The rapid increase in sequence data in combination with a greater understanding of the forces regulating protein structure has been the impetus for an upsurge in the development of theoretical prediction methods. These methods have afforded protein chemists the ability to identify and quantify the various secondary structures along the protein chain. Concurrently, various physico-chemical techniques have been developed such as nuclear Overhauser enhancement n.m.r. and laser Raman spectroscopy. In addition, traditional methods such as infrared and circular dichroism spectroscopy have been refined. Although both predictive and physico-chemical techniques are limited in the types of secondary structure they are capable of determining, they have provided valuable information with regards to protein folding and topology in the absence of X-ray data, and have formed the basis for the development of improved methods for secondary structure determination. This paper reviews some of the predictive and physico-chemical methods presently used to determine protein secondary structure.

ABSTRACT The use of resonance properties, measured by dynamic oscillation using a constant stress rheometer, as textural parameters for excised pericarp tissue of tomato ( Lycopersicon esculentum Mill cv Trust) fruit during 21 days of ambient temperature ripening was assessed relative to the large‐deformation behavior measured by flat‐plate compression. The resonance frequency (f r ) of excised tissue discs decreased significantly (P < 0.05) over the first 9–12 days, and then approached a relatively low and constant value with subsequent ripening. A significant (P < 0.05) increase in associated resonance strain amplitude (y max ) became apparent after the first 9–12 days of tomato fruit ripening. Ripening‐associated changes in bioyield and failure strengths, toughness, and apparent elastic and failure deformability moduli derived from flat‐plate compression tests were similar to those observed for tissue f r . Results were consistent with a change in the dominant mode of tissue failure from cell relaxation and rupture to debonding after 9–12 days of fruit ripening. A significant (P < 0.05) semilogarithmic relationship was observed between f r , but not y max , and compression parameters, allowing for reasonable prediction of large‐deformation behavior on the basis of measurement of only f r . The results demonstrate that resonance frequency may provide a useful parameter in the evaluation of tomato pericarp tissue texture.

The effects of pH (5.0 to 7.0), NaCl concentration (0 to 20 g/L) and metmyoglobin concentration (1 to 50 g/L) on the thermal stability of metmyoglobin in a model aqueous system were investigated using differential scanning calorimetry and a central composite response surface experimental design. Significant predictive models were developed for onset and peak denaturation temperatures that yielded similar response surfaces; the model developed for denaturation enthalpy was statistically inadequate. The effects of treatment factors on metmyoglobin thermal stability were highly interdependent. Maximum stability occurred at high pH and low concentrations of NaCl and metmyoglobin within the ranges studied. Metmyoglobin denaturation was accentuated above a critical concentration by increasing NaCl concentration and/or pH. Results suggest that aggregation may have played a key role in reducing the thermal stability of metmyoglobin in the model system as mediated by pH, NaCl and metmyoglobin concentration effects.

Texture is a major quality attribute that determines the acceptance of plant foods. However, the term is still often poorly defined and applied. As texture is dictated by the underlying composition and organization of plant tissues, it is crucial for food scientists to be aware of the structure of plant foods. The texture of plant foods can be attributed mainly to the structural integrity of the cell wall and middle lamella, as well as to the turgor pressure generated within cells by osmosis. Recent models of the cell wall envision a cellulose-hemicellulose structural domain embedded in a second domain consisting of pectic substances, while a third domain contains covalently crosslinked protein units. Textural problems in plant foods, arising from diffusion, ripening and processing factors, for example, are directly related to the architecture of the plant cell. Thus, an increased understanding of the structural basis of texture and also of the fundamentals of texture measurement should assist in overcoming quality problems in plant foods.

