This may explain why high CO2 stored raspberries were perceived as firmer by our sensory panelists. However, firmer fruit tasted less sweet to the sensory panelists. Stec et al. reported that firmer kiwifruit tasted less sweet than softer ones. This finding aligned with the general notion that softer fruit have more ripe fruit characteristics such as sweetness, juiciness and higher aroma intensity . This can explain the negative correlation of juiciness with firmness in our experiment. In addition, storage under high CO2 atmospheres also might have inhibited further ripening of the fruit which would inhibit fruit softening. High CO2 atmospheres also reduced development of leakiness and color darkening. Exposure to CO2 atmospheres can induce fermentative metabolism , likely due to its capacity to disrupt enzyme systems . Elevated CO2 has been reported to induce development of alcoholic flavors in fruit if the concentration is too high for longer times . High CO2 enhances the activity of pyruvate decarboxylase and alcohol dehydrogenase, but reduces activity of alcohol acetyltransferase.
As a result, acetaldehyde and ethanol accumulate and this trigger further production of ethyl esters and reduction of other esters, thus, hydroponic bucket enhancing alcoholic flavor . Larsen suggested that accumulation of ethyl acetate was linked to development of off-flavor in raspberries in some cultivars. High CO2 atmospheres can impact the lipoxygenase pathway which is involved in the formation of aromatic volatile compounds through effects on enzymes or by limiting substrates due to production of fermentative volatiles . In our study, fermentative volatiles were strongly associated with storage in the higher CO2 atmospheres early in storage, but after 10 days of storage, raspberries stored in air also accumulated fermentative volatiles, likely due to over-ripening. In addition, very low O2 atmospheres can contribute to off-flavors. Joles et al. reported that raspberries stored in 3% O2 developed off-flavor because fermentative respiration occurs when O2 levels drop below this critical level . However, this might not be the case for our experiment, because our lowest O2 level was ≥ 6 kPa. The one exception might be the 15 kPa atmosphere. The combination of 15 kPa CO2 with 6 kPa O2 could have resulted in additional impacts on fruit metabolism given the relatively low O2 concentration, resulting in a stronger impact of the 15 kPa atmosphere on fruit quality.
The concentration of individual fermentative volatiles was as much as 1000-fold higher than the aromatic volatiles, and the fermentative volatiles were 4 to 300-fold higher in raspberries stored under 15 kPa CO2 compared to fruit stored in air. Accumulation of acetaldehyde, ethanol and ethyl acetate can contribute to objectionable changes in taste . However, in our experiment, the off flavor sensory score was lower in fruit stored in 15 or 8 kPa atmosphere than in 5 kPa atmosphere, and was moreclosely clustered on the bi-plot with aromatic volatiles than fermentative volatiles. Limonene was most highly correlated with off-flavor. The concentrations of ethanol detected in our raspberry samples appear to be below the corresponding odor threshold of 990 µl /L, . This may explain why our sensory panelists did not sense any off-flavor in raspberries stored in 15 kPa atmosphere, even when fermentative volatile concentrations were significantly higher in those fruit than in fruit stored in lower CO2 atmospheres. The odor threshold is the lowest concentration of a volatile that can be smelled and can vary as much as 106 to 108 among volatiles in fruit . Therefore, the most abundant volatile is not necessarily always the dominant fruit aroma. Alcohols usually have considerably higher threshold values, near 990 µl /L , and therefore contribute less to aroma building than their corresponding aldehydes . The signature raspberry flavor comes from aromatic volatiles, mostly composed of a mixture of ketones and terpenes . α-Ionone and β-ionone are carotenoid-derived aromatic volatiles that are mostly responsible for floral notes in fruit ; these compounds usually intensify as raspberries ripen. Also, α-terpineol, which has a sweet, flowery aroma was found to have a positive correlation with sweetness in our study.
