enhances the antioxidant capacity of grapes as previously indicated in the literature (21). Finally, some of the participants reported side effects during the experimental protocol when they were oriented to consume GPF: 7 reported an increase of intestinal gas, 6 reported heartburn, 7 reported regularization of intestinal transit, 6 reported softer stools, 3 reported increased appetite, 2 reported dyspepsia and 2 reported gastroesophageal reflux (19). GPF has high polyphenol content and non-soluble dietary fiber that is resistant to digestion by human gastrointestinal enzymes. This fact may have contributed to the observed side effects. In addition, polyphenols, such as proanthocyanidins, when partially fermented by bacterial microflora, may generate intestinal gas. Further studies are needed to test this hypothesis (19). Although GPF consumption demonstrated beneficial effects on human individuals as described in the previous studies included in this review, the authors highlight that there was some limitations in those studies, including: the low number of participants that were able to finish the protocol, the bias that might have occurred due to the open-label scope of the protocols, the differences of calorie intake and dietary composition of each participant that influences the final results mentioned above (19,20). Those enforce the need of developing new studies that involve larger numbers of participants and investigate different features such as diabetes and cardiovascular diseases on human scopes in order to better understand the potential effects of GPF supplementation.
Applications of grape pomace flour in the food industry
In 2014, Walker et al. (22) headed a study to evaluate the effects of fortification of baked good, including breads, muffins, and brownies with Pinot Noir (RGPF) or Pinot Grigio (WGPF) grape pomace flour in concentration range 5-20%. RGPF and WGPF substituted wheat flour at concentration of 5%, 10%, and 15% for bread, 10%, 15%, 20%, and 25% RGPF for brownies, and 5%, 10%, and 15% RGPF or 10%, 15%, and 20% WGPF for muffins. The final products were then evaluated for total phenolic content, radical scavenging activity, dietary fiber content and physicochemical and sensory proprieties. For the sensory evaluation, the participants received two samples of each baked good and rated the likeness of established parameters on a 9-point hedonic scale. Physicochemical properties and bioactive compounds analyses were performed according to the stablished methodology to each parameter. Regarding to physicochemical and sensory characteristics, both breads and muffins (5% or 10% GPF) and brownies (15% RGPF) were found to be acceptable and to be accepted by consumers when compared to control. About the bioactive composition, it could be observed that the total phenolic content and radical scavenging activity of breads increased as the GPF percent raised as well. For the bread and muffins, the total phenolic content followed the same trend as the increase of GPF was presented, except that the WGPF samples had significantly lower radical scavenging values when compared to RGPF ones. Similarly, the radical scavenging activity of brownies was only significantly higher in 10 and 25% RGPF fortified samples. In general, breads and muffins fortified with 10% RGPF increased their total phenolic content and radical scavenging activity by 5.86% and 194.38%, and 176.42% and 1144.87%, respectively, compared to the control. As for the dietary fiber content, there was a trend of increase as the percentage of GPF increased as well. Bread and muffins fortified with 10% RWGP increased their content 31.61% and 15.02%, respectively, when compared to the control. At 15% RGPF fortification, brownies had a 6.94% increase in dietary fiber content. Finally, the consumer acceptance of RGPF fortified baked good demonstrated that 5 and 10% breads presented a lower rating for mouth feel, what suggests that these samples was a little dry, as expected after the water activity and holding. Regarding the muffin samples, the 5% formulation presented the most acceptable color as the 10% demonstrated significantly higher scores and the control lower, indicating that these samples were too dark and light, respectively. As for the aroma rating, the 5% RGPF muffins presented the highest scores of the fortified samples, while the most appreciated was the control one. For the brownies, the less appreciated sample regarding texture was the 20% RGPF, which according to the panelists, presented big particle size of the pomace which interfered with this attribute.
