David B. Min

Chemistry and reactions of reactive oxygen species in foods

Reactive oxygen species (ROS) are formed enzymatically, chemically, photochemically, and by irradiation of food. They are also formed by the decomposition and the inter-reactions of ROS. Hydroxy radical is the most reactive ROS, followed by singlet oxygen. Reactions of ROS with food components produce undesirable volatile compounds and carcinogens, destroy essential nutrients, and change the functionalities of proteins, lipids, and carbohydrates. Lipid oxidation by ROS produces low molecular volatile aldehydes, alcohols, and hydrocarbons. ROS causes crosslink or cleavage of proteins and produces low molecular carbonyls from carbohydrates. Vitamins are easily oxidized by ROS, especially singlet oxygen. The singlet oxygen reaction rate was the highest in β-carotene, followed by tocopherol, riboflavin, vitamin D, and ascorbic acid.

Pulsed electric field processing effects on flavor compounds and microorganisms of orange juice

Abstract The headspace flavor compounds of fresh squeezed orange juice processed by pulsed electric field (PEF) at 30 kV/cm for 240 or 480 μs, or heat at 90°C for 1 min were isolated by a solid phase microextraction (SPME) coating and separated by gas chromatography. The average losses of flavor compounds in orange juice processed by 240, 480 μs PEF and heat process were 3.0%, 9.0% and 22.0%, respectively ( P

Effects of thermally oxidized triglycerides on the oxidative stability of soybean oil

Soybean oil purified by silicic acid column chromatography did not contain peroxides, free fatty acids, phospholipids or oxidized polar compounds. The purified soybean oil was thermally oxidized at 180°C for 96 hr in the presence of air. The thermally oxidized compounds (31.3%) were separated from the purified soybean oil by gradient elution silicic acid chromatography. Thermally oxidized compounds contained hydroxyl groups, carbonyl groups andtrans double bonds according to the infrared spectrum. Thermally oxidized compounds were added to soybean oil and purified soybean oil at 0, 0.5, 1.0, 1.5 and 2.0% to study the effects of these compounds on the oxidative stability of oil. The oxidative stabilities of oils were determined by gas chromatographic analysis of volatile compound formation and molecular oxygen disappearance in the headspace of oil bottles. The thermally oxidized compounds showed prooxidant effects on the oxidative stabilities of both refined, bleached and deodorized soybean oil and purified soybean oil. Duncan’s Multiple Range Test showed that thermally oxidized compounds had a significant effect on the volatile compound formatiion and oxygen disappearance in the headspace of oil at α=0.05.

Effects of yeast extract and glucose on xanthan production and cell growth in batch culture of Xanthomonas campestris

Abstract Although available kinetic data provide a useful insight into the effects of medium composition on xanthan production by Xanthomonas campestris, they cannot account for the synergetic effects of carbon (glucose) and nitrogen (yeast extract) substrates on cell growth and xanthan production. In this work, we studied the effects of the glucose/yeast-extract ratio (G/YE) in the medium on cell growth and xanthan production in various operating modes, including batch, two-stage batch, and fed-batch fermentations. In general, both the xanthan yield and specific production rate increased with increasing G/YE in the medium, but the cell yield and specific growth rate decreased as G/YE increased. A two-stage batch fermentation with a G/YE shift from an initial low level (2.5% glucose/0.3% yeast extract) to a high level (5.0% glucose/0.3% yeast extract) at the end of the exponential growth phase was found to be preferable for xanthan production. This two-stage fermentation design both provided fast cell growth and gave a high xanthan yield and xanthan production rate. In contrast, fed-batch fermentation with intermittent additions of glucose to the fermentor during the stationary phase was not favorable for xanthan production because of the relatively low G/YE resulting in low xanthan production rate and yield. It is also important to use a moderately high yeast extract concentration in the medium in order to reach a high cell density before the culture enters the stationary phase. A high cell density is also important to the overall xanthan production rate.

Xanthan Gum Fermentation by Xanthomonas campestris Immobilized in a Novel Centrifugal Fibrous-Bed Bioreactor

Current industrial fermentations in conventional stirred‐tank fermentors for production of xanthan gum and other polysaccharides are energy‐intensive and costly, mainly because the high broth viscosity causes agitation and aeration to be difficult and limits the final product concentration and productivity. In this work, a novel, centrifugal, packed‐bed reactor (CPBR) was developed for viscous xanthan gum fermentation. Xanthomonas campestris cells were immobilized in a rotating fibrous matrix by natural attachment to the fiber surfaces. The mixing and aeration problems were overcome by continuously pumping and circulating the medium broth through the rotating fibrous matrix to ensure intimate contact of gas and liquid with the cells and to separate the xanthan polymer from the cells. Almost all of the cells were immobilized on the fiber surfaces as was indicated by the absence of suspended cells in the fermentation broth. Thus, cell‐free xanthan broth was obtained, with ∼85% xanthan yield from glucose. The bioreactor was operated in repeated batch mode to study the feasibility and performance of long‐term xanthan gum production using the immobilized cells in the fibrous bed. Consistent xanthan production rate and gum quality were obtained for eight consecutive batches studied during a total operation period of over 3 weeks. The volumetric xanthan productivity achieved in the reactor was ∼1 g/(L·h) based on the total liquid volume and ∼3 g/(L·h) based on the fibrous‐bed volume. In contrast, conventional batch fermentation in a stirred‐tank reactor (STR) with free cells usually has a productivity of only 0.5 g/(L·h) or lower. The high productivity in CPBR was attributed to the relatively high cell density, ∼7 g/L, in the reactor. The specific xanthan productivity was lower in CPBR than in STR, however. This was because of the relatively low cell viability (∼60%) and limitation in oxygen transfer in CPBR, which can be improved by increasing the medium recirculation rate and the rotational speed of the fibrous matrix.

