Metabolite dynamics and phytochemistry of a soy whey-based beverage bio-transformed by water kefir consortium

Food process contaminants: formation, occurrence, risk assessment and mitigation strategies - a review

Thermal treatment of food can lead to the formation of potentially harmful chemicals, known as process contaminants. These are adventitious contaminants that are formed in food during processing and preparation. Various food processing techniques, such as heating, drying, grilling, and fermentation, can generate hazardous chemicals such as acrylamide (AA), advanced glycation end products (AGEs), heterocyclic aromatic amines (HAAs), furan, polycyclic aromatic hydrocarbons (PAHs), N-nitroso compounds (NOCs), monochloropropane diols (MCPD) and their esters (MCPDE) which can be detrimental to human health. Despite efforts to prevent the formation of these compounds during processing, eliminating them is often challenging due to their unknown formation mechanisms. It is critical to identify the potential harm to human health in processed food and understand the mechanisms by which harmful compounds form during processing, as prolonged exposure to these toxic compounds can lead to health problems. Various mitigation strategies, such as the use of diverse pre- and post-processing treatments, product reformulation, additives, variable process conditions, and novel integrated processing techniques, have been proposed to control these food hazards. In this review, we summarize the formation and occurrence, the potential for harm to human health produced by process contaminants in food, and potential mitigation strategies to minimize their impact.

Untargeted metabolomics analysis of probiotic jujube juice and its anti-obesity effects on high-fat-diet-induced obese mice

BACKGROUND

Dietary intervention, including polyphenol consumption, is recognized as an effective strategy to prevent obesity. Although fermented jujube juice (FJJ) with lactic acid bacteria has been shown to be rich in polyphenols and have strong antioxidant properties, little is known about its anti-obesity properties.

RESULTS

Untargeted metabolomics was employed to identify and analyze the differential metabolites between FJJ and raw jujube juice. A total of 431 metabolites belonging to diverse classes and with various functional active ingredients were quantitatively identified. The animal experiments results showed that FJJ administration for 13 weeks significantly inhibited high-fat-diet-induced body and epididymal adipose weight gain, and improved the serum lipid parameters in obese mice. Additionally, DNA-sequencing results revealed that FJJ treatment increased Akkermansia abundance in the gut and changed the composition of fecal microbiota by decreasing the Firmicutes/Bacteroidota ratio and Helicobacter pylori abundance.

CONCLUSION

These findings suggest that FJJ contributes to regulating lipid accumulation and gut microbiota composition in high-fat-diet-induced obese mice, which helps to prevent obesity. Hence, FJJ has the potential to be a beneficial beverage for controlling obesity. © 2024 Society of Chemical Industry.

Comprehensive analysis of the dynamic changes of volatile and non-volatile metabolites in boletus edulis during processing by HS-SPME-GC-MS and UPLC-MS/MS analysis

In order to investigate the dynamic changes of flavor compounds, Ultra Performance Liquid Chromatography Tandem Mass Spectrometry (UPLC-MS/MS) combined with Headspace Solid Phase Microextraction Gas Chromatography Mass Spectrometry (HS-SPME-GC-MS) was used to detect the metabolites in different drying processes. A total of 80 volatile compounds and 1319 non-volatile compounds were identified. The trend in the changes of C-8 compounds and sulfur-containing compounds were generally consistent with the trend of key enzyme activities. 479 differential metabolites were identified and revealed that metabolic profiles of compounds in Boletus edulis were altered with increased organic acids and derivatives and lipids and lipid-like molecules. Fatty acids and amino acids were transformed into volatile compounds under the action of enzymes, which played a significant role in the formation of the distinctive flavor of Boletus edulis. Our study provided a theoretical support for fully comprehending the formation mechanism of flavor from Boletus edulis during drying processes.

