Effect of pretreatment severity on xylan solubility and enzymatic breakdown of the remaining cellulose from wheat straw.

The effect of process conditions used for wheat straw pretreatments on the liquor- and residue-composition was studied. Hereto, the pretreatment conditions were expressed in a 'combined severity R(0)(')-factor'. The higher the combined severity factor (R(0)(')) the more xylan was released from the wheat straw, but the more xylan decomposed and furfural formation occurred. The percentage of residual xylan present after pretreatment appeared to be a good indicator concerning cellulose degradability or bio-ethanol production. Namely, cellulose degradation by using commercial enzymes was higher at higher severities corresponding to a lower amount of residual xylan. The xylan release and degradation was studied in more detail by using HPSEC and MALDI-TOF mass spectrometry. The more severe the treatment the more (acetylated) xylose oligomers with a DP lower than nine were analysed. The presence of (acetylated) xylans with a DP of 9-25 increased slightly from low to medium severity. The quantification of the DP-distribution of the (acetylated) xylans released proved to be a good tool to predict cellulose degradability.

Real-time flavor release from French fries using atmospheric pressure chemical ionization-mass spectrometry.

Flavor release from French fries was measured with atmospheric pressure chemical ionization-mass spectrometry (APCI-MS) using both assessors (in vivo) and a mouth model system (in vitro). Several volatiles measured with APCI were identified with MS-MS. The effect of frying time, salt addition, and an alternative process using superheated steam was determined on I(max) (maximum intensity of compounds) and on t(max) (time of maximum intensity). In vitro a "chewing" frequency of 0.60 Hz caused an increased t(max) for low molecular weight compounds compared to the other frequencies tested. Above 0.93 Hz further increase in the frequency did not affect t(max). Trends observed with in vivo experiments could be verified with in vitro experiments. I(max) correlated well with frying time. Addition of salt resulted in a decreased t(max), suggesting a salting-out effect. The alternative process caused a layer of oil on the surface, and this resulted in a higher t(max), but no effect on I(max) was found. This phenomenon may be critical for the sensory quality and would not have been observed with static volatile measurements, demonstrating the value of flavor release measurements.

Effect of whey protein on the in vivo release of aldehydes.

Retention of aldehydes by whey proteins in solutions buffered at a range of pH values was studied under static and dynamic headspace conditions and in vivo in exhaled air. Static headspace measurements showed a clear increase in retention in the presence of whey proteins for aldehydes with longer carbon chains and for buffer solutions with higher pH values. For in vivo aldehyde release measurements, these effects were much less pronounced. The presence of saliva or the binding of aldehydes to the surface of the oral cavity was not responsible for this effect. This difference can be explained by the highly dynamic conditions of in vivo aroma release of liquid products, due to the relatively large flow of air during exhalation. After swallowing, a thin film of aldehyde solution remains in the pharynx; subsequent exhalation will release both the free aldehydes present in this film and those reversibly bound to the whey protein.

Characterisation of cell wall polysaccharides from okra (Abelmoschus esculentus (L.) Moench).

Okra pods are commonly used in Asia as a vegetable, food ingredient, as well as a traditional medicine for many different purposes; for example, as diuretic agent, for treatment of dental diseases and to reduce/prevent gastric irritations. The healthy properties are suggested to originate from the high polysaccharide content of okra pods, resulting in a highly viscous solution with a slimy appearance when okra is extracted with water. In this study, we present a structural characterisation of all major cell wall polysaccharides originating from okra pods. The sequential extraction of okra cell wall material yielded fractions of soluble solids extractable using hot buffer (HBSS), chelating agent (CHSS), dilute alkaline (DASS) and concentrated alkaline (CASS). The HBSS fraction was shown to be rich in galactose, rhamnose and galacturonic acid in the ratio 1.3:1:1.3. The degree of acetylation is relatively high (DA=58) while the degree of methyl esterification is relatively low (DM=24). The CHSS fraction contained much higher levels of methyl esterified galacturonic acid residues (63% galacturonic acid; DM=48) in addition to minor amounts of rhamnose and galactose. The ratio of galactose to rhamnose to galacturonic acid was 1.3:1.0:1.3 and 4.5:1.0:1.2 for HBSS and CHSS, respectively. These results indicated that the HBSS and CHSS fractions contain rhamnogalacturonan type I next to homogalacturonan, while the latter is more prevailing in CHSS. Also the DASS fraction is characterised by high amounts of rhamnose, galactose, galacturonic acid and some arabinose, indicating that rhamnogalacturonan I elements with longer arabinose- and galactose-rich side chains were part of this fraction. Partial digestion of HBSS and CHSS by pectin methyl esterase and polygalacturonase resulted in a fraction with a lower Mw and lower viscosity in solution. These samples were subjected to NMR analysis, which indicated that, in contrast to known RG I structure, the acetyl groups in HBSS are not located on the galacturonic acid residues, while for CHSS only part of the acetyl groups are located on the RG I galacturonic acid residues. The CASS fraction consisted of XXXG-type xyloglucan and 4-methylglucuronoxylan as shown by their sugar (linkage) composition and enzymatic digestion.