Ortega-Heras et al. 2019 (23) designed a study to evaluate the adequacy of sensory attributes, nutritional, color, and texture proprieties of muffins fortified with red (RGPF) and white (WGPF) grape pomace flour. Five formulations of muffins were analyzed: a control muffin composed of 100% whole-wheat flour and muffins made with 10 and 20% of RGPF or WGPF (free of seeds). For the sensory analysis, participants received one muffin of each formulation and were asked to indicate the degree of preference using a nine-point hedonic scale, ranging from “disliked extremely” to “like extremely”. The evaluated attributes were surface color, crumb color, sweetness, hardness, chewiness, and flavor using a five-point bipolar scale. In addition, were evaluated the nutritional composition, height increase and weight loss, color and texture profile analysis of the muffins, according to each previously proposed methodology. In terms of nutritional composition, it was observed that the GPF-fortified muffins had higher fiber content, which ranged from 5.67 ± 0.44 in the control formulation to 9.56 ± 0.55 and 11.9 ± 0.3 in the 10 and 20% WGPF formulations, respectively and 8.24 ± 0.64 and 11.2 ± 0.7 in the 10 and 20% RGPF formulations, respectively, and fat which ranged from 26. 7 ± 2.in the control muffins to 31.0 ± 3.9 and 32.4 ± 1.2 in 10 and 20% WGPF, respectively and 33.3 ± 1.9 and 33.8 ± 1.1 in 10 and 20% RGPF formulations, while the control muffins showed the highest height increase (23.0 ± 2.6) and the 20% fortified muffins showed the highest weight loss scores (14.8 ± 1.7 for WGPF and 13.8 ± 1.4 for RGPF). Regarding colour and texture parameters, muffins prepared with GPF were darker and firmer when compared to the control muffin. Furthermore, the addition of GPF resulted in an increase in chewiness (from 3.08 ± 1.02 in the control muffins and 9.10 ± 5.88 in the muffins fortified with 20% WGPF) and a decrease in resilience as the percentage of GPF increased (0.224 ± 0.006 in 10% WGPF, 0.194 ± 0.014 20% WGPF, 0.225 ± 0.018 10% RGPF and 0.197 ± 0.033 20% RGPF). Finally, the 20% GPF concentrations resulted in a significant reduction in muffin elasticity (from 0.609 ± 0.009 in 10% WGPF to 0.541 ± 0.014 in 20% WGPF and 0.604 ± 0.015 in RGPF 10% to 0.561 ± 0.081 in RGPF 20%). After sensory analysis, the authors observed that the most liked muffin was the control (7.05 ± 1.11), followed by the 10% RGPF or WGPF formulations (6.34 ± 1.29 and 6.24 ± 1.42, respectively). The organoleptic analyses showed that the fortified muffins had "much more" color and flavor, especially in the formulation with RGPF 20% (41% e 29%). According to the perception of sweetness, the WGPF formulations showed poorer results.
In 2019, Cilli et al. (24) evaluated the antioxidant potential of GPF in a frozen salmon burger. Volunteers were recruited to participate in a sensory evaluation in which they received approximately 10 g of three burgers with different formulations: the control, and 1 or 2% GPF incorporation. Each was evaluated in terms of color, odor, taste, texture, appearance, and overall quality using a 9-point verbal hedonic scale. Participants were also asked to give their opinion on purchase intention using a 5-point scale. The authors also analyzed the TBARS content and in vitro cytotoxicity of GPF according to the methodology accepted in the literature. According to the cytotoxicity, the GPF showed a good cytocompatibility when exposed to mouse fibroblasts cells, exerting cytotoxic effects only at 800 mg of GPF/ml, which means that the 1 and 2% formulations of burgers are safe as represents 100 and 200 mg of GPF/ml, respectively. The proximate composition of the burgers showed that GPF addition improved the dietary fiber content from 4.60 on control formulation to 4.90 and 5.20 in supplemented burgers, respectively, and darkness, which was evaluated through lightness diminution, from 70.77 ± 0.47 for the control sample to 66.29 ± 0.46 in the 100 mg and 63.08 ± 0.32 in the 200 mg of GPF/ml supplementation. As for the sensory evaluation of salmon burgers, the researchers could observe that both the supplementation of GPF was less appreciated for appearance from 7.75 ± 0.12 on the control to 6.55 ± 0.15 and 6.20 ± 0.18 on the supplemented, from 7.73 ± 0.12 to 6.57 ± 0.15 and 6.25 ± 0.18 of color, and overall quality of salmon burgers from 7.79 ± 0.10 to 7.12 ± 0.14 and 6.85 ± 0.16 as compared to the negative control. In general, the 1% GPF formulation of burger was more accepted by the consumers and presented a higher purchase intention, representing 23.64% of the participants that would certainly buy it, when compared to the 2% samples (20.91%). Finally, the authors could observe that GPF supplementation in the salmon burgers resulted in a decrease of TBARS content during storage without negatively compromising the nutritional or microbiological characteristics.
In 2020, Palma et al. (5) performed a sensory evaluation of salty biscuits fortified with GPF at percentages of 5 and 10%. The participants received five different formulations of biscuits: flour of the Arinto and Touriga Nacional variety (5 and 10% of the amount of wheat flour), and a control of the same formulation, without GPF. The attributes evaluated by the volunteers were: color, aroma, flavor, texture, and global impression using a 5-point hedonic scale. Parameterized purchase intention was also assessed according to a 5-point scale. Participants were also asked to choose which biscuits they "like the most" and which they "like the least". At the end of the sensory evaluation, the authors could observe that the Arinto 10% biscuit had the highest scores of which represented 4.32 points, 3.72 for aroma, 4.32 for flavor, 3.92 for texture, 4.32 for of overall appreciation and 4.32 of purchase intention when compared to the other biscuits, although none of these results are statistically significant. It could also be observed that the formulations with the lowest concentrations of GPF were less appreciated by the participants (Arinto 5% and Touriga Nacional 5% representing 30.20% and 41.50% of the participants, respectively). Finally, the most and least liked biscuits choice question resulted in a highest number of votes to GPF Touriga Nacional 10% as the “best liked” biscuit, representing 39.60% of the participants and GPF Touriga Nacional 5% as the “least liked," representing 41.50%.