Exploring peptidomes of by-products generated during chhurpi production using Lactobacillus delbrueckii WS4 for identification of novel bioactive peptides

The considerable value of whey is evident from its significant potential applications and contributions to the functional food and nutraceutical market. The by-products were individually obtained during functional chhurpi and novel soy chhurpi cheese production using defined lactic acid bacterial strains of Sikkim Himalaya's traditional chhurpi. Hydrolysis of substrate proteins by starter proteinases resulted in a comparable peptide content in whey and soy whey which was associated with antioxidant and ACE inhibition potential. Peptidome analysis of Lactobacillus delbrueckii WS4 whey and soy whey revealed the presence of several bioactive peptides including the multifunctional peptides PVVVPPFLQPE and YQEPVLGPVRGPFPIIV. In silico analyses predicted the antihypertensive potential of whey and soy whey peptides with strong binding affinity for ACE active sites. QSAR models predicted the highest ACE inhibition potential (IC50) for the β-casein-derived decapeptide PVRGPFPIIV (0.95 μM) and the Kunitz trypsin inhibitor protein-derived nonapeptide KNKPLVVQF (16.64 μM). Chhurpi whey and soy whey can be explored as a valuable source of diverse and novel bioactive peptides for applications in designer functional foods development.

Widely targeted metabolomics-based analysis of the impact of L. plantarum and L. paracasei fermentation on rosa roxburghii Tratt juice

Rosa roxburghii Tratt fruits (RRT) exhibit extremely high nutritional and medicinal properties due to its unique phytochemical composition. Probiotic fermentation is a common method of processing fruits. Variations in the non-volatile metabolites and bioactivities of RRT juice caused by different lactobacilli are not well understood. Therefore, we aimed to profile the non-volatile components and investigate the impact of L. plantarum fermentation (LP) and L. paracasei fermentation (LC) on RRT juice (the control, CG). There were both similarities and differences in the effects of LP and LC on RRT juice. Both of the two strains significantly increased the content of total phenolic, total flavonoid, and some bioactive compounds such as 2-hydroxyisocaproic acid, hydroxytyrosol and indole-3-lactic acid in RRT juice. Interestingly, compared with L. paracasei, L. plantarum showed better ability to increase the content of total phenolic and these valuable compounds, as well as certain bioactivities. The antioxidant capacity and α-glucosidase inhibitory activity of RRT juice were notably enhanced after the fermentations, whereas its cholesterol esterase inhibitory activity was reduced significantly. Moreover, a total of 1466 metabolites were identified in the unfermented and fermented RRT juices. There were 278, 251 and 134 differential metabolites in LP vs CG, LC vs CG, LC vs LP, respectively, most of which were upregulated. The key differential metabolites were classified into amino acids and their derivatives, organic acids, nucleotides and their analogues, phenolic acids and alkaloids, which can serve as potential markers for authentication and discrimination between the unfermented and lactobacilli fermented RRT juice samples. The KEGG enrichment analysis uncovered that metabolic pathways, purine metabolism, nucleotide metabolism and ABC transporters contributed mainly to the formation of unique composition of fermented RRT juice. These results provide good coverage of the metabolome of RRT juice in both unfermented and fermented forms and also provide a reference for future research on the processing of RRT or other fruits.

Effects of different strains fermentation on the sensory and nutritional properties of soy-based yogurt

BACKGROUND

Soy protein is the only full plant protein that is comparable to animal protein. Soy whey contains a variety of nutrients including isoflavones and oligosaccharides. Additionally, different strains have profound influence on functional metabolism. Most prospective studies used mixed strain fermented yogurt, but few studies on single strain fermented yogurt. The present study aimed to ferment nutritious and healthy soy-based yogurt using a single strain and to provide technical support for the reuse of soy whey.

RESULTS

Streptococcus salivarius subsp. thermophilus HCS07-002, Bifidobacterium animalis subsp. lactis HCS04-001 and Lactiplantibacillus plantarum HCS03-084 had strong growth and metabolic activities. Bifidobacterium animalis subsp. lactis HCS04-001 fermentation raised the texture and rheological properties of yogurt and enriched the flavor substances in the yogurt, resulting in a good quality soy-based yogurt. The contents of arginine, glutamic and proline were greatly affected by the fermentation of different strains. The fermentation of B. animalis subsp. lactis HCS04-001 was beneficial to the enrichment of soy isoflavone and equol in yogurt, and enhanced antioxidant activity of yogurt.

CONCLUSION

Our findings indicated that yogurt fermented with B. animalis subsp. lactis HCS04-001 had the best sensory and nutritional properties, which provides valuable insights into the selection of suitable strains to improve the quality and nutritional value of plant yogurt. © 2023 Society of Chemical Industry.

