Laurence D. Melton

Creating Proteins with Novel Functionality via the Maillard Reaction: A Review

Proteins are widely utilized to add functional properties, such as gelling and emulsification to foods. These attributes depend on a number of factors such as molecular structure of the protein, the pH, and the composition of its chemical environment. There is substantial evidence to suggest that the functional properties of food proteins can be further improved by derivatization. Covalent bonding of proteins to polysaccharides and smaller reducing sugars via the Maillard reaction has been shown to alter the functionality of proteins without requiring the addition of chemical reagents. Establishment of a technologically feasible method for preparing the conjugates and optimization of the processing conditions, however, is needed to promote their development as functional food ingredients. This paper provides a state-of-the-art contribution to the impact of the Maillard reaction on protein functionality. It presents a deeper understanding of the influence of processing conditions and reactant formulation on improving desirable properties of proteins. In particular attention is given to how potential improvements could be achieved in the emulsifying, textural, and solubility properties of proteins to add value to commodity food ingredients. Elements that are considered to be critical to the design of functional Maillard conjugates are highlighted and suggestions proposed to facilitate progress in this area.

Cell-wall polysaccharides of kiwifruit (Actinidia deliciosa): Chemical features in different tissue zones of the fruit at harvest

Abstract Cell-wall material (CWM) was isolated from cryo-milled (− 196°) powders prepared from 4 different tissue zones of kiwifruit ( Actinidia deliciosa ). Polysaccharides were solubilised by stepwise extraction with cyclohexane- trans -1,2-diaminetetra-acetate (CDTA), 0.05 m Na 2 CO 3 , 6 m guanidinium thiocyanate (GTC), and 4 m KOH. A heterogeneous mixture of pectic galactans accounted for 40–50% of the CWMs, while hemicelluloses, the bulk of which were xyloglucans, accounted for 15–25%. Each tissue zone contained similar types of polysaccharide. Variability in their amount and sugar composition are thought to reflect different stages in the physiological development of the fruit at harvest, in the 4 zones. Polymers from the outer pericarp tissue were fractionated by anion-exchange chromatography and subjected to methylation analysis. The CDTA- and Na 2 CO 3 -soluble polymers were rhamnogalacturonans substituted to varying degrees with galactan and arabinogalactan side-chains containing 4-, 2,4-, 3,4- and 4,6-linked galactose and 5- and 3,5-linked arabinose. Side chains were terminated by galactose and arabinose and lesser amounts of rhamnose, fucose, xylose, and galacturonic acid. The pectic polysaccharides of the GTC- and KOH-soluble fractions had more highly branched rhamnogalacturonan backbones than the CDTA- and Na 2 CO 3 -soluble polymers and contained hemicellulosic elements. The major hemicellulose was a xyloglucan, but lesser amounts of a 4- O -methylglucuronoxylan and a branched mannan were partially characterised. Several polymers were associated with proteins low in hydroxyproline. Evidence is presented that a polysaccharide of the rhamnogalacturonan II type is associated with the pectic polymers of kiwifruit.

State diagram of apple slices : glass transition and freezing curves

Abstract The state diagram of apple flesh was developed by measuring and modeling its freezing points and glass transition temperatures. The freezing curve and glass transition lines were developed using Clausias–Clapeyron and Gordon–Taylor models, respectively. The state diagram of apple pieces developed in this work can be used in determining the stability during frozen storage and in dried conditions as well as in designing drying and freezing processes.

Celery (Apium graveolens L.) parenchyma cell walls examined by atomic force microscopy: effect of dehydration on cellulose microfibrils.

Abstract. Atomic force microscopy (AFM) was used to image celery (Apium graveolens L.) parenchyma cell walls in situ. Cellulose microfibrils could clearly be distinguished in topographic images of the cell wall. The microfibrils of the hydrated walls appeared smaller, more uniformly distributed, and less enmeshed than those of dried peels. In material that was kept hydrated at all times and imaged under water, the microfibril diameter was mainly in the range 6–25 nm. The cellulose microfibril diameters were highly dependent on the water content of the specimen. As the water content was decreased, by mixing ethanol with the bathing solution, the microfibril diameters increased. Upon complete dehydration of the specimen we observed a significant increase in microfibril diameter. The procedure used to dehydrate the parenchyma cells also influenced the size of cellulose microfibrils with freeze-dried material having larger diameters than air-dried material.