Similarly, in 2021, Palma et al. (25) aimed to evaluate the acceptability of sweet biscuits fortified with Arinto or Touriga Nacional GPF, with a higher percentage of incorporation and with a wider range of volunteers than the previous work. The participants received five biscuits with different formulations: GPF from the Arinto variety and GPF from the Touriga Nacional variety in 15 and 20% incorporations and control biscuits without GPF supplementation. The tests were conducted on two different days and the participants were given a questionnaire to evaluate parameters such as color, taste, texture, aroma and general impression, purchase intention and preference, in which the participants were asked to rate the biscuits as "most liked", "liked more or less" and "least liked". In this study, the highest scores were assigned to the control biscuit. Meanwhile, both Touriga Nacional GPF formulations presented a lower score related to color and flavor (scoring 3,83 out of 5 points), however they presented the highest texture scores. According to purchase intention, the highest percentages were attributed to the Arinto GPF incorporations of 15 and 20% (representing 32,1% each) and the biscuits with the lowest purchase intention were those of the control formulation (6,4% of the participants referred that would certainly not buy it). Finally, the choice preferences showed that the most appreciated biscuit was the control, and among the fortified biscuits, the highest scores were those of Touriga Nacional GPF 15 and 20% followed by Arinto GPF 20 and 15%.
Based on the results presented by the previous mentioned studies, it could be observed that different incorporation ratio of GPF on different recipes of baked goods (salty and sweet ones) and salmon burgers demonstrated a heterogeneous preference for the control or fortified samples. In general, the addition of GPF interferes with the organoleptic characteristics such as color, in which the final product tends to be darker as the percentage of flour increases and to present a feature similar to the grape cast, which means that it acquires a purple to red color when fortified with RGPF and a yellow to brown color when added the WGPF. The capacity of GPF to influence the color of the products represents a beneficial attribute because, according to the literature, consumers tend to associate darker baked goods as healthier ones, being the preferred choice when seeking functional food (22). Furthermore, GPF presented a higher content of dietary fiber when compared to the control formulations investigated in the studies mentioned, increasing as the percentage of GPF was increased. According to the literature, dietary fiber intake is associated with decreased risk of cardiovascular disease (26) and other chronic diseases, in addition to an association with lower body weight, what represents benefits to health (27).
Furthermore, the chemical composition analyses demonstrated that GPF is rich in bioactive compounds, as its concentrations were generally higher than those in control formulations. As for the final products the phenolic composition tends to increase as the percentage of GPF is increased. Other characteristics of fortified baked goods are hardness and chewiness, in which the panelists indicated that those samples presented a dryer texture, which can be explained by the flour’s lower water activity and water retention, which results in incomplete hydration of the flour and might attribute to its harder texture (22).
Finally, the authors could observe that even though a fortified baked good presented lower single characteristics such as color and texture scores, when compared to the control, the same samples could achieve higher values of purchase intention or likeness. According to the literature, it could be explained as consumers do not always answer “like and dislike” questions the same as specific ones due to emotional influence (5,25). In this sense, the need of evaluating the same formulation with different methods and the necessity of new studies investigating different concentrations and preparations using GPF and a larger scope of volunteers is emphasized to better understand general preferences for the introduction of its consumption as a functional food.
GPF is a by-product of the wine industry that represents a valuable source of important nutrients with healthy antioxidant, cardio protective, and anti-hyperglycemic-promoting activities. GPF also represents an alternative to inadequate GP disposal, which might lead to environmental problems. Recent evidence is demonstrating that this by-product may have an important effect in the optimization of health benefits and minimizing possible negative health markers. In addition, the incorporation of fiber-rich GPF in different recipes results in higher nutritional value and sensory properties of the final product. Although there are preclinical and clinical studies observing that some mechanisms played by its chemical composition may act as cardio and neuroprotective, more investigations are needed regarding the targets and pathways by which the effects of GPF can act to improve health and food nutritional quality to better understand its molecular interaction on systemic biochemical parameters and food shelf life and sensory characteristics.
Authors Contributions Statement
RCP - study design, data analyses, writing; PP - supervision, revision; MN- supervision, revision; MLP -supervision, revision; RGT - study design, supervision, final revision
Conflict of Interests
The authors declare that there is no financial or personal relationship that could present a potential conflict of interests.
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