Bibliometric analysis of water kefir and milk kefir in probiotic foods from 2013 to 2022: A critical review of recent applications and prospects

Although milk kefir and water kefir have different physical, chemical and microbiological characteristics, several microbial species that make up kefir stand out with probiotic functions. Furthermore, because it is suitable for a variety of substrates, kefir and the species of probiotic microorganisms that make it up are seen as a promising alternative in the development of probiotic and health-promoting foods. The aim of this study was to carry out a bibliometric analysis of water kefir and milk kefir in probiotic foods and to critically analyze recent applications and prospects. Using the Scopus database, 202 documents published between 2013 and 2022 were identified and submitted to bibliometric analysis using the VOSviewer software. Regarding recent applications, 107 documents published between 2021 and June 2023 were identified. It was observed that, in the literature consulted, no study used bibliometric analysis to evaluate the use of water kefir and milk kefir in probiotic foods. Due to the presence of probiotic species, kefir has been listed as an alternative for the production of new probiotic food matrices that are beneficial to health. Recent applications show kefir's potential in the development of probiotic products based on fruit and fruit juice, whey beverages, fermented milks and derivatives, and alcoholic beverages such as beers.

Multi-omics characterization of the microbial populations and chemical space composition of a water kefir fermentation

In recent years, the popularity of fermented foods has strongly increased based on their proven health benefits and the adoption of new trends among consumers. One of these health-promoting products is water kefir, which is a fermented sugary beverage based on kefir grains (symbiotic colonies of yeast, lactic acid and acetic acid bacteria). According to previous knowledge and the uniqueness of each water kefir fermentation, the following project aimed to explore the microbial and chemical composition of a water kefir fermentation and its microbial consortium, through the integration of culture-dependent methods, compositional metagenomics, and untargeted metabolomics. These methods were applied in two types of samples: fermentation grains (inoculum) and fermentation samples collected at different time points. A strains culture collection of ∼90 strains was established by means of culture-dependent methods, mainly consisting of individuals of Pichia membranifaciens, Acetobacter orientalis, Lentilactobacillus hilgardii, Lacticaseibacillus paracasei, Acetobacter pomorum, Lentilactobacillus buchneri, Pichia kudriavzevii, Acetobacter pasteurianus, Schleiferilactobacillus harbinensis, and Kazachstania exigua, which can be further studied for their use in synthetic consortia formulation. In addition, metabarcoding of each fermentation time was done by 16S and ITS sequencing for bacteria and yeast, respectively. The results show strong population shifts of the microbial community during the fermentation time course, with an enrichment of microbial groups after 72 h of fermentation. Metataxonomics results revealed Lactobacillus and Acetobacter as the dominant genera for lactic acid and acetic acid bacteria, whereas, for yeast, P. membranifaciens was the dominant species. In addition, correlation and systematic analyses of microbial growth patterns and metabolite richness allowed the recognition of metabolic enrichment points between 72 and 96 h and correlation between microbial groups and metabolite abundance (e.g., Bile acid conjugates and Acetobacter tropicalis). Metabolomic analysis also evidenced the production of bioactive compounds in this fermented matrix, which have been associated with biological activities, including antimicrobial and antioxidant. Interestingly, the chemical family of Isoschaftosides (C-glycosyl flavonoids) was also found, representing an important finding since this compound, with hepatoprotective and anti-inflammatory activity, had not been previously reported in this matrix. We conclude that the integration of microbial biodiversity, cultured species, and chemical data enables the identification of relevant microbial population patterns and the detection of specific points of enrichment during the fermentation process of a food matrix, which enables the future design of synthetic microbial consortia, which can be used as targeted probiotics for digestive and metabolic health.

The metabolome and bacterial composition of high-moisture Italian ryegrass silage inoculated with lactic acid bacteria during ensiling

BACKGROUND

With its high nutritional value and productivity, Italian ryegrass as a biomass feedstock constantly supplies rumen degradable nitrogen and digestible fiber to ruminants. However, biofuel production is easily reduced during ensiling due to the high-moisture content of Italian ryegrass, leading to economic losses. Lactic acid bacteria inoculants could improve lignocellulosic degradation and fermentation quality and decrease dry matter loss during the bioprocessing of silage. Therefore, this study analyzed the effects of Lactobacillus buchneri TSy1-3 (HE), Lactobacillus rhamnosus BDy3-10 (HO), and the combination of HE and HO (M) on fermentation quality, bacterial community and metabolome in high-moisture Italian ryegrass silage during ensiling.

