D.W. Stanley

Membrane effects in postharvest physiology

Abstract Plant tissue deterioration is of prime importance to food and horticultural scientists. Experimental evidence continues to be gathered supporting membrane damage as the key event leading to a cascade of biochemical reactions culminating in tissue deterioration and economic loss. In this paper, the sequence of occurrences in plant membranes during natural senescence and resulting from postharvest stresses are reviewed and compared. The picture emerging from numerous studies devoted to this topic is of membrane lipids undergoing extensive chemical degradation. Biophysical changes in membrane lipids and enzymatic and non-enzymatic lipid peroxidation leads to altered membrane properties and result in defects such as ion leakage and cellular decompartmentation. Postharvest stresses including chilling injury and freezing injury often lead to similar patterns of deterioration. However, it is too early to conclude positively that a truly universal mechanism for both senescence and postharvest stress-induced membrane damage exists.

Perspectives in the textural evaluation of plant foods

Abstract 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.

Texture-structure relationships in foamed dairy emulsions

Favorable textural qualities of foamed dairy emulsions result from the successful incorporation of air bubbles surrounded by partially coalesced fat globules into a stable product. Stabilizers, usually hydrocolloids, are often added to improve texture by controlling such factors as, in ice cream, the agglomeration of ice crystals and, in whipped cream, protection of the product against excessive drainage during storage. The mechanism of stabilizer action is not yet fully known and a clearer understanding of their role is needed to improve functionality and extend shelf life. This review focuses on recent work employing microstructural and rheological techniques aimed at achieving a better appreciation of the role of stabilizers in foamed dairy emulsions. Results from studies of ice cream and model systems have led to the conclusion that the growth of ice crystals in this product is governed by the kinetic properties of the freeze-concentrated viscoelastic liquid surrounding them and hydrocolloid stabilizers are effective due to their ability to increase viscosity of this liquid, possibly through the interaction of their chains. In whipped cream, preliminary data suggest that structure formation is a result of coated air bubbles surrounded by a viscous matrix of partially coalesced fat globules, perhaps held in place by proteins. Stabilizers increase viscosity of the serum phase leading to lower overrun but greater resistance of the product to drainage.

Textural and Microstructural Changes in Corn Starch as a Function of Extrusion Variables

Abstract The effect of extrusion variables (temperature, feed moisture and screw speed) on the expansion, breaking strength and microstructure of starch extrudates was studied using response surface methodology. For expansion the most significant variable was found to be feed moisture which bore an inverse relation to the expansion response. All the extrusion variables studied were significantly related to expansion and accounted for 95.7% of the total variation. Screw speed and feed moisture were the most significant variables for breaking strength, all three variables accounting for 89.7% of the total break strength variation. Microstructure of the extrudates could be related to the extrusion variables, the expansion and breaking strength responses. The porosity of the extrudates increased with decreasing moisture, with a concomitant increase in expansion and a decrease in breaking strength. Regression equations generated from composite rotatable response surface design experiments could be used to accurately predict the responses studied.

New dimensions in microstructure of food products

Abstract Food microstructure is the organization of elements within a food and their interaction. Food scientists break down and build up microstructure during food processing, which should be viewed as a series of restructuring and reassembling operations. Researchers in this field have benefited from newer techniques available for examining food microstructure and analyzing images. Microstructural knowledge is critical if food properties are to be controlled properly since there is a causal connection between structure and functionality. The microstructural approach to food processing and engineering is based on the realization that microstructural changes underlie product properties and microstructural techniques are necessary to understand structure-property relationships.

HPLC Separation of Bitter Peptides from Cheddar Cheese

Abstract High performance liquid chromatography was used to separate bitter peptides extracted from Cheddar cheese coagulated with chicken pepsin. A reversed phase octadecylsilane type column coupled with linear gradient elution from water to 91% methanol enabled the bitter extract to be separated into at least seventy-one compounds shown to be peptides. Gel filtration was also used but the one bitter fraction, as determined by sensory testings, obtained in this manner was separated into at least forty-four different components by high performance liquid chromatography. The chromatograms from both methods were divided into bitter and nonbitter fractions, each containing numerous peaks, according to sensory analysis. The average molecular weight of the bitter fraction obtained by gel filtration was established to be approximately 190 daltons. Ultracentrifugation of the high performance liquid chromatography fractions supported this estimate. Fractions collected by both separation techniques were subjected to sensory and chemical analyses. The two bitter high performance liquid chromatography fractions had slightly higher average hydrophobicity values than the nonbitter fractions, but this did not hold for the bitter fraction obtained by gel chromatography. Valine and leucine occurred at a higher level in the high performance liquid chromatography separated bitter fractions than in the nonbitter fractions, while lysine values were elevated in the bitter fraction separated by gel chromatography. There was an increase in retention time with increasing average hydrophobicity of the bitter extract fractions, but no such trend appeared for the pure proteins and peptides tested; there was, however, an apparent positive relationship between retention time and molecular weight.