Cell-wall polysaccharides of kiwifruit (Actinidia deliciosa) : effect of ripening on the structural features of cell-wall materials

Abstract Cell-wall polysaccharides were solubilised from cell-wall materials (CWMs) isolated from kiwifruit at 2 ripening stages, after treatment for 1 and 7 days with ethylene. The fractions soluble in cyclohexane- trans -1,2-diaminetetra-acetate (CDTA), Na 2 CO 3 , guanidinium thiocyanate (GTC), and KOH were purified by ion-exchange and gel-permeation chromatography, and subjected to methylation analysis. The pectic polymers in the 7-day fractions had increased proportions of 4-linked galactosyl, 2- and 2,4-linked rhamnosyl, and terminal arabinosyl, xylosyl, and galactosyl residues, and decreased proportions of 4-linked galacturonosyl residues. In the major Na 2 CO 3 -soluble pectic fraction, there was an increase in the proportion of branched polymers of high molecular weight during ripening. Therefore, a large proportion of the polyuronides may have been solubilised without significant structural modification, indicating that the action of endo-polygalacturonase was not involved. There was a decrease in M w of the xyloglucan fraction during ripening, but no detectable changes in the primary structure or in the 4- O -methylglucuronoxylan and galactoglucomannan fractions.

Cell wall changes in kiwifruit following post harvest ethylene treatment

Abstract The composition of the cell walls from four tissue zones of kiwifruit at three times after harvest have been investigated. Cell wall breakdown triggered by ethylene did not occur simultaneously in different tissue zones of kiwifruit, being more pronounced in the outer pericarp and inner pericarp but much less in the locule wall and core tissues. Cell wall material (CWM) from the outer pericarp lost 80 and 64% of the CDTA (cyclohexanediamine tetraacetic acid) and sodium carbonate-soluble fractions respectively. However, the amounts of these fractions remaining in the CWM of ripe fruit contained higher proportions of galactose, arabinose and rhamnose, than fruit at harvest. These sugars occur at branchpoints or in sidechains in kiwifruit pectic polymers; a fact that pointed to a non-random degradation of the wall polysaccharides and the preferential release of the less branched parts of the rhamnogalacturonan backbone of the pectic substances. There was a 50% decrease in outer pericarp cell wall galactose, 80% of which occurred in the KOH-soluble and CWM residue fractions. Comparison of whole kiwifruit at harvest and seven days after ethylene treatment showed a net decrease in galactose (33%) and arabinose (22%). Absence of sufficient free galactose or galactose-containing oligosaccharides to account for the galactose released from the wall polysaccharides during ripening, implied post harvest metabolism of cell wall derived galactose. In contrast, the uronic acid in the cell wall was partially converted to soluble forms during ripening without any apparent loss due to metabolism.

Ripening-related changes in cell wall polysaccharides of strawberry cortical and pith tissues

Abstract Cell wall composition of cortical and pith tissues of strawberry ( Fragaria x ananassa Duchesne cv. Yolo) was investigated at three stages of ripeness: unripe, partially ripe and ripe. Cell walls from these tissues, isolated using Hepes-buffered phenol (pH 6.5), were sequentially extracted with aqueous 0.1 M Hepes buffer, 0.05 M CDTA, 0.05 M Na 2 CO 3 , 1 M KOH and 4 M KOH. The uronic acids, neutral sugars and cellulose contents in these fractions were estimated colorimetrically, while monosaccharide composition of the neutral noncellulosic polysaccharides was determined by gas chromatography. During ripening, total cell wall polysaccharides (CWP) of cortical tissues decreased more (1.7 to 1.0 g/100 g frozen tissue) than that of the pith tissues (1.5 to 1.2 g/100 g frozen tissue). Concomitantly, cortical CWP showed greater changes in the extent of solubilisation compared to that of the pith—this appeared to be due to highly soluble pectic substances and the disappearance of highly branched pectic material. For pith, changes in CWP appeared to be associated with a marked decrease in non-esterified pectins. Monosaccharide composition of both tissue types indicated that losses in highly branched pectic substances were mainly due to a decrease in arabinose residues. By comparison, the cell walls of both tissue types exhibited only small decreases in hemicellulose during ripening. Cellulose appeared to be a minor cell wall component of strawberry fruit and its amount remained relatively constant throughout ripening.

