Acetaldehyde: an intermediate in the formation of ethanol from glucose by lactic acid bacteria

Metabolomic Markers of Storage Temperature and Time in Pasteurized Milk

The current date labeling system for pasteurized milk is based on the predicted growth of spoilage microorganisms, but inherent inaccuracies and the inability to account for environmental factors (e.g., temperature fluctuations) contribute to household and retail food waste. Improved shelf-life estimation can be achieved by monitoring milk quality in real-time. In this study, we identify and quantify metabolites changing over storage temperature and time, the main factors affecting milk stability. Pasteurized 2% fat milk was stored at 4, 10, 15, and 20 °C. Metabolite change was analyzed using untargeted and targeted nuclear magnetic resonance (NMR) metabolomics approaches. Several metabolites correlated significantly to storage time and temperature. Citric acid decreased linearly over time at a temperature-dependent rate. Ethanol, formic acid, acetic acid, lactic acid, and succinic acid increased non-linearly after an initial period of minimal increase. Butyric acid exhibited strong inverse temperature dependencies. This study provides the first analysis of the effect of time and temperature on the concentration of key metabolites during milk storage. Candidate molecules for shelf-life monitoring have been identified, and the results improve our understanding of molecular changes during milk storage. These results will inform the development of real-time shelf-life indicators for milk, helping to reduce milk waste.

Effects of Ensiling Oxytropis glabra with Whole-Plant Corn at Different Proportions on Fermentation Quality, Alkaloid Swainsonine Content, and Lactic Acid Bacteria Populations

Simple Summary

In Inner Mongolia, developing innovative forages is an indispensable way to ease the shortage of animal feed. Oxytropis glabra (OG) has become a dominant population, with a high nature yield in the degraded grassland of Inner Mongolia. As a typical legume, it is rich in amino acids and trace elements, so using OG to feed livestock is a valuable strategy. However, it contains swainsonine (SW), which hinders the utilization of OG. This study was conducted to evaluate silage fermentation and SW removal from OG ensiled with whole-plant corn at different proportions, and the result showed ensiling a mixture of OG and corn could be a valuable approach for producing livestock feed, as it had a positive effect on fermentation quality and SW detoxification; the optimal ratio was 1:1. In the silages, Lactobacillus plantarum was the most common microorganism present in all mixture silages, and Lactobacillus amylovorus and Lactobacillusbrevis were prevalent at lower ratios of corn to OG. All representative strains were able to remove SW from OG fermentation, and the strains of L. amylovorus and L. plantarum had a higher SW removal rate. These mixtures of silages could make OG useable as a feed source in ruminant production.

Abstract

Oxytropis glabra (OG) is a leguminous forage that is potentially valuable for solving the shortage of feed for livestock production, while, in large quantities, it may be toxic because of its swainsonine (SW) content. In this study, OG was ensiled with whole-plant corn (Zea mays L.) at 10:0, 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, and 0:10 ratios on a fresh matter basis, and, after 60 d of ensiling, the chemical composition, fermentation characteristic, SW removal rate, lactic acid bacteria (LAB) populations, and their capabilities for SW removal were analyzed. As the proportion of corn in the silage increased, the pH, as well as the propionic acid, ammonia-N, dry matter, crude protein, and SW contents, decreased linearly, while the lactic acid, neutral detergent fiber, and residual water-soluble carbohydrate contents increased linearly. Lactobacillus plantarum was the most common microorganism present in all mixture silages. Lactobacillus amylovorus and Lactobacillusbrevis were prevalent at lower ratios of corn to OG. Meanwhile, the LAB strains belong to L. amylovorus and L. plantarum had a higher SW removal rate. Our results suggested that ensiling OG with whole-plant corn improves fermentation and decreases SW content, and that 5:5 is the optimal ratio, so this type of mixed silage could make OG useable for ruminant production.