Texture — Strueture Relationships in Texturized Soy Protein II. Textural Properties and Ultrastructure of an Extruded Soybean Product

Abstract Thermoplastic extrusion was employed to produce an experimental texturized soybean product. Physical properties relative to process temperature were examined by test methods which yielded information on product density, shear strength, work of shearing, break elongation, breaking strength, stress relaxation and water regain. Light and scanning electron microscopy were employed to examine product structure. Shear force and work (Kramer Shear Press) and product density were affected markedly by process temperature, showing an increase in the former two and a decrease in the latter with increasing process temperature. Breaking strength (Instron), shear force (Warner-Bratzler Shear) and water regain were also affected but demonstrated more complex relationships. The photomicrographs obtained showed clearly the physical changes which occurred during processing and indicated that, under our experimental conditions, maximum texturization occurred between 175°C and 192°C.

Natural Compounds for Pest and Weed Control

The control of insect pests and invasive weeds has become more species-selective because of activity-guided isolation, structure elucidation, and total synthesis of naturally produced substances with important biological activities. Examples of isolated compounds include insect pheromones, antifeedants, and prostaglandins, as well as growth regulators for plants and insects. Synthetic analogues of natural substances have been prepared to explore the relationships between chemical structure and observed biological activity. Recent scientific advances have resulted from better methods for the chemical synthesis of target compounds and better analytical methods. The capability of analytical instrumentation continues to advance rapidly, enabling new insights.

Texture-Structure Relationships in Texturized Soy Protein IV. Influence of Process Variables on Extrusion Texturization

Abstract The effects of several variables on texture (as measured by Warner-Bratzler values), extruder pressure, extruder torque, extruder throughput, product moisture level and microstructure of extruded soymeal were investigated using response surface methodology. Variables studied included extrusion temperature, screw speed, and protein and moisture levels of the meal. Protein level and extrusion temperature were found to be the most important factors in determining Warner-Bratzler values, accounting for 64% of the variation. When the effects of protein and moisture were separated by adding soy protein isolate to the feed material 77% of the variation was explained by protein. Warner-Bratzler values increased with increasing protein and temperature levels but while the effects of protein were both linear and quadratic, temperature had predominantly a linear effect Feed moisture had little influence on Warner-Bratzler values at constant protein levels; extruder pressure and torque decreased with increasing feed moisture. Scanning electron microscopy showed that protein level had a pronounced influence on product micro-structure. Increasing protein levels led to a better orientation and contexture of the protein which was in agreement with regression analyses of process parameters and Warner-Bratzler values.

Rheological properties of dextran related to food applications

Chemical, physical and mechanical properties of dextran were evaluated to assess the potential use of this microbial polysaccharide in food applications. Dextran demonstrated high solubility characteristics and promoted low solution viscosities. Newtonian behaviour was observed at concentrations 1.5% w/w. Dilute and concentrated solution behaviour indicated the conformation of dextran in solution is dependent on both molecular weight and polysaccharide concentration. At low concentrations dextran (mol. wt 500 000) demonstrated properties typical of a ‘random coil’ polysaccharide. Increased concentration resulted in the polymer chain adopting a more compact coil geometry. Non-Newtonian behaviour observed in ‘native’ dextran solutions (>1.5% w/w) is attributable to the formation of inter-chain entanglements through polymer size effects and unique branching properties. Dextran (mol. wt 500 000) demonstrated two critical concentrations (c* = 4.7% and c** = 19% w/v); quasi-elastic light scattering measurements verified the coil overlap region (c* ~ 4.6% w/v). Dynamic oscillatory evaluations indicated dextran (mol. wt 500 000) solutions exhibited dilute solution characteristics at concentrations <20% (w/w) and obeyed the empirical Cox—Merz rule. ‘Native’ dextran solutions (10–15% w/w) demonstrated mechanical spectra typical of concentrated polysaccharide solutions. Calorimetric analysis of ternary dextran:sucrose:water solutions demonstrated that dextran addition modified the frozen system behaviour of sucrose solutions. Increasing dextran concentration effectively increased the onset of melting temperatures (Tg) by as much as 12°C.