The extractability of polygalacturonase (PG) activity from alcohol-insoluble solids (AIS) prepared from red-ripe tomato pericarp tissue was investigated using acetate (pH 4.5), citrate (pH 4.5) and MES (pH 6.0) buffers. The acetate buffer was also investigated with increasing concentrations of NaCl, CaCl2, MgCl2, ethylenediaminetetraacetic acid (EDTA) or ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid (EGTA). Heat treatment of AIS to prepare control material suppressed PG extractability but not in situ PG activity, as evidenced by the release of significant quantities of soluble uronides into extraction media. With only a single extraction, acetate buffer containing > I M NaCl yielded the greatest amount of PG activity (> 50% of maximum extractability), with MES buffer being only marginally less efficient However, 3 successive extractions over a 2 h period yielded significantly greater amounts of PG activity. The greatest yields of PG activity were obtained by successive extraction with parent MES and acetate buffers. There appeared to be little benefit in adding NaCl or a chelating agent to the extraction medium. Use of these extractants is suggested to have led to losses of PG activity during dialysis, via coprecipitation of PG protein with otherwise soluble uronide material that was released in greater quantities when these extractants were used. Increasing CaCl2 and MgCl2 concentrations reduced the amount of extracted PG activity similarly, to about 50% of maximum levels with successive extractions.

ABSTRACTThe influence of normal ripening and chilling stress on viscoelastic properties of tomato pericarp tissue were investigated by measuring creep behaviour of tissue from fruit stored at 22C (nonchilled) or 5C (chilled) for 28 days, or at 5C for 16 days prior to transfer to 22C for an additional 12 days (prechilled). Creep compliance of tissue from all treatments subjected to a constant shear stress of 150 Pa for 5 min was best represented by a 6‐element Burgers model containing two discrete Voigt‐Kelvin units characterizing fast and slow rate viscoelastic properties. The magnitude of instantaneous elastic, viscoelastic and steady‐state viscous compliances each increased steadily and contributed to the overall softening of nonchilled and prechilled tissues during ripening, but remained unchanged during chilling of tomato fruit. Increased fluidity of ripening tissues occurred at the expense of elasticity, consistent with a decrease in molecular weight‐size distribution of structural elements contributing to respective viscoelastic properties. The physico‐mechanical changes in prechilled tissue preceded those in nonchilled tissue by several days, and occurred at a faster rate. The 6‐element Burgers model defining the creep behaviour of tomato pericarp tissue was interpreted with respect to general plant cell wall structure and biochemical changes known to occur during ripening of tomato fruits. Multiple mechanisms of softening were thereby consolidated into a single physico‐mechanical model.

ABSTRACTMature‐green tomatoes chilled 15 days (5°C; RH >85%) were softer than nonchilled during subsequent ripening (22°C) by both whole fruit and pericarp tissue puncture (p<0.05), but not by flat‐plate compression. No differences in total polygalactnronase (PG) or PG isozyme activity were evident although total activity was greater in nonchilled after 10 days ripening. Softening of nonchilled fruit correlated (p<0.05) with extracted PGI activity, while chilling‐associated softening correlated (p<0.05) with higher initial extracted pectinmethylesterase (PME) activity. Extracted peroxidase remained constant throughout ripening but was greater (p<0.05) in pre‐chilled fruit consistent with chill‐induced membrane dysfunction. Transmission electron microscopy showed the middle lamella from pre‐chilled tomatoes was swollen and less defined. Loss of turgor from translocation of water to the PME‐modified cell wall was suggested to be responsible for softening as a consequence of chilling.

ABSTRACTThe puncture properties (firmness, toughness) of tomato skin and its influence on the overall puncture properties of mature‐green fruit during ambient temperature (22C) ripening or chilled (5C) storage were estimated from the difference between whole fruit puncture force‐deformation behaviour and that measured with the skin removed. Although the firmness of the skin of nonchilled fruit did not change significantly (p > 0.05) over 28 days of ripening, its contribution to overall puncture firmness increased from about 25 to 70%. Both skin toughness and its contribution to overall puncture toughness doubled within the first week of ripening of nonchilled fruit. The firmness of chilled fruit skin decreased (p > 0.05) only after 16 days of storage, but its contribution to overall puncture firmness remained constant at 25–30%. Both the toughness of the skin of chilled fruit and its contribution to overall puncture toughness remained constant during storage. Possible artifacts in the measurement of puncture properties of chilled fruit are discussed. The results are consistent with a toughening of tomato skin during early stages of ripening of nonchilled fruit that somewhat compensates for a rapid reduction in pericarp tissue integrity, and with an increase in firmness or rigidity of tomato skin and underlying tissue(s) during chilled storage of fruit.