RESULTS

The results showed that the pH value was significantly lower in the HO groups than in the other treatments at the end of ensiling, and the dry matter and acetic acid contents were significantly higher in the HO group than in the other inoculated groups. All inoculants decreased the diversity of the bacterial community and significantly increased the relative abundance of Lactobacillus. Inoculation with HO significantly improved the concentrations of organic acids, dipeptides, ferulic acid, apigenin, and laricitrin. Compared with Lactobacillus buchneri TSy1-3 (HE), HO significantly upregulated the flavonoid compounds in the flavone and flavonol biosynthesis pathway.

CONCLUSIONS

Overall, these findings suggest that inoculation with HO was beneficial for the development of Italian ryegrass as a biomass feedstock, improving fermentation quality, accelerating changes in bacterial community composition and increasing biofunctional metabolites in high-moisture Italian ryegrass silage.

Study on the fermentation effect of Rhodotorula glutinis utilizing tofu whey wastewater and the influence of Rhodotorula glutinis on laying hens

Background

Tofu whey wastewater (TWW) is the wastewater of tofu processing, which is rich in a variety of nutrients. Rhodotorula glutinis can make full use of TWW to ferment and reproduce yeast cells, produce carotenoids and other nutrients, improve the utilization value of TWW, and reduce environmental pollution and resource waste.

Methods

In this study, the nutrient composition changes of TWW treated by Rhodotorula glutinis were analyzed to reformulate TWW medium, and the optimal composition and proportion of TWW medium that can improve the biomass and carotenoids production of Rhodotorula glutinis were explored. Meanwhile, the Rhodotorula glutinis liquid obtained under these conditions was used to prepare biological feed for laying hens, and the effect of Rhodotorula glutinis growing on TWW as substrate on laying performance and egg quality of laying hens were verified.

Results

The results showed that the zinc content of TWW after Rhodotorula glutinis fermentation increased by 62.30%, the phosphorus content decreased by 42.31%, and the contents of vitamin B1, B2 and B6 increased to varying degrees. The optimal fermentation conditions of Rhodotorula glutinis in the TWW medium were as follow: the initial pH was 6.40, the amount of soybean oil, glucose and zinc ions was 0.80 ml/L, 16.32 g/L, and 20.52 mg/L, respectively. Under this condition, the biomass of Rhodotorula glutinis reached 2.23 g/L, the carotenoids production was 832.86 μg/g, and the number of effective viable yeast count was 7.08 × 10⁷ cfu/ml. In addition, the laying performance and egg quality of laying hens fed Rhodotorula glutinis biological feed were improved.

Discussion

In this study, we analyzed the composition changes of TWW, optimized the fermentation conditions of Rhodotorula glutinis in TWW medium, explored the influence of Rhodotorula glutinis utilizing TWW on laying layers, and provided a new idea for the efficient utilization of TWW.

Evaluating the effect of lactic acid bacteria fermentation on quality, aroma, and metabolites of chickpea milk

Legumes are an attractive choice for developing new products since their health benefits. Fermentation can effectively improve the quality of soymilk. This study evaluated the impact of Lactobacillus plantarum fermentation on the physicochemical parameters, vitamins, organic acids, aroma substances, and metabolites of chickpea milk. The lactic acid bacteria (LAB) fermentation improved the color, antioxidant properties, total phenolic content, total flavonoid content, lactic acid content, and vitamin B6 content of raw juice. In total, 77 aroma substances were identified in chickpea milk by headspace solid-phase microextraction with gas chromatography/mass spectrometry (HS-SPME-GC-MS); 43 of the 77 aroma substances increased after the LAB fermentation with a significant decrease in beany flavor content (p < 0.05), improving the flavor of the soymilk product. Also, a total of 218 metabolites were determined in chickpea milk using non-targeted metabolomics techniques, including 51 differentially metabolites (28 up-regulated and 23 down-regulated; p < 0.05). These metabolites participated in multiple metabolic pathways during the LAB fermentation, ultimately improving the functional and antioxidant properties of fermented soymilk. Overall, LAB fermentation can improve the flavor, nutritional, and functional value of chickpea milk accelerating its consumer acceptance and development as an animal milk alternative.

