Baraem Ismail

Invited review: Plasmin protease in milk: Current knowledge and relevance to dairy industry

Plasmin is by far the predominant and most completely studied endogenous protease in bovine milk. Plasmin-induced proteolysis can have either beneficial or detrimental effects on the texture and flavor of dairy products, depending on the extent of hydrolysis and type of dairy product. In cheese, the breakdown of protein can help develop desirable flavors and texture during ripening, whereas in pasteurized milk and ultra-high-temperature milk, proteolysis causes undesirable gelation. Plasmin is part of a complex protease-protease inhibitor system in milk that consists of active and inactive forms of the enzyme, activators, and inhibitors. Considerable research has been done to isolate and characterize components of the plasmin system, determine how they interact, develop and compare quantitation methods, and determine how they are affected by cow characteristics, processing conditions, other milk components, storage conditions, and bacterial proteases. Considerable research has focused on enhancing or minimizing the activity of plasmin system components. The intent has been to control protease activity in casein and whey fractions, depending on the final food or ingredient application. Controlling the activity of plasmin has a great potential to improve dairy product quality and reduce their processing costs.

Development of a total quality scoring system based on consumer preference weightings and sensory profiles: application to fruit dates (Tamr)

A scoring system for evaluating the total quality of a food product (Tamr) based on consumer preferences was developed and tested for validity. The development process started with a survey among adult date consumers in the United Arab Emirates to identify their perception of characterizing total quality of date fruit among specified attributes. The consumer subjective ranking of eleven specified attributes was used to derive a quantitative weighted factor for each attribute. Based on the weighted factor a quantitative scoring guide was developed. A panel evaluation of five date varieties (Khlas, Barhee, Boumaan, Fard and Ruzeiz) was conducted as an application of the developed scoring system. Panel evaluation results and consumer ranking and preference data of the same varieties compared well. In both panel evaluation and consumer ranking, Khlas variety was perceived to have the best quality by far among the tested varieties. Barhee and Boumaan varieties showed no significant variation between them in both panel evaluation and consumer ranking, and both were in the second order of preference. Variety Ruzeiz was in the lowest order of preference in both panel evaluation and consumer ranking. Testing of the method indicated its appropriateness in predicting total quality of a food product as would be perceived by consumers.

The Role of Salivary Proteins in the Mechanism of Astringency

Understanding astringency has focused on the interaction of tannins with the salivary proline-rich proteins (PRPs), although it remains unclear if other astringents precipitate the PRPs or how this interaction relates to sensory perceptions of astringency. We used 2 approaches to compare how distinct classes of astringent compounds interacted with the salivary PRPs and mucins. Using sodium dodecyl sulfate polyacrylamide gel electrophoresis, we evaluated protein patterns and characterized the salivary proteins present in the supernatants and pellets of pooled saliva assayed with tannin, alum, and hydrochloric acid solutions. Tannins and alum precipitated many of the PRPs, but acid did not. Mucins were precipitated by both the acid and alum, but not by the tannins. From our research, it appears that the precipitation of salivary proteins may be involved in the mechanism of astringency, but the precipitation of PRPs is not requisite for the development of astringency. We also measured mucin and deoxyribonucleic acid content of expectorated solutions of astringents that panelists swished in their mouths to determine if astringency was associated with a loss of oral lubricating films.

Angiotensin converting enzyme inhibitory activity of soy protein subjected to selective hydrolysis and thermal processing.

Soy protein isolate (SPI) and β-conglycinin- and glycinin-rich fractions were hydrolyzed using papain and pepsin. Protein denaturation, profiling, and peptide identification were carried out following DSC, SDS-PAGE, and liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis. The in vitro antihypertensive activity of the hydrolysates was compared by determining the angiotensin converting enzyme (ACE) inhibitory activity. SDS-PAGE and LC-MS/MS analysis confirmed pepsin selectivity to glycinin and papain partial selectivity to β-conglycinin when the protein is least denatured. Both the papain-hydrolyzed SPI and the papain-hydrolyzed β-conglycinin-rich fraction had more than double the ACE inhibitory activity of that of pepsin-hydrolyzed SPI and pepsin-hydrolyzed glycinin-rich fraction. This observation indicated that β-conglycinin is a better precursor for antihypertensive peptides than glycinin. Additionally, the inhibitory activity of the papain-hydrolyzed SPI was thermally stable. This work demonstrated, for the first time, that selective hydrolysis to release peptides with ACE inhibitory activity can be accomplished without inducing extensive hydrolysis and performing unnecessary fractionation.