ABSTRACTThe effects of turgor pressure on puncture and viscoelastic properties of mature‐green tomato pericarp were examined using tissue discs soaked in a range of osmotica (0.0–0.6 M mannitol) for at least 36 h at 4C. Turgor pressure was estimated from the osmotic potential of soaking solutions that induced incipient plasmolysis. Based on volume changes, the osmotic potential and turgor pressure of fresh tissue were estimated to be −0.56 ± 0.08 MPa and 0.20 MPa, respectively. However, puncture and viscoelastic properties corresponded to a turgor pressure of 0.15 MPa. The discrepancy between calculated and actual turgor pressures was attributed to the presence of apoplastic solutes. The data from this study revealed a general increase in cell wall stress, strain and elasticity with increasing turgor. With increases in turgor above that of untreated tissue both wall extensibility and elasticity became limiting and thus cell wall stiffness increased. Conversely, a decrease in turgor below that of untreated tissue led to an increase in viscoelasticity. Increases in bioyield and pseudoplastic bioyield strains with a variation in turgor from that of untreated tissue were consistent with cell debonding as a dominant mechanism of tomato tissue bioyielding. The reduced failure force, deformation and firmness with increasing turgor were consistent with cell rupture as a predominant mechanism of failure of mature‐green tomato pericarp tissue.

Softening effects of chelating and ion-exchange agents in bean soaking solutions have been attributed to improved solubilization of pectates. The present results indicate a mechanism based on protein destabilization also contributes to softening. Solubility and calorimetric data established softening to be correlated with increased protein denaturation of isolated protein bodies and not with pectin solubilization during cooking, although pH 10, but not pH 7, carbonate buffer did extract significant (p ≤ 0·05) quantities of both pectin and protein. Carbonate ions reduced onset temperature of protein body denaturation by > 10°C. Rates of protein and pectin solubilization in hardened beans were faster then softening rates, suggesting that softening was not totally attributable to thermal degradation of macromolecular components or middle lamella dissolution.

ABSTRACTFailure mechanisms of tomato pericarp tissue were investigated by measuring puncture and viscoelastic properties of tissue from fruit stored at 22C (nonchilled) or 5C (chilled) for 28 days, or at 5C for 15 days prior to transfer to 22C for an additional 13 days (prechilled) to facilitate ripening. The puncture properties of tissue remained unchanged during 28 days of chilling. However, tissue shear strength and rigidity increased with chilling, as demonstrated by increases in storage and loss moduli, and decreases in oscillatory strain and the loss tangent. Puncture failure force, failure firmness and storage and loss moduli of nonchilled and prechilled tissues decreased to low, constant levels by 12 and 9 days, respectively. Simultaneously, pseudoplastic and failure strains and oscillatory strain each increased. These data suggest that the loss of tissue compressive strengths and transition in the dominant mode of failure from cell relaxation and rupture to cell debonding occurred at a faster rate during ripening of prechilled (i.e., chill‐injured) compared to nonchilled fruit.

ABSTRACTThe influence of tomato fruit ripeness on area‐ and perimeter‐dependent properties and dominant failure mechanisms of pericarp tissue were investigated. Tissue discs from mature‐green and red‐ripe fruit were punctured with a flat‐ended cylindrical probe and compressed with a flat plate at a constant rate of deformation. Approximately linear force‐deformation curves were obtained to tissue failure by both puncture and flat plate compression, interrupted by a region of pseudoplastic deformation at a relatively low initial bioyield force. Based on estimated area‐ and perimeter‐dependent coefficients and firmness (force/deformation) values, initial bioyielding of tissue appeared to be associated with an abrupt increase in cell‐to‐cell compaction. Puncture of mature‐green tissue led to premature failure induced by shearing or rupture of tissue at the probe perimeter. The contribution to puncture of perimeter‐dependent or shear‐associated forces and a putative “zone of influence” increased markedly with ripening, while the contribution of area‐dependent or compression associated forces generally decreased. A concomitant decrease in failure force and firmness with ripening reflected a general loss of both tissue compression and shear strengths. These results suggested that the dominant mode of tissue failure changed with ripening, from cell relaxation and rupture to cell debonding. The data obtained in this study emphasize the need to exercise caution in the interpretation of force‐deformation parameters derived from puncture tests alone.

