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Table of Contents:
Invisible text
The tables of contents for the Food Microstructure and Food Structure journals are now also accessible at the following URL: http://www.foodsci.uoguelph.ca/dairyedu/journal2.htm by courtesy of professor Dr. H. D. Goff at the University of Guelph, Ontario, Canada. The new URL will be available even if this website ceases to be functional. A review entitled Conventional Scanning Electron Microscopy of Bacteria was published by the author in collaboration with A. F. Yang and Denise Chabot on pages 42-61 in the June 2008 issue of Infocus, Proceedings of the Royal Microscopical Society in the UK. The review presents experience of the authors with conventional SEM of
microorganisms. Erwinia
carotovora, Listeria monocytogenes, Mycobacterium smegmatis, Salmonella
spp., Serratia marcescens, Shigella sonnei, Staphylococcus aureus, and Streptococcus thermophilus bacteria have been on this site for some time. For a variety of microorganisms and their micrographs, please see the links in the table below. Fresh spinach was removed from food stores in North America in 2006 because it was contaminated with toxigenic bacteria. Also lettuce was found to have caused food poisoning - similar to some other leaf vegetables which are consumed raw. SEM shows Escherichia coli bacteria on the leaves in the Talking about electron microscopy of foods. Some 20 years ago, when research findings were published in journals, pairs of SEM stereograms were printed on paper if there was a need to show the three-dimensional structure of the specimen. They were then examined visually using a simple optical stand, but some researchers have been able to cross their eyes and see the 3-D structures directly. This was relatively easy if the two images were properly spaced with respect to the distance between human eyes of about 68 mm. It is still possible but not as easy nowadays when micrographs are shown on monitors or projected on a screen. Yet it is sometimes useful to show the 3-D structure, particularly where minute particles are interacting with each other - not necessarily in foods, e.g., blood platelets with bacteria or bacteria with magnetic beads, etc.
Anaglyphs are pairs of stereograms within a single frame - one image is red and the other is cyan, green, or blue. They have to be viewed through (plastic) glasses of the corresponding colours. If you have them, you may find the new contribution interesting. In addition to several examples, it shows how to make anaglyphs using a scanning electron microscope.
Images of microorganisms shown in bold letters in the pink table below are available for viewing. Restoration of these sites has been slower than anticipated
One of the earlier additions to this site is a finding that a double sticky tape
is not as good a mounting material as it has been believed to be. After
a few days, bacteria on Nuclepore filters could not be examined again
because they seemed to be obscured by some unknown material. Something
oozing was through the filter...
Is it possible to obtain SEM images of stainless steel surfaces used in the manufacture of foods? Such surfaces get into contact with a variety of food ingredients and unless they are frequently cleaned, they could become sources of food-borne pathogens. Could scratches in smooth surfaces provide hiding places for some hardened microorganisms? Although it is not possible to place large subjects into electron microscopes, it is possible to replicate surfaces of interest and examine the replicas of small surface areas. A replication procedure now added to these Web sites makes it possible to show the details of steel surfaces including bacterial contamination. Does cryofixation of hydrated foods produce more accurate images of their structures than chemical fixation? It may, under certain conditions, which, however, are not easy to meet. Otherwise the structure of the sample may be distorted by the development of ice crystals. Several food microscopists have lately asked for advice on the fixation of fat for transmission electron microscopy. Although osmium tetroxide reacts with unsaturated fatty acids, the product hydrolyzes and forms a diol - osmium is released in the form of osmium trioxide. There is a way, however, to keep osmium attached to the fatty acid. The information may be found at the Food Microscopy site. Micrographs of microorganisms may be accessed from the table below, unless the sites are on the failed server. The accessible sites are shown in bold letters. The micrographs are protected by copyright. For technical and scientific information about the images featured please contact the author. Commercial interest in images of microorganisms should be directed to photo banks such as CMSP, PR, MS, VU, and others.
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The author has used electron microscopy since the seventies in the past century to study the development of milk products such as yogurt and cheeses. Some thirty years ago, he was supposed to process skim milk powder in such a way that the resulting product would resemble a "hot dog". Very little was known at that time about factors which affect various important properties of foods. It was long before 100% vegetarian meatless alternatives started to be introduced onto the food market. Needless to say that the milk-based hot dog resembled a piece of strange cheese than a juicy hot dog. Electron microscopy showed what was the problem: Meat-based hot dogs consist of meat fibres and solidified gelatin gel with fat particles dispersed in it. On heating, gelatin liquefies and produces the "juice". The meat fibres give the product an elastic feeling. In contrast, gelled milk was found to consist of densely packed casein micelles, i.e., protein particles so small that they cannot be viewed under optical microscopes and a much higher resolution of the electron microscope must be used. The milk has enough mystery in its ability to form a great variety of milk products that it became very interesting to study the ways in which such products are made. It is fascinating to see and taste a great variety of cheeses and also follow the milk on its way from the cow, goat or ewe through the process of cheese making to a Brick cheese, Cottage cheese, Old Cheddar cheese, Camembert cheese, Mozzarella cheese etc. And then there are other milk products such as yogurt, cultured buttermilk, and kefir. Interestingly all these products involve the presence of so-called lactic acid bacteria and some other microorganisms. Studies of food microstructure help to understand some of very important physical properties of foods such as elasticity or firmness and sensory attributes such as grittiness. Industrial production of foods and, in particular, the development of new foods can use the information collected and analyzed by food microscopists. . In the past several years, the author's interest has changed from foods to foodborne microorganisms. Some of them are useful (probiotic), which means that their presence in the gastrointestinal tract of animals and humans is beneficial, in part suppressing detrimental microbes. Lactic acid bacteria and bifidobacteria are good examples. However, there are also harmful (pathogenic) microorganisms such as Salmonella sp., Campylobacter jejuni, Listeria monocytogenes, Shigella sp., Clostridium sp., Staphylococcus aureus etc., which cause food poisoning and other health problems. During the past several years, the "intestinal rod" (Escherichia coli) as a common part of the intestinal microflora developed a toxigenic mutant (E. coli 0157:H7) which is of great concern to food processors. Several years ago, these toxigenic bacteria resulted in the recall of fresh spinach and lettuce from the market. Some of the microorganisms may be viewed from the pink table above. Several years ago, Dr. Petr Dejmek at the University of Lund in Sweden made it possible for the author to place many micrographs on the university server. This possibility does not exist any more and the variety of information has been reduced to fit into the server space allotted to residential subscribers. Yet, there are several interesting topics featured on this website as the table at the top shows. Work of several food microscopists is featured under the titles of "Guest Food Microscopists 1", "2" and "3". Titles of scientific papers published in the Food Microstructure and Food Structure journals are also available. Some links for other sites on food microscopy are shown below:
Food Microscopy (Microscopy Handbooks) (Paperback)
by O. Flint
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