Basic Principles of Chromatography

Chromatography has many applications for analysis of various foods components. It differs from other methods of separation in that a wide variety of materials, equipment, and techniques can be used. This chapter focuses on the principles of chromatography, as background information for the detailed principles and application of high-performance liquid chromatography (HPLC) and gas chromatography (GC) covered in Chap. 13 and 14, respectively. This chapter gives the basics of chromatography as a separation method based on the partitioning of a solute between a mobile phase and a stationary phase. Based on the physicochemical characteristics of the analyte and the availability of instrumentation, a chromatographic system is chosen to separate, identify and quantify the analyte. Chromatographic modes include adsorption, partition, hydrophobic interaction, ion exchange, affinity, and size exclusion chromatography. Factors to be considered when developing a separation include mobile phase variables, as well as column efficiency, selectivity, and capacity. Following detection, a chromatogram provides both qualitative and quantitative information via retention time and peak area data.

The effects of processing and extraction conditions on content, profile, and stability of isoflavones in a soymilk system.

The effect of processing temperature and pH as well as enzyme-assisted extraction on the content and profile of isoflavones in a soymilk system was investigated. Isoflavone content in thermally treated soymilk at pH 7 and pH 9 was determined following a standard solvent extraction or an enzyme-assisted extraction protocol. Upon thermal processing, at both pH 7 and pH 9, significant interconversions were noted, indicated by the observed decrease in malonylglucosides with the concurrent increase in beta-glucosides. Enzyme-assisted extraction resulted in enhanced isoflavone extraction efficiency and revealed significant loss in total isoflavone content upon processing. This observation suggested that protein-isoflavone interactions, which are dependent on the protein structure and isoflavone form, affect isoflavone extractability, leading to underestimation of any loss that might have occurred in previously reported thermal studies. Accurate isoflavone measurements are essential to determine the processing conditions that result in the least loss of the biologically relevant isoflavone content.

Analysis of Food Contaminants, Residues, and Chemical Constituents of Concern

Consumer concerns and government regulations focused on the safety of foods dictate the need for analysis of various food contaminants, residues, and chemical constituents of concern.These compounds include pesticide residues, mycotoxins, antibiotic residues, GMOs, allergens, food adulterants, packaging material hazardous chemicals, environmental contaminants, and certain other chemicals.This chapter covers the screening methods and quantitative methods that are commonly used for the detection and quantification of several food hazards, in addition to some recently developed methods for the detection of newly identified and emerging food hazards.Included are both rapid screening methods and more time consuming quantitative methods required to meet the needs of industry and government, in an effort to ensure a safe and reliable food supply.Sampling and sample preparation are covered because they can be a significant challenge due to the low levels of the chemicals and the complex food matrices.Methods referred to in this chapter include many techniques covered in other chapters (e.g., high-performance liquid chromatography, gas chromatography, mass spectrometry, immunoassays, enzyme inhibitor assays), and others not previously covered (DNA methods using polymerase chain reaction, certain colorimetric assays).This chapter on chemical contamination is intended to compliment that of Chap. 35 on Food Forensic Investigations.

Enzymes Indigenous to Milk | Plasmin System in Milk

Proteolysis in milk caused by the indigenous protease plasmin is one of the most important contributors to the quality of milk and its products. Plasmin-induced proteolysis can have either beneficial or detrimental effects, depending on the purpose of processing and processing conditions. In cheese, the breakdown of protein can help develop desirable flavors and texture during ripening, while in pasteurized milk and ultra-high temperature milk, proteolysis causes undesirable gelation. Plasmin is part of a complex system including its inactive form plasminogen, plasminogen activators, and plasminogen inhibitors. The components of the plasmin system interact together and with other components of milk to promote or inhibit proteolysis, mainly of the caseins, in milk and milk products depending on the thermal processing employed and on the storage conditions. Many factors such as pH, heat, mineral content, whey proteins, and storage temperature can influence the kinetics of plasmin-induced hydrolysis. The components of the plasmin system have been successfully isolated and characterized. Thermal stability of each of the components of the plasmin system as well as their interactions with caseins and whey proteins as affected by thermal processing has been investigated. A brief overview of the present state of knowledge and future research requirements is presented in this article.