Bovine β-Lactoglobulin Is Dimeric Under Imitative Physiological Conditions: Dissociation Equilibrium and Rate Constants over the pH Range of 2.5–7.5

The oligomerization of β-lactoglobulin (βLg) has been studied extensively, but with somewhat contradictory results. Using analytical ultracentrifugation in both sedimentation equilibrium and sedimentation velocity modes, we studied the oligomerization of βLg variants A and B over a pH range of 2.5-7.5 in 100 mM NaCl at 25°C. For the first time, to our knowledge, we were able to estimate rate constants (k(off)) for βLg dimer dissociation. At pH 2.5 k(off) is low (0.008 and 0.009 s(-1)), but at higher pH (6.5 and 7.5) k(off) is considerably greater (>0.1 s(-1)). We analyzed the sedimentation velocity data using the van Holde-Weischet method, and the results were consistent with a monomer-dimer reversible self-association at pH 2.5, 3.5, 6.5, and 7.5. Dimer dissociation constants K(D)(2-1) fell close to or within the protein concentration range of ∼5 to ∼45 μM, and at ∼45 μM the dimer predominated. No species larger than the dimer could be detected. The K(D)(2-1) increased as |pH-pI| increased, indicating that the hydrophobic effect is the major factor stabilizing the dimer, and suggesting that, especially at low pH, electrostatic repulsion destabilizes the dimer. Therefore, through Poisson-Boltzmann calculations, we determined the electrostatic dimerization energy and the ionic charge distribution as a function of ionic strength at pH above (pH 7.5) and below (pH 2.5) the isoelectric point (pI∼5.3). We propose a mechanism for dimer stabilization whereby the added ionic species screen and neutralize charges in the vicinity of the dimer interface. The electrostatic forces of the ion cloud surrounding βLg play a key role in the thermodynamics and kinetics of dimer association/dissociation.

Rapid enumeration of Listeria monocytogenes in artificially contaminated cabbage using real-time polymerase chain reaction.

A quantitative real-time polymerase chain reaction (PCR) detection method specific for Listeria monocytogenes was developed, and studies involving pure culture showed that the response of the assay was linear over 7 log10 (log) cycles. The method was then applied to the detection of L. monocytogenes artificially inoculated onto cabbage, a vegetable chosen because it is a major component of coleslaw, which has been associated with an outbreak of listeriosis. After being allowed to attach to the food, cells were washed from the cabbage leaf surface and recovered by centrifugation. The DNA was purified by an organic solvent extraction technique and analyzed by real-time PCR. In this matrix, the method again produced a linear response over 7 log cycles from 1.4 x 10(2) to 1.4 x 10(9) CFU of L. monocytogenes in 25 g of cabbage, and analysis of the reproducibility of the system showed that log differences in L. monocytogenes numbers added to cabbage could be reliably distinguished. The system allowed quantitative results to be obtained within 8 h and was relatively inexpensive, showing good potential for routine analytical use.

Application of headspace solid-phase microextraction to volatile flavour profile development during storage and ripening of kiwifruit

Kiwifruit volatile flavour compounds were evaluated with headspace solid-phase microextraction (SPME) as a sample concentration technique. Gas chromatography–mass spectrometry (GC–MS) was used for qualitative and semi-quantitative analysis after SPME. Components such as heptanal, ethyl hex-3-enoate, 6-methylhept-5-en-2-one, acetic acid, c/t-2-nonenal (mixture) and c/t-2-decenal (mixture), which were previously found only in kiwifruit juice, were detected by headspace SPME–GC–MS. Other compounds (pent-4-enal, t,t-nona-2,4-dienal, 2-nonanone, ethyl octanoate, butyrolactone and 2-propenyl butanoate), which had not been found previously, were identified. Flavour volatiles of kiwifruit were very sensitive to storage time and state of ripeness.