Miloslav Kalab

Microscopy and other imaging techniques in food structure analysis

Abstract Microscopy and imaging techniques are the most appropriate techniques for evaluating food structure because they are the only analytical methods that produce results in the form of images rather than numbers. However, images may now also be converted into numerical data to allow for statistical evaluation. Advances in microscopy and imaging techniques are made, for the most part, outside the field of food science, drawing from the fields of materials science, biology and medicine. Such techniques cannot, in most cases, be directly applied to study food structure. They must be adapted because the processing conditions that turn biological raw materials into food cause structural and textural changes which, in turn, change the innate properties and behaviour of the foods. This necessitates the development of appropriate methods and also the specialization of researchers. Future developments in this field can be divided into the use of new equipment developed for use in other fields, and the application of techniques modified to solve specific food science problems, such as the development of new foods with particular properties and texture or the detection of defects in foods.

Processed Cheese Products

Processed cheese is produced by blending shredded natural cheeses of different types and degrees of maturity with emulsifying agents, and by heating the blend under a partial vacuum with constant agitation until a homogeneous mass is obtained. In addition to natural cheeses, other dairy and non-dairy ingredients may be included in the blend.

Staphylococcus epidermidis forms biofilms under simulated platelet storage conditions

BACKGROUND: Staphylococcus epidermidis grows slowly in platelet (PLT) preparations compared to other bacteria, presenting the possibility of missed detection by routine screening. S. epidermidis is a leading cause of nosocomial sepsis, with virulence residing in its ability to establish chronic infections through production of slime layers, or biofilms, on biomedical devices. This study aims to establish biofilm formation (BF) as a mode of growth by S. epidermidis in PLT preparations.

Effect of whey protein denaturation on structure of casein micelles and their rennetability after ultra-high temperature processing of milk with or without ultrafiltration

Abstract Whole milk and skim milk were ultrafiltered to 3× concentration and heated to 72°C, 89°C, 106°C, 123°C, or 140°C in an ultra-high temperature (UHT) plate heat exchanger. Whey protein denaturation increased with heating temperature and was higher in concentrated milk than in the corresponding unconcentrated milk. There was no difference in the patterns of whey protein denaturation and rennet coagulation times between whole milk and skim milk. Rennet coagulation time increased and gel strength decreased with milk processing temperature. Concentrating milk by ultrafiltration shortened coagulation time and increased gel firmness. UHT processed milk did not coagulate when rennet was added. Its 3× concentrated counterpart did coagulate although only a weak gel was formed. UHT heating caused the casein micelles to increase in size with additional protein material adhering to their surface, especially in the 3× skim milk heated to 140°C. This diffuse layer of material around casein micelles was not observed in 3× whole milk. It is suggested that this denatured protein is adsorbed onto the fat-water interfaces during homogenization.

The manufacture of set-type natural yoghurt containing different oils: 2. Rheological properties and microstructure

Abstract Yoghurts rich in poly- and mono-unsaturated fatty acids were manufactured from reconstituted skimmed milk powder using vegetable oils (olive, groundnut, sunflower or maize) to replace the milk fat. They were compared with a yoghurt-containing anhydrous milk fat [AMF, 1.5% ( w w )]. The vegetable oil-based yoghurts [at a 1.5% ( w w ) level of addition] were different from the control yoghurt containing AMF. Whey separation was higher and firmness was lower for all vegetable oil-containing yoghurts than for the product containing AMF. Scanning electron microscopy revealed similar microstructure, including the distribution of void spaces filled with milk serum (porosity of the protein matrices), whereas transmission electron microscopy showed the interaction of the vegetable oil droplets with casein micelles and their participation in the formation of the gel matrices.

Serratia marcescens strains implicated in adverse transfusion reactions form biofilms in platelet concentrates and demonstrate reduced detection by automated culture.

Background and Objectives  Serratia marcescens is a Gram‐negative bacterium that has been implicated in adverse transfusion reactions associated with contaminated platelet concentrates. The aim of this study was to investigate whether the ability of S. marcescens to form surface‐attached aggregates (biofilms) could account for contaminated platelet units being missed during screening by the BacT/ALERT automated culture system.