Beverage and Food Fragrance Biotechnology, Novel Applications, Sensory and Sensor Techniques: An Overview

Flavours and fragrances are especially important for the beverage and food industries. Biosynthesis or extraction are the two main ways to obtain these important compounds that have many different chemical structures. Consequently, the search for new compounds is challenging for academic and industrial investigation. This overview aims to present the current state of art of beverage fragrance biotechnology, including recent advances in sensory and sensor methodologies and statistical techniques for data analysis. An overview of all the recent findings in beverage and food fragrance biotechnology, including those obtained from natural sources by extraction processes (natural plants as an important source of flavours) or using enzymatic precursor (hydrolytic enzymes), and those obtained by de novo synthesis (microorganisms' respiration/fermentation of simple substrates such as glucose and sucrose), are reviewed. Recent advances have been made in what concerns "beverage fragrances construction" as also in their application products. Moreover, novel sensory and sensor methodologies, primarily used for fragrances quality evaluation, have been developed, as have statistical techniques for sensory and sensors data treatments, allowing a rapid and objective analysis.

Protective effects of Lactococcus chungangensis CAU 28 on alcohol-metabolizing enzyme activity in rats

In this study, we investigated the beneficial effects of Lactococcus chungangensis CAU 28, a bacterial strain of nondairy origin, on alcohol metabolism in rats treated with ethanol, focusing on alcohol elimination and prevention of damage and comparing the effects with those observed for Lactococcus lactis ssp. lactis ATCC 19435. Male Sprague-Dawley rats were orally administered 20% ethanol and 3 substrates (freeze-dried cells, cream cheese, and yogurt) containing Lc. chungangensis CAU 28 or Lc. lactis ssp. lactis ATCC 19435, which were provided 1 h before or 1 h after ethanol ingestion. Blood samples were collected from the tail veins of the rats at 1, 3, 6, 12, and 24 h after ingestion of ethanol, Lc. chungangensis CAU 28 substrate, or Lc. lactis ssp. lactis ATCC 19435 substrate. Alcohol and acetaldehyde concentrations in the Lc. chungangensis CAU 28 substrate-treated rats were significantly reduced in a time-dependent manner compared with those in the Lc. lactis ssp. lactis ATCC 19435 substrate-treated rats. Among the experimental groups, treatment with cream cheese before ingestion of 20% ethanol was found to be the most effective method for reducing both alcohol and acetaldehyde levels in the blood. Alanine aminotransferase and aspartate aminotransferase activities in the Lc. chungangensis CAU 28 substrate-treated rats were significantly lower than those in the positive controls. Moreover, in the Lc. chungangensis CAU 28 cream cheese-treated group, rats showed a reduction of liver enzymes by up to 60%, with good effectiveness observed for both pre- and post-ethanol ingestion. These results suggested that intake of lactic acid bacteria, particularly in Lc. chungangensis CAU 28-supplemented dairy products, may reduce blood alcohol and acetaldehyde concentrations, thereby mitigating acute alcohol-induced hepatotoxicity by altering alcohol-metabolizing enzyme activities.

Microbial Quality of and Biochemical Changes in Fresh Soft, Acid-Curd Xinotyri Cheese Made from Raw or Pasteurized Goat's Milk

The microbiological quality of and changes in the main physicochemical parameters, together with the evolution of proteolysis, lipolysis and volatile profiles of soft Xinotyri, a traditional Greek acid-curd cheese (pH≈4.4, moisture 65%, salt 1%) made from raw (RMC) or pasteurized (PMC) goat's milk without starters, were evaluated during aerobic storage at 4 ^oC for 60 days. No statistically significant differences between the total nitrogen (TN) and nitrogen fraction (% of TN) contents, the degradation of intact α_s- or β-caseins, total free amino acid (FAA) contents, and the ratio of hydrophilic and hydrophobic peptides in the water-soluble fraction of RMC and PMC were found. Threonine, alanine and lysine were the principal FAAs. Oleic, palmitic, capric and caprylic acids, and ethyl hexonate, ethyl octanoate, ethyl decanoate, ethanol, 3-methyl butanol, phenyl ethyl alcohol and acetone were the most abundant free fatty acids and volatile compounds, respectively. Cheese lipolysis evolved slowly at 4 ^oC, and milk pasteurization had no significant effect on it. Mesophilic lactic acid bacteria (LAB) were predominant in fresh cheese samples. PMC samples had significantly lower levels of enterococci and enterobacteria than RMC samples, while yeasts grew at similar levels during storage at 4 ^oC. All cheese samples (25 g) were free of Salmonella and Listeria monocytogenes. Coagulase-
-positive staphylococci exceeded the 5-log safety threshold in fresh RMC samples, whereas they were suppressed (<100 CFU/g) in all PMC samples. Consequently, pasteurization of raw goat milk's and utilization of commercially defined or natural mesophilic LAB starters are recommended for standardizing the biochemical, microbial and safety qualities of fresh soft Xinotyri cheese.