Standard assays do not predict the efficiency of commercial cellulase preparations towards plant materials.

Commercial cellulase preparations are potentially effective for processing biomass feedstocks in order to obtain bioethanol. In plant cell walls, cellulose fibrils occur in close association with xylans (monocotyls) or xyloglucans (dicotyls). The enzymatic conversion of cellulose/xylans is a complex process involving the concerted action of exo/endocellulases and cellobiases yielding glucose and xylanases yielding xylooligomers and xylose. An overview of commonly measured cellulase-, cellobiase-, and xylanase-activity, using respectively filter paper, cellobiose, and AZCL-dyed xylan as a substrate of 14 commercially available enzyme preparations from several suppliers is presented. In addition to these standardized tests, the enzyme-efficiency of degrading native substrates was studied. Grass and wheat bran were fractionated into a water unsoluble fraction (WUS), which was free of oligosaccharides and starch. Additionally, cellulose- and xylan-rich fractions were prepared by alkaline extraction of the WUS and were enzymatically digested. Hereby, the capability of cellulose and xylan conversion of the commercial enzyme preparations tested was measured. The results obtained showed that there was a large difference in the performance of the fourteen enzyme samples. Comparing all results, it was concluded that the choice of an enzyme preparation is more dependent on the characteristics of the substrate rather than on standard enzyme-activities measured.

Mode of action of pectin lyase A of Aspergillus niger on differently C(6)-substituted oligogalacturonides.

A thorough investigation of the mode of action of Aspergillus niger (4M-147) pectin lyase A (PLA) on differently C(6)-substituted oligogalacturonides is described. PLA appeared to be very specific for fully methyl-esterified oligogalacturonides: removal of the methyl-ester or changing the type of ester (ethyl esterification) or transamidation resulted in (almost) complete loss of conversion. The PLA activity increased with increasing length of the substrate up to a degree of polymerization (DP) of 8 indicating the presence of at least eight subsites on the enzyme. Product analysis demonstrated the formation of several Delta 4,5 unsaturated products and their saturated counterparts. The Delta 4,5 unsaturated trimer was the main product up to DP 8. For DP 9 and 10 Delta 4,5 unsaturated tetramer was the major product. Based upon the bond cleavage frequencies, a provisional subsite map was calculated, which supports the presence of eight subsites. By limited alkaline de-esterification of fully methyl-esterified pentamer and hexamer two sets of partially methyl-esterified pentamers (x and y methyl groups) and hexamers (a and b methyl groups) were prepared. Matrix-assisted laser desorption/ionization time of flight mass spectroscopy (MALDI-TOF MS) analysis demonstrated that the methyl-ester distribution was fully random. Using these partially methyl-esterified oligogalacturonides as substrates for PLA a 10-fold decrease in reaction rate was recorded compared with the fully methyl-esterified counterparts. Analysis of the methyl-ester distribution of the products showed that PLA tolerates carboxyl groups in the substrate binding cleft. At either subsite +2, +4, or -1 to -4 a free carboxyl group could be tolerated, whereas methyl-esters were obligatory at subsite +1 and +3. So PLA is capable to cleave the bond between a methyl-esterified and a non-esterified galacturonic acid residue, where the newly formed Delta 4,5 unsaturated non-reducing end residue always contains a methyl-ester.