The effects of chilling on tomato (Lycopersicon esculentum Mill cv. Caruso) texture were investigated using fruit stored at 22°C (nonchilled) or 5°C (chilled) for 28 days. or at 5°C for 15 days before transfer to 22°C to facilitate ripening during and additional 13 days (prechilled). Prechilled fruit exhibited symptoms of slight chilling injury, i.e. development of mealiness, accelerated softening relative to that of nonchilled fruit and nonuniform surface colour development. The firmness of all fruit decreased during ripening and chilled storage when measured by flat plate compression and puncture, especially during the early stages of ripening of nonchilled and prechilled fruit. The compression firmness of pericarp tissue similarly decreased during ripening of nonchilled and prechilled fruit, but was maintained during chilling. Total moisture content (ca 94%) of tissue, uronide content (32‐35% w/w) and extracted β‐galactosidase activity did not differ significantly (P > 0.05) among fruit during ripening and chilled storage. The degree of uronide methyl esterification in ethanol‐insoluble solids prepared from pericarp tissue (EIS) was relatively low for all fruit. i.e. <40%. EIS from which greater levels of pectinesterase were extracted (i.e. nonchilled>chilled>prechilled) exhibited decreased levels of uronide methyl esterification. Markedly elevated levels of β‐glucosidase activity were extracted from prechilled EIS. Total polygalacturonase activity (mainly as PGI) and autolysis of enzyme‐extracted EIS were inversely correlated (P≤ 0.05) only with the loss of nonchilled fruit and tissue firmness and prechilled fruit firmness. Results suggest a possible role for β‐glucosidase in textural changes of prechilled fruit and tissue (e.g. loss of firmness, development of mealiness) and also implicate loss of skin strength in the softening of whole fruit during chilling.

Flat-plate compression, constant area compression, and puncture tests were examined for their sensitivity in differentiating the firmness of previously chilled (6C, 85% RH, 15 days) and nonchilled mature-green tomato (Lycopersicon esculentum Mill cv . Caruso) fruit during 10 days of ripening at 22C. Firmness, as measured by each of the three methods, progressively decreased (P < 0.001) with ripening. Previously chilled tomatoes were initially softer (P < 0.01) than nonchilled tomatoes, as measured by puncture of whole fruit and constant area compression of pericarp tissue sections, but not by flat-plate compression of whole fruit. Flat-plate compression was therefore found to be a relatively insensitive method by which to measure differences in tomato firmness that are characteristic of slightly chilling-injured fruit.

Several multivariate statistical techniques were applied to ultraviolet absorption, fluorescence emission and extrinsic fluorescence data to investigate the relationship between functionality and structure-related properties of whey-potato and whey-pea protein composites over the pH range of 4 to 8. Multiple regression analysis resulted in highly significant (p<0.001) equations predicting solubility, thermocoagulability, foaming capacity and emulsion properties. Cluster and principal component analyses delineated variables into three distinct groups reflecting solubility and/or surface-active properties, surface hydrophobicity and tryptophan exposure. Stepwise linear discriminant analysis led to correct differentiation of individual samples into their respective composites, based solely on significant (p<0.05) contributions to discriminant functions by variables characterizing surface hydrophobicity and solubility and/or surface-active properties. Results from this study support the hypothesis that hydrophobic residues play a key role in functional properties displayed by whey-vegetable protein composites.