Aromatic volatiles become prominent during fruit ripening and tend to increase towards senescence, ultimately developing the aroma and flavor for the fruit . Guichard reported that in raspberries all the terpenes and sesquiterpenes concentrations significantly increased during ripening with an increase in α -ionone followed by a slight increase in β-ionone. In our research, α-ionone concentration increased over time in air and 5 kPa atmosphere storage, but decreased in raspberries stored in 15 kPa atmosphere, perhaps due to slowing of further ripening. α-Terpineol, limonene, linalool and hexanoic acid also decreased 3-4-fold with increasing CO2 concentration in storage. Linalool, α-terpineol, limonene and hexanoic acid showed increases in concentration with time in air storage. In our study, the aromatic volatiles were mostly associated with raspberries stored in lower CO2 atmospheres . This may be largely due to little or no inhibitory effects on further ripening in these atmospheres. Ripening of fruit is usually accompanied by softening and production of flavor and aroma volatiles Some fruity/floral volatiles are known to enhance the perception of sweetness . Volatiles such α-ionone, linalool, and α-terpineol have a sweet floral aroma . This may explain why we observed a positive correlation of sweetness with these particular volatiles in our study. While raspberries held in higher CO2 atmospheres had lower concentrations of aromatic volatiles, most, but not all, of the differences can be explained by ripening inhibition. High CO2/low O2 atmospheres also restrict enzyme activity, diminishing generation of certain organic volatiles, and reducing the effects of ethylene on CA-stored produce . Off-flavor’s association with low CO2 atmosphere storage may be related to the concentration of limonene which was higher in low CO2 stored raspberries and positively correlated with off-flavor. Elmaci et al. also reported an association of off-flavor with increasing percentage of limonene during storage of mandarins. It is possible that off-flavor was also linked to development of decay or leakiness because the rate of decay and leakiness was higher in fruit stored in low CO2 atmospheres due to the lack of fungistatic conditions or inhibition of metabolism.Raspberries stored in 15 kPa atmosphere maintained better firmness over other atmospheres. However, raspberries stored in 8 kPa atmosphere performed better in sensory evaluations in terms of raspberry flavor, juiciness, and sweetness. Raspberries stored in air or 5 kPa atmosphere lost almost all their sensory quality by 10 days. Selection of modified atmospheres for raspberries should be based on the storage time and desired quality. While 15 kPa atmosphere prolonged shelf life the longest, 8 kPa atmosphere prolonged shelf life to 10 days while maintaining sensory quality. Based on these findings, modified atmosphere conditions can be formulated and applied during transportation to further investigate the impacts on quality under commercial conditions. Also, stackable planters synthesis of volatile compounds and associated gene expression as effected by high CO2 atmospheres would be an interesting area for further exploration.Metabolic syndrome , characterized by concurrence of at least three of five risk factors , is a global epidemic that increases the risk of developing type 2 diabetes and cardiovascular disease . Evidence strongly suggests that chronic low-grade inflammation promoted by complex interactions between an individual’s diet and their gut microbiota is an important factor underlying chronic disorders such as MetS . In addition to adipose tissue, the intestine has emerged as an important source of inflammatory mediators that disrupt insulin signaling leading to whole-body insulin resistance and hyperglycemia . Mice fed a high-fat diet showed increased levels of inflammatory cytokines in ileum, colon, and surrounding mesenteric fat but not in other fat depots, even before the development of obesity . Gnotobiotic or germ-free mice are generally protected from HFD-induced obesity, insulin resistance, and intestinal inflammation; however, when colonized with the microbiota of obese mice, germ-free mice rapidly developed these features of MetS , indicating a critical role for the gut microbiota in the development of metabolic disease. Obesity-related MetS is also associated with chronically higher levels of proinflammatory and gut microbiota– derived lipopolysaccharide in circulation, an event defined as metabolic endotoxemia .