Effects of oxidized α-, γ- and δ-tocopherols on the oxidative stability of purified soybean oil

Oxidized α-, γ- and δ-tocopherols were prepared in methanol containing methylene blue for 30 h under light. The effects of 0, 100, 250, 500 and 1000 ppm of oxidized α-, γ- or δ-tocopherol on the oxidative stability of purified soybean oil in the dark at 55°C were studied by measuring the peroxide value and headspace oxygen of sample bottles. As the concentrations of oxidized tocopherols increased, the peroxide values increased and headspace oxygen decreased. Tukeys test showed that the oxidized α-, γ- and δ-tocopherols had prooxidant effects (P < 0.05) on the peroxide value and headspace oxygen of soybean oil. Although oxidized α-tocopherol had the greatest prooxidant effect, oxidized γ- and δ-tocopherols had similar but lesser effects.

Kinetic and feasibility studies of ultrafiltration of viscous xanthan gum fermentation broth

Abstract Ultrafiltration of xanthan gum solution as an alternative method to alcohol precipitation for xanthan gum recovery from dilute fermentation broth was studied. A polysulfone membrane hollow fiber (with 500 000 MWCO) tubular cartridge was used. The xanthan fermentation broth, which is highly viscous at its normal concentration of ∼2.5 (w/v)%, was concentrated to 13.5 (w/v)% or higher, with a recovery yield of ∼95% or higher. During ultrafiltration, the flux remained almost constant for xanthan concentrations up to ∼8%. It then decreased dramatically as the xanthan concentration increased beyond 8%. The decreased filtrate flux can be attributed to the decreased pumping (shear) rate and the increased viscosity that resulted at higher xanthan concentrations. When the xanthan concentration was held constant during ultrafiltration, the filtrate flux remained almost unchanged for the entire 2-h period studied, suggesting that the process was stable and no significant fouling occurred. In general, the filtrate flux decreased with increasing xanthan concentration and increased with increasing pumping (shear) rate and trans-membrane pressure difference. Changing the solution pH had a slight effect on the viscosity of xanthan solution, but did not affect the filtration performance. Even under high-shear-rate conditions, ultrafiltration did not give any observed adverse effects on the rheological properties and molecular weight of the xanthan polymer. Thus, ultrafiltration can be used to concentrate xanthan broth from fermentation by a factor of five or higher, thereby reducing the amount of alcohol needed for xanthan recovery by at least 80%.

Chemistry of Lipid Oxidation

The scientific knowledge on the chemistry of lipid oxidation has made giant progress during the last 30 years. This paper reviews the progress on the chemistry of singlet and triplet oxygen, quantum mechanics and kinetics of lipid oxidation, mechanisms and stereochemistry of lipid hydroperoxidation, mechanisms and analysis of volatile compounds, and flavor evaluation of oxidized oils by gas chromatography of volatile compounds.

Changes of headspace volatiles in milk with riboflavin photosensitization.

Effects of fluorescent light, riboflavin, ascorbic acid, sodium azide, and butylated hydroxyanisole (BHA) on the volatiles in milk at 4 degrees C were determined using a combination of headspace-solid phase microextraction (HS-SPME), gas chromatography (GC), and mass spectrometry (MS). Pentanal, hexanal, heptanal, and dimethyl disulfide were formed only in the milk stored under light and increased significantly as the duration of light exposure increased from 0 to 8 h and the concentration of added riboflavin increased from 5 to 50 ppm (P < 0.05). As fat content in milk increased, peak areas of pentanal, hexanal, and heptanal increased significantly (P < 0.05) while those of dimethyl disulfide did not change significantly (P > 0.05). Sodium azide prevented the formation of dimethyl disulfide in milk, implying that dimethyl disulfide can be formed through singlet oxygen oxidation (type II pathway). Addition of ascorbic acid and BHA reduced the formation of hexanal, heptanal, and dimethyl disulfide significantly (P < 0.05). Generation mechanisms of pentanal seem to be different from those of hexanal and heptanal in milk. Both singlet oxygen oxidation (type II pathway) and free radicals (type I pathway) play important roles in the formation of light-induced volatiles in milk.

Production of fruity aroma by Neurospora from beiju

Eight strains of Neurospora sp., isolated from beiju in various regions of the state of Maranhao, Brazil, and the culture medium of the Neurospora sp., have shown a pleasant fruity odour. Strains of Neurospora from NRRL collections and the strains from soil in the Sao Paulo area did not produce fruity aromas. It was established by dynamic headspace/gas chromatography that the nature of the fruity aroma was ethyl hexanoate. Furthermore, the strains of Neurospora sp. produced 3-methyl-1-butanol, 1-octen-3-ol, ethyl acetate and ethanol.