Role of milk protein-based products in some quality attributes of goat milk yogurt

Goat milk yogurts were manufactured with the fortification of 2% (wt/vol) skim goat milk powder (SGMP), sodium caseinate (NaCn), whey protein concentrate (WPC), whey protein isolate (WPI), or yogurt texture improver (YTI). Yogurts were characterized based on compositional, microbiological, and textural properties; volatile flavor components (with gas chromatography); and sensory analyses during storage (21d at 5 °C). Compared with goat milk yogurt made by using SGMP, the other goat milk yogurt variants had higher protein content and lower acidity values. Goat milk yogurts with NaCn and WPC, in particular, had better physical characteristics. Using WPI caused the hardest structure in yogurt, leading to higher syneresis values. Acetaldehyde and ethanol formation increased with the incorporation of WPI, WPC, or YTI to yogurt milk. The tyrosine value especially was higher in the samples with NaCn and YTI than in the samples with WPC and WPI. Counts of Streptococcus thermophilus were higher than the counts of Lactobacillus delbrueckii ssp. bulgaricus, possibly due to a stimulatory effect of milk protein-based ingredients other than SGMP on the growth of S. thermophilus. Yogurt with NaCn was the best accepted among the yogurts. For the parameters used, milk protein-based products such as NaCn or WPC have promising features as suitable ingredients for goat milk yogurt manufacture.

Metabolic engineering of lactic acid bacteria for the production of industrially important compounds

Lactic acid bacteria (LAB) are receiving increased attention for use as cell factories for the production of metabolites with wide use by the food and pharmaceutical industries. The availability of efficient tools for genetic modification of LAB during the past decade permitted the application of metabolic engineering strategies at the levels of both the primary and the more complex secondary metabolism. The recent developments in the area with a focus on the production of industrially important metabolites will be discussed in this review.

Effect of oxidoreduction potential on aroma biosynthesis by lactic acid bacteria in nonfat yogurt

The aim of this study was to investigate the effect of oxidoreduction potential (Eh) on the biosynthesis of aroma compounds by lactic acid bacteria in non-fat yogurt. The study was done with yogurts fermented by Lactobacillus bulgaricus and Streptococcus thermophilus. The Eh was modified by the application of different gaseous conditions (air, nitrogen, and nitrogen/hydrogen). Acetaldehyde, dimethyl sulfide, diacetyl, and pentane-2,3-dione, as the major endogenous odorant compounds of yogurt, were chosen as tracers for the biosynthesis of aroma compounds by lactic acid bacteria. Oxidative conditions favored the production of acetaldehyde, dimethyl sulfide, and diketones (diacetyl and pentane-2,3-dione). The Eh of the medium influences aroma production in yogurt by modifying the metabolic pathways of Lb. bulgaricus and Strep. thermophilus. The use of Eh as a control parameter during yogurt production could permit the control of aroma formation.

The uptake of amino acids and peptides byStreptococcus lactis

Peptide and amino acid transport inStreptococcus lactiswas both energy-and temperature-dependent. Separate systems were present for the transport of amino acids, dipeptides and oligopeptides as well as for individual amino acids. Both tri- and tetra-peptides were readily transported while an apparent size restriction at the point of uptake resulted in a relatively low rate of uptake of the pentapeptide. The effects of pH, temperature and salt concentration on the uptake of amino acids and peptides were studied. Exchange reactions played a significant role in the uptake of both peptides and amino acids.