Changes and Driving Mechanism of Microbial Community Structure during Paocai Fermentation

Fermentation of paocai is a dynamic process of the microbial community structure, and the interaction between community structure and physicochemical factors endows paocai with unique taste and flavor. The study of bacterial and fungal community structure changes and the driving mechanism of physicochemical factors induced changes in community structure, showing that Pseudomonas belonging to Proteobacteria and Lactobacillus belonging to Firmicutes were the dominant bacteria in the process of paocai fermentation. The correlation analysis of physicochemical factors with bacterial community showed that titratable acid was significantly positively correlated with Lactobacillus and negatively correlated with Pseudomonas, while nitrite was the opposite. Redundancy analysis (RDA) showed that pH was positively correlated with the bacterial community in the early fermentation stage, amino acid nitrogen was positively correlated with the bacterial community in the middle fermentation stage, and titratable acid was positively correlated with the bacterial community in the late fermentation stage. Variance partitioning analysis (VPA) showed that environmental factors, pH and metabolites, were the main driving forces of bacterial community diversity, which jointly explained 32.02% of the bacterial community structure variation. To study the glucolysis and nitrogen metabolism in the process of paocai fermentation, we found that in the early stage of the fermentation, the nitrite reductase enzyme of Pseudomonas activity was high, with high nitrite content in the prophase, but by the end of fermentation, lactic acid bacteria rapidly increased, the content of L−lactic acid through the glycolysis pathway, making paocai fermentation environment become acidic, then Pseudomonas decreased. Ascomycota and Basidiomycota were the main phylum fungi in the fermentation process. RDA analysis showed that the fungal community was positively correlated with pH, nitrite, and soluble protein at the early fermentation stage, amino acid nitrogen was positively correlated with the fungal community at the middle fermentation stage, titratable acid and reducing sugar were positively correlated with the fungal community at the late fermentation stage. VPA analysis showed that metabolites were the main driving force of fungal community diversity and accounted for 45.58% of fungal community diversity. These results had a certain guiding significance for the production and preservation of naturally fermented paocai.

Effects of different combinations of probiotics on rheology, microstructure, and moisture distribution of soy materials-based yogurt

The effects of different probiotics on the texture, rheological properties, microstructure, and water distribution of yogurt fermented with soy powder, soy isolate protein powder, soy umbilical powder, and soy whey as the main raw materials were studied. The soy materials-based yogurt fermented by Danisco mixed probiotic reached the end of fermentation after 4 h, which significantly shortened the fermentation time compared with other probiotic combinations. The fermentation with Danisco mixed probiotic and Kefir mixed probiotic respectively resulted in good texture and a denser and more homogeneous microstructure, which was consistent with the sensory evaluation results. Both fermentations had a high water holding capacity of 90.92% and 78.30%, respectively, in agreement with the results of moisture distribution tests. However, the elastic and viscous behaviors were weaker at certain shear frequency. This study achieved a high value-added utilization of soy whey and the development of a new soy materials-based yogurt that met the consumption needs of people with lactose intolerance and high cholesterol. PRACTICAL APPLICATION: In this study, high value-added utilization of soy whey was realized, which solved the problems of resource waste and environmental pollution. Meanwhile, the research and development of soy materials-based yogurt provided another nutritional and healthy consumption demand for lactose intolerant people.

Lacticaseibacillus rhamnosus GG Survival and Quality Parameters in Kefir Produced from Kefir Grains and Natural Kefir Starter Culture

The study aimed to determine the effect of starter cultures (kefir grains and natural kefir starter culture without grains) on Lacticaseibacillus rhamnosus GG (LGG) survival and on the quality characteristics of kefir. To this end, the viability of probiotic L. rhamnosus GG strain and the rheological properties and quality parameters of kefir beverages were tested during storage over 21 days at 4 °C. The final LGG counts were 7.71 and 7.55 log cfu/mL in natural kefir starter culture and kefir grain, respectively. When prepared with probiotic bacteria, the syneresis values of kefir prepared using natural kefir starter culture was significantly lower (p < 0.05) than that of kefir made using grains. However, the viscosity indices, hysteresis loop, and dynamic moduli were similar between kefir made with natural kefir starter culture and other kefir formulations (p > 0.05). Moreover, all samples showed shear-thinning behavior. The flavor scores for kefir prepared using natural kefir starter culture were significantly higher than for the other samples (p < 0.05), but overall acceptability was similar at the 10-day assessment across both starters (with and without grain) after the addition of probiotic bacteria (p > 0.05). Overall, the results indicate that natural kefir starter culture could be a potential probiotic carrier.

