Functional characterization of phospholipid N-methyltransferases from Arabidopsis and soybean.

Phospholipid N-methyltransferase (PLMT) enzymes catalyze the S-adenosylmethionine-dependent methylation of ethanolamine-containing phospholipids to produce the abundant membrane lipid phosphatidylcholine (PtdCho). In mammals and yeast, PLMT activities are required for the de novo synthesis of the choline headgroup found in PtdCho. PLMT enzyme activities have also been reported in plants, yet their roles in PtdCho biosynthesis are less clear because most plants can produce the choline headgroup entirely via soluble substrates, initiated by the methylation of free ethanolamine-phosphate. To gain further insights into the function of PLMT enzymes in plants, we isolated PLMT cDNAs from Arabidopsis and soybean (Glycine max) based upon primary amino acid sequence homology to the rat PLMT, phosphatidylethanolamine N-methyltransferase. Using a heterologous yeast expression system, it was shown that plant PLMTs methylate phosphatidylmonomethylethanolamine and phosphatidyldimethylethanolamine but cannot utilize phosphatidylethanolamine as a substrate. Identification of an Arabidopsis line containing a knock-out dissociator transposon insertion within the single copy AtPLMT gene allowed us to investigate the consequences of loss of PLMT function. Although the accumulation of the PLMT substrates phosphatidylmonomethylethanolamine and phosphatidyldimethylethanolamine was considerably elevated in the atplmt knock-out line, PtdCho levels remained normal, and no obvious differences were observed in plant morphology or development under standard growth conditions. However, because the metabolic routes through which PtdCho is synthesized in plants vary greatly among differing species, it is predicted that the degree with which PtdCho synthesis is dependent upon PLMT activities will also vary widely throughout the plant kingdom.

Extended shelf life of soy bread using modified atmosphere packaging.

This study investigated the use of modified atmosphere packaging (MAP) to extend the shelf life of soy bread with and without calcium propionate as a chemical preservative. The bread samples were packaged in pouches made from low-density polyethylene (LDPE) as the control (film 1), high-barrier laminated linear low-density polyethylene (LLDPE)-nylon-ethylene vinyl alcohol-nylon-LLDPE (film 2), and medium-barrier laminated LLDPE-nylon-LLDPE (film 3). The headspace gases used were atmosphere (air) as control, 50% CO2-50% N2, or 20% CO2-80% N2. The shelf life was determined by monitoring mold and yeast (M+Y) and aerobic plate counts (APC) in soy bread samples stored at 21 degrees C +/- 3 degrees C and 38% +/- 2% relative humidity. At 0, 2, 4, 6, 8, 10, and 12 days of storage, soy bread samples were removed, and the M+Y and APC were determined. The preservative, the films, and the headspace gases had significant effects on both the M+Y counts and the APC of soy bread samples. The combination of film 2 in the 50% CO2-50% N2 or 20% CO2-80% N2 headspace gases without calcium propionate as the preservative inhibited the M+Y growth by 6 days and the APC by 4 days. It was thus concluded that MAP using film 2 with either the 50% CO2-50% N2 or 20% CO2-80% N2 was the best combination for shelf-life extension of the soy bread without the need for a chemical preservative. These MAP treatments extended the shelf life by at least 200%.

Texture, proteolysis and viable lactic acid bacteria in commercial cheddar cheeses treated with high pressure.

High pressure processing was investigated for controlling Cheddar cheese ripening. One-month-or 4-month-old Cheddar cheeses were subjected to pressures ranging from 200 to 800 MPa for 5 min at 25 C. The number of viable Lactococcus lactis (starter) and Lactobacillus (nonstarter) cells decreased as pressure increased. Subsequent storage of the control and pressure-treated cheeses at 10 degrees C caused viable cell counts to change in some cases. Free amino acid content was monitored as an indicator of proteolysis. Cheeses treated with pressures > or = 400 MPa evolved free amino acids at significantly lower rates than the control. No acceleration in free amino acid development was observed at lower pressures. Pressure treatment did not accelerate the rate of textural breakdown compared with the non-pressure treated control. On the contrary, pressure treatment at 800 MPa reduced the time-dependent texture changes. Results indicate that high pressure may be useful in arresting Cheddar cheese ripening.

Lactobacillus growth and membrane composition in the presence of linoleic or conjugated linoleic acid.

Five Lactobacillus strains of intestinal and food origins were grown in MRS broth or milk containing various concentrations of linoleic acid or conjugated linoleic acid (CLA). The fatty acids had bacteriostatic, bacteriocidal, or no effect depending on bacterial strain, fatty acid concentration, fatty acid type, and growth medium. Both fatty acids displayed dose-dependent inhibition. All strains were inhibited to a greater extent by the fatty acids in broth than in milk. The CLA isomer mixture was less inhibitory than linoleic acid. Lactobacillus reuteri ATCC 55739, a strain capable of isomerizing linoleic acid to CLA, was the most inhibited strain by the presence of linoleic acid in broth or milk. In contrast, a member of the same species, L. reuteri ATCC 23272, was the least inhibited strain by linoleic acid and CLA. All strains increased membrane linoleic acid or CLA levels when grown with exogenous fatty acid. Lactobacillus reuteri ATCC 55739 had substantial CLA in the membrane when the growth medium was supplemented with linoleic acid. No association between level of fatty acid incorporation into the membrane and inhibition by that fatty acid was observed.