An HFD decreases expression of intestinal tight junction proteins, leading to greater intestinal epithelium permeability and increased leakage of LPS into circulation . Transport of LPSby gut enterocyte-derived chlyomicrons also contributes to the increased levels of systemic LPS . HFD-induced metabolic endotoxemia provided a key concept linking diet-induced changes in the gut microbiota and intestinal barrier function with the chronic low-grade inflammation that ultimately leads to insulin receptor dysfunction, insulin resistance, and glucose intolerance . No single or combination drug therapy has been effective in curtailing the prevalence of MetS, signifying the need for new approaches. Numerous epidemiological, clinical, and preclinical studies indicate that dietary polyphenols can protect against MetS . Grapes and grape products are a major source of dietary polyphenols that have been shown to attenuate many symptoms of obesity-related MetS, including chronic low-grade inflammation . Anthocyanins comprise the most abundant class of polyphenols in Concord grape berries and juice , while monomeric flavan-3-ols and their oligomers, B-type proanthocyanidins , are the major classes contained in grape seeds . We have previously demonstrated that Concord grape polyphenols can be stably sorbed to a protein-rich food matrix and that this complex induces antihyperglycemic effects in HFD-fed mice . ACNs and PACs confer protection against symptoms of MetS despite their limited absorption in circulation . In rodent studies, 88–94% of the administered radiolabeled ACN or PAC compounds were recovered in the gastrointestinal tract and feces . While polyphenols are known to be biotransformed by gut microbiota into simpler phenolic compounds that may be absorbed , the levels and bioactivities of circulating metabolites may not be sufficient to explain the pharmacological effects of polyphenols. More than 75% of PACs in grapes are polymers having more than four degrees of polymerization; however, the ability of microbes to catabolize PACs declines with increased molecular size . For example, the yield of phenolic acids in rat gut was 10% and 7% for catechin monomer and PAC dimers but just 0.7% and 0.5% for PAC trimers and polymers . One possibility is that grape polyphenols act by remodeling the gut microbiota, leading to reduced inflammation and improved metabolic function. The current study provides compelling evidence in support of this hypothesis.Interaction between the gut microbiota and HFD was reported to promote expression of proinflammatory cytokines in intestinal tissues before development of obesity and insulin resistance . Compared with the SPI diet group, mice fed the GP-SPI diet had significantly lower expression of the inflammatory mediators TNFa and iNOS in ileum tissue as well as lower expression of TNFa and IL-6 in colon tissue . Fiaf is secreted by the ileum and functions as a circulating inhibitor of LPL, restraining its ability to import and store fatty acids in peripheral tissues . Elevated Fiaf levels have previously been shown to protect against dietinduced obesity . Compared with SPI diet–fed mice, Fiaf expression was significantly increased by twofold in ileum tissue of the GP-SPI mice, indicating that GP may suppress fatty acid storage . HFD has been reported to reduce expression of tight junction proteins that control the permeability of the intestinal epithelium . Compared with the SPI diet–fed group, mice fed the GP-SPI diet showed a significant increase in occludin gene expression consistent with GP playing a role in maintaining intestinal barrier integrity . Compared with the SPI diet group, jejunal expression of the plaque protein ZO-1 trended higher but was not significantly increased in the GP-SPI group . Proglucagon , expressed in enteroendocrine L cells of the ileal epithelium, is the precursor of glucagon-like peptide 1 and GLP-2 proteins . GLP-1 promotes insulin production and secretion from pancreatic b-cells, while GLP-2 promotes mucosal and gut barrier integrity . The GP-SPI diet group showed significantly higher proglucagon expression in ileum tissue compared with the SPI diet group , suggesting higher GLP-1 and GLP-2 protein levels, which could contribute to the improved glucose tolerance and decreased presence of LPS in serum, respectively. Finally, compared with the SPI diet group, jejunum tissue of the GP-SPI diet group showed significantly lower gene expression of Glut2 , the main GLUT of the small intestine , suggesting an additional mechanismfor the observed improvement in glucose tolerance and lower body weight in these animals.