Water kefir: Factors affecting grain growth and health-promoting properties of the fermented beverage

Nowadays, the interest in the consumption of healthy foods has increased as well as the homemade preparation of artisanal fermented product. Water kefir is an ancient drink of uncertain origin, which has been passed down from generation to generation and is currently consumed practically all over the world. Considering the recent and extensive updates published on sugary kefir, this work aims to shed light on the scientific works that have been published so far in relation to this complex ecosystem. We focused our review evaluating the factors that affect the beverage microbial and chemical composition that are responsible for the health attribute of water kefir as well as the grain growth. The microbial ecosystem that constitutes the grains and the fermented consumed beverage can vary according to the fermentation conditions (time and temperature) and especially with the use of different substrates (source of sugars, additives as fruits and molasses). In this sense, the populations of microorganisms in the beverage as well as the metabolites that they produce varies and in consequence their health properties. Otherwise, the knowledge of the variables affecting grain growth are also discussed for its relevance in maintenance of the starter biomass as well as the use of dextran for technological application.

Metabolomics as a Tool to Study Underused Soy Parts: In Search of Bioactive Compounds

The valorization of agri-food by-products is essential from both economic and sustainability perspectives. The large quantity of such materials causes problems for the environment; however, they can also generate new valuable ingredients and products which promote beneficial effects on human health. It is estimated that soybean production, the major oilseed crop worldwide, will leave about 597 million metric tons of branches, leaves, pods, and roots on the ground post-harvesting in 2020/21. An alternative for the use of soy-related by-products arises from the several bioactive compounds found in this plant. Metabolomics studies have already identified isoflavonoids, saponins, and organic and fatty acids, among other metabolites, in all soy organs. The present review aims to show the application of metabolomics for identifying high-added-value compounds in underused parts of the soy plant, listing the main bioactive metabolites identified up to now, as well as the factors affecting their production.

Kefir and Its Biological Activities

Kefir is a fermented beverage with renowned probiotics that coexist in symbiotic association with other microorganisms in kefir grains. This beverage consumption is associated with a wide array of nutraceutical benefits, including anti-inflammatory, anti-oxidative, anti-cancer, anti-microbial, anti-diabetic, anti-hypertensive, and anti-hypercholesterolemic effects. Moreover, kefir can be adapted into different substrates which allow the production of new functional beverages to provide product diversification. Being safe and inexpensive, there is an immense global interest in kefir’s nutritional potential. Due to their promising benefits, kefir and kefir-like products have a great prospect for commercialization. This manuscript reviews the therapeutic aspects of kefir to date, and potential applications of kefir products in the health and food industries, along with the limitations. The literature reviewed here demonstrates that there is a growing demand for kefir as a functional food owing to a number of health-promoting properties.

Neuroprotective Potency of Tofu Bio-Processed Using Actinomucor elegans against Hypoxic Injury Induced by Cobalt Chloride in PC12 Cells

Fermented soybean products have attracted great attention due to their health benefits. In the present study, the hypoxia-injured PC12 cells induced by cobalt chloride (CoCl₂) were used to evaluate the neuroprotective potency of tofu fermented by Actinomucor elegans (FT). Results indicated that FT exhibited higher phenolic content and antioxidant activity than tofu. Moreover, most soybean isoflavone glycosides were hydrolyzed into their corresponding aglycones during fermentation. FT demonstrated a significant protective effect on PC12 cells against hypoxic injury by maintaining cell viability, reducing lactic dehydrogenase leakage, and inhibiting oxidative stress. The cell apoptosis was significantly attenuated by the FT through down-regulation of caspase-3, caspases-8, caspase-9, and Bax, and up-regulation of Bcl-2 and Bcl-xL. S-phase cell arrest was significantly inhibited by the FT through increasing cyclin A and decreasing the p21 protein level. Furthermore, treatment with the FT activated autophagy, indicating that autophagy possibly acted as a survival mechanism against CoCl₂-induced injury. Overall, FT offered a potential protective effect on nerve cells in vitro against hypoxic damage.