Effect of platelet additive solution on bacterial dynamics and their influence on platelet quality in stored platelet concentrates

BACKGROUND: Platelet additive solutions (PASs) are an alternative to plasma for the storage of platelet concentrates (PCs). However, little is known about the effect of PAS on the growth dynamics of contaminant bacteria. Conversely, there have been no studies on the influence of bacteria on platelet (PLT) quality indicators when suspended in PAS.

The manufacture of set-type natural yoghurt containing different oils — 1. Compositional quality, microbiological evaluation and sensory properties

Yoghurts rich in poly- and mono-unsaturated fatty acids were manufactured from reconstituted skimmed milk powder using vegetable oils (olive, groundnut, sunflower or maize) to replace the milk fat. They were compared with a yoghurt containing anhydrous milk fat [AMF, 1.5% (ww)]. The vegetable oil-based yoghurts [at a 1.5% (ww) level of addition] were different from the control yoghurt containing AMF. The microbiological quality of all the yoghurts was excellent, and the yoghurt starter organisms were in abundance. The sensory panel identified significant differences (P < 0.05) between products containing AMF and vegetable oils in terms of whey separation and some flavour and aroma attributes (acidic, oxidised, unclean and after-taste). Copyright

Effects of freezing and frozen storage on the microstructure and texture of ewe's milk cheese

Semi-hard ewes milk cheeses, frozen immediately after manufacture either slowly at −35 °C or rapidly at −80 °C and stored at −20 °C for 4 months were studied for microstructural and textural characteristics during subsequent ripening. Two control groups were used to establish the effect of freezing: the fresh unfrozen cheese and cheese thawed immediately after freezing. Freezing proper did not result in any marked changes in the textural parameters of the cheeses, but considerable changes were found in slowly frozen cheeses after 4 months of frozen storage. Shear strength values were lower in all frozen and stored cheeses, particularly in cheese samples frozen slowly compared to those in the unfrozen control batch. This parameter and firmness values were significantly lower in both slowly and rapidly frozen cheeses at the completion of ripening. Ripening tended to offset differences in elasticity, noticeable in the cheeses during the first 30 days of ripening. Light microscopy and electron microscopy revealed small cracks and ruptures in the cheeses which could not be observed by the naked eye. More extensive damage to the cheese microstructure was found in slowly frozen cheese samples stored frozen for 4 months.

Biofilm formation by Staphylococcus capitis strains isolated from contaminated platelet concentrates

Bacterial contamination of platelet concentrates (PCs) poses the greatest infectious risk in modern transfusion medicine despite the implementation of measures such as improved skin disinfection and first aliquot diversion. The majority of PC contaminants are commensal skin flora introduced by venipuncture at the time of blood collection. The predominant organisms are Gram-positive coagulase-negative staphylococci such as Staphylococcus capitis. This bacterium has been implicated in numerous instances of infection and sepsis, likely for its ability to form surface-associated communities of micro-organisms encased in extracellular materials, known as biofilms. In the present study, five strains of S. capitis isolated from contaminated PCs were assessed for their ability to produce extracellular polysaccharide (slime), a canonical indicator of biofilm-formation ability, on Congo red agar plates. Biofilm formation was evaluated in both glucose-enriched trypticase soy broth (TSBg) and in PCs by using a crystal violet staining assay. The chemical nature of the biofilms was evaluated by disruption assays using sodium metaperiodate and proteinase K. In addition, biofilm architecture was observed by scanning electron microscopy. The presence of the biofilm-associated icaR and icaADBC genes was also examined by PCR. While only two out of the five S. capitis strains formed biofilms in TSBg, all strains formed biofilms in PCs. The ability of strains to produce extracellular polysaccharide and their possession of wild-type ica genes were not exclusive predictors of biofilm formation in TSBg or PCs; different profiles of biofilm markers were observed among isolates. This is likely due to the proteinaceous composition of the S. capitis biofilm matrix. Interestingly, an ica-negative, non-slime-producing isolate was capable of biofilm formation in PCs. Together, these data indicate that the platelet storage environment stimulates biofilm formation in S. capitis in the absence of extracellular polysaccharide production and that multiple bacterial factors and regulatory elements are likely involved in biofilm formation in this milieu.