Several whey-potato and whey-pea protein composites were prepared by wet-blending and their ultraviolet (UV) absorption and fluorescence properties were measured at pH 4 to 8. A significant (p<0.05) interaction between the effects of pH and the proportion of vegetable proteins combined with acid whey proteins was observed for all UV absorption variables. In general, however, a blue shift and reduction of intensity of both UV absorption and fluorescence emission spectra occurred as pH decreased and the proportion of vegetable protein increased. Extrinsic fluorescence of proteins, measured at pH 6 to 8, also decreased with increasing proportions of vegetable protein. Increased protein-protein interaction and/or changes in protein conformation are implicated as proportions of vegetable protein in composites increased, and as pH decreased to near the pi of parent source proteins.

Journal of Food BiochemistryVolume 13, Issue 2 p. 127-153 CHILLING INJURY. A REVIEW OF POSSIBLE MECHANISMS K. L. PARKIN, K. L. PARKIN Department of Food Science, University of Wisconsin, Madison, Wisconsin 53706Search for more papers by this authorA. MARANGONI, A. MARANGONI Department of Food Science, University of Guelph, Guelph, Ontario, Canada N1G 2W1Search for more papers by this authorR. L. JACKMAN, R. L. JACKMAN Department of Food Science, University of Guelph, Guelph, Ontario, Canada N1G 2W1Search for more papers by this authorR. Y. YADA, R. Y. YADA Department of Food Science, University of Guelph, Guelph, Ontario, Canada N1G 2W1Search for more papers by this authorD. W. STANLEY, D. W. STANLEY Department of Food Science, University of Guelph, Guelph, Ontario, Canada N1G 2W1Search for more papers by this author K. L. PARKIN, K. L. PARKIN Department of Food Science, University of Wisconsin, Madison, Wisconsin 53706Search for more papers by this authorA. MARANGONI, A. MARANGONI Department of Food Science, University of Guelph, Guelph, Ontario, Canada N1G 2W1Search for more papers by this authorR. L. JACKMAN, R. L. JACKMAN Department of Food Science, University of Guelph, Guelph, Ontario, Canada N1G 2W1Search for more papers by this authorR. Y. YADA, R. Y. YADA Department of Food Science, University of Guelph, Guelph, Ontario, Canada N1G 2W1Search for more papers by this authorD. W. STANLEY, D. W. STANLEY Department of Food Science, University of Guelph, Guelph, Ontario, Canada N1G 2W1Search for more papers by this author First published: April 1989 https://doi.org/10.1111/j.1745-4514.1989.tb00389.xCitations: 113AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat ABSTRACT Chilling injury (CI) is a physiological defect of plants and their products that results in reduced quality and loss of product utilization following exposure to low but nonfreezing temperatures. To design more effective control strategies and maximize shelf-life, it is necessary to develop an understanding of the biochemical mechanism(s) responsible for the initiation of CI. Despite considerable efforts in this field of study, there is no general agreement on the cause or nature of CI, or even the primary event(s) triggering low temperature damage. The first unified theory to explain CI was founded on low temperature induced membrane lipid phase transitions leading to a loss of membrane integrity and physiological dysfunction. This was modified to account for the observation that the level of certain high melting phospholipids appears to be related to the chill sensitivity of many plant tissues. Membranes and changes in their physical characteristics are further implicated as having a role in CI by the discovery that chilling stress evokes an elaborate membrane retailoring response that leads to increased fluidity at reduced temperatures. Others have postulated that CI results from the direct effect of reduced temperatures on enzymes or the indirect effect of membrane perturbations on intrinsic enzymes. The redistribution of cellular calcium has most recently been advanced as the primary transducer of CI. The weight of this theory rests on the role of calcium as a secondary messenger for many cellular functions. In this review it is also speculated that lipid peroxidation may have a role in the development of irreversible injury during low temperature stress. Its effect would be similar to the senescent processes of free radical damage to tissue and progressive membrane rigidification. Citing Literature Volume13, Issue2April 1989Pages 127-153 RelatedInformation