Fermentation of goat's milk by two DL-type mixed strain starters

Variations in the biochemical performance of 2 DL-type mixed strain starters CH 01 and FDn grown in a sample of skimmed goat's milk I (obtained 2–3 weeks after parturition) and of goat's milk II (8 months after parturition) were studied, and results compared with those of the cultures grown in skimmed cow's milk. The initial levels of citrate differed in the 3 milks, with goat's milk II containing 0·7 mg/ml, goat's milk I 1·4 mg/ml and cow's milk 1·8 mg/ml. The production of diacetyl, α-acetolactic acid and acetoin was very low in goat's milk II. In goat's milk I the production of these compounds was higher, but still considerably lower than in cow's milk. Production of ethanol (EtOH) was similar in the 3 milks incubated with starter CH 01, but with FDn the production of EtOH was distinctly higher in goat's milk II. The total amount of CO2produced corresponded with the initial level of citrate; consequently the highest production was found in cow's milk and the lowest in goat's milk II. The use of different starters for fermented products made from goat's milk in early and late lactation is suggested.

Effect of threonine and glycine on acetaldehyde formation in goats' milk yogurt

The effect of threonine on the biochemical activity of yogurt starter was investigated by the addition of L-threonine to cows' and goats' milk in two concentrations (5 and 10mg/100g). A separate experiment was conducted to investigate the effect of free glycine on the formation of acetaldehyde from yogurt starter. Increasing amounts of glycine (1, 2, 3, 4 and 5 mg/100 g) were added to samples of cows' milk. Addition of threonine to milk samples did not influence acid production, total bacterial growth, or the balance between cocci and rods in the yogurts. Raised levels of threonine resulted in increased production of acetaldehyde. This increase was more pronounced in goats' milk than in cows' milk. The production of acetaldehyde in cows' milk without added threonine was significantly greater than in goats' milk without addition of threonine. Addition of threonine did not influence the formation of diacetyl and α-acetolactic acid, whereas production of acetoin in both milks was reduced. No marked difference in the formation of C02 was observed in samples with or without addition of threonine. Addition of free glycine to cows' milk had a marked negative effect on the production of acetaldehyde by yogurt starter. Addition of 3 mg glycine/100 g cows' milk gave approximately the same amount of acetaldehyde as obtained in goats' milk yogurt. It is suggested that the relatively low amount of acetaldehyde usually observed in goats' milk yogurt is caused by a feed-back inhibition of threonine aldolase produced by the relatively high amount of free glycine present in goats' milk.

Acetate utilization in Lactococcus lactis deficient in lactate dehydrogenase: a rescue pathway for maintaining redox balance

Acetate was shown to improve glucose fermentation in Lactococcus lactis deficient in lactate dehydrogenase. 13C and 1H nuclear magnetic resonance studies using [2-13C]glucose and [2-(13)C]acetate as substrates demonstrated that acetate was exclusively converted to ethanol. This novel pathway provides an alternative route for NAD+ regeneration in the absence of lactate dehydrogenase.

Changes in chemical composition and sensory qualities of peanut milk fermented with lactic acid bacteria

The effects of fermentation of aqueous extracts of peanuts (peanut milk) with Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus salivarius ssp. thermophilus, separately and in combination, on selected chemical and sensory qualities were investigated. Changes in pH, titratable acidity and viable cell populations indicated that there was a synergistic interaction between L. delbrueckii ssp. bulgaricus and S. salivarius ssp. thermophilus during fermentation. Analysis of headspace volatiles revealed that hexanal, which is one of the compounds responsible for undesirable green/beany flavor in peanut milk, completely disappeared as a result of fermentation. S. salivarius ssp. thermophilus was more effective than L. delbrueckii ssp. bulgaricus in reducing the hexanal content. The acetaldehyde content of peanut milk increased during fermentation. Changes in concentrations of these volatile compounds were correlated with sensory evaluation scores which showed that a significant (P less than or equal to 0.05) decrease in green/beany flavor and a significant increase in creamy flavor occurred as a result of fermentation.

Effect of threonine and glycine concentrations on threonine aldolase activity of yogurt microorganisms during growth in a modified milk prepared by ultrafiltration

To evaluate the combined effects of threonine and glycine concentrations during growth on threonine aldolase activity (EC 2.1.2.1) of yogurt microorganisms, Streptococcus thermophilus and Lactobacillus bulgaricus, a modified milk growth medium was prepared using UF to deplete the free amino acid level. Threonine and glycine were added according to a 2x2x2 factorial design at 5 or 195 microg.ml(-1) along with a standard amino acid mixture. Acetaldehyde production and threonine aldolase activity were evaluated utilizing headspace gas chromatography. Results showed that threonine and glycine concentrations did not affect growth or titratable acidity. The high concentration of threonine in combination with low glycine in the growth medium resulted in increased acetaldehyde synthesis by both microorganisms. Conversely, high glycine with low threonine decreased acetaldehyde synthesis. High threonine and low glycine increased threonine aldolase activity of cell-free extracts from S. thermophilus and L. bulgaricus, whereas high glycine and low threonine reduced threonine aldolase activity of both microorganisms.

Proteolysis and electrophoretic pattern of casein of some fermented milks

Proteolysis and electrophoretic pattern of casein were determined in Friesian cows' skim milk, buffaloes' skim milk and in these milks fermented spontaneously (leben rayeb) and by Streptococcus lactis subsp. diacetylactis DRC3 (nonfat buttermilk) or by zabadi starter (zabadi). The highest proteolysis, as evidenced by the amount of released leucine, occurred in zabadi and the lowest in leben rayeb. The highest amount of tyrosine was liberated in the nonfat buttermilk. Proteolysis seems not to depend on the kind of milk used. A band of a relatively high density appeared to be released from alpha s-casein by the cell-wall proteinase of the microflora of raw milk and by heating at 90 degrees C for 1 min. Cows' skim milk, raw or heated, showed the presence of 2 probable proteose-peptone components; buffaloes' skim milk 3 and 2. These components underwent slow or rapid degradation, depending on the type of fermented milk during skim milk coagulation.

Oxygen dependent lactate utilization by Lactobacillus plantarum

Lactobacillus plantarum P5 grew aerobically in rich media at the expense of lactate; no growth was observed in the absence of aeration. The oxygen-dependent growth was accompanied by the conversion of lactate to acetate which accumulated in the growth medium. Utilization of oxygen with lactate as substrate was observed in buffered suspensions of washed whole cells and in cell-free extracts. A pathway which accounts for the generation of adenosine triphosphate during aerobic metabolism of lactate to acetate via pyruvate and acetyl phosphate is proposed. Each of the enzyme activities involved, nicotinamide adenine dinucleotide independent lactic dehydrogenase, nicotinamide adenine dinucleotide dependent lactic dehydrogenase, pyruvate oxidase, acetate kinase and NADH oxidase were demonstrated in cell-free extracts. The production of pyruvate, acetyl phosphate and acetate was demonstrated using cell-free extracts and cofactors for the enzymes of the proposed pathway.

Proteolytic systems in lactic acid bacteria

The proteolytic systems of lactic acid bacteria are important as a means of making protein and peptide N available for growth and as part of the curing or maturation processes which give foods their characteristic rheological and organoleptic properties. The proteolytic systems of lactic acid bacteria are described in relation to their growth and their functions in protein-rich foods. Their role in the manufacture of milk products is discussed.

Formation of acetaldehyde from threonine by lactic acid bacteria

Group N streptococci were found to cleave threonine to form acetaldehyde and glycine. Threonine aldolase, the enzyme catalysing this reaction, was found in all strains except Streptococcus cremoris Z8, an organism which had been shown previously to have a nutritional requirement for glycine. The enzyme was strongly inhibited by glycine and cysteine. The inhibition showed characteristics of allosteric inhibition and was pH-dependent. Inhibition by glycine, but not by cysteine, was highly specific. Analogues and derivatives of cysteine which contained a thiol group and a free amino group inhibited the activity of threonine aldolase. The presence of a carboxyl group was not necessary for inhibition. The cleavage of threonine by whole-cell suspensions was stimulated by either an energy source to aid transport or by rendering the cells permeable to substrate with oleate. Threonine did not appear to be degraded by enzymes other than threonine aldolase, as threonine dehydratase activity was low and NAD- and NADP-dependent threonine dehydrogenases were absent.