Atomic force microscopy has been used to image the bacterial polysaccharides xanthan, acetan and gellan. Images were obtained under constant force conditions in a liquid cell. Drops of dilute solutions of the polysaccharides were deposited onto freshly cleaved surfaces of mica and allowed to dry in air. The deposits were then imaged under butanol. Xanthan and acetan form entangled networks upon deposition. Individual molecules can be identified. Under suitable circumstances it has been possible to image the helical structure of acetan. Gellan forms gels upon concentration during drying and images have been obtained of the gel network.

The role of fat crystals in the stabilisation of air bubbles in cake batters was studied with aqueous phase aerations prepared by the "all in one" method using shortening containing emulsifier. During mixing, fat crystals become coated with an interfacial layer of adsorbed protein (crystal-water interface). They adsorb to the surface of bubbles (that have been transiently stabilised by egg proteins) by a process which involves the fusion of the crystal-water interface with the air-water interface. The adsorption of fat crystals helps to stabilise large numbers of small bubbles which must expand without rupturing during baking if the batter is not to collapse. During heating, the fat crystals melt, the oil runs over the internal surface of the bubble and leaves behind the crystal-water interface which can be used by the bubble to provide extra interface during its expansion. Implicit in this mechanism is the explanation for: a) how fat moves from the aqueous phase to the inside surface of air bubbles during cooking; b) why oil does not produce the same degree of cake aeration as the same weight of crystalline fat; and c) the fact that shortenings containing crystals of the beta' polymorph perform better than those containing the larger beta polymorph. A similar mechanism may operate in other aerated cereal products, such as bread, in which crystalline fat is known to produce similar improvements in volume and crumb structure.

Foods containing aqueous solutions of proteins readily foam when air is introduced into them. When fat is also present, interaction between the two compo­ nents at the air/ water interface may produce a stable foam with characteristic bulk properties. In the case of dairy foams (such as whipped cream), bubbles produced in the whipping process are initially stabilised by the adsorption of protein at the air/water interfaces. Commonly encountered defects in whipping cream arise when large triglyceride crystals , formed in masses of free fat, adsorb to the air/ water interface during whipping at the expense of fat globules. In other food systems, fat crystal adsorption is part of the normal stabilizing mechanism. In cake bat­ ters produced by 'creaming' with shortening, air is held in the fat phase and stabilised by .B'-gystals; however, bubbles move into the aqueous phase during cooking. Studies with batters in which the air is in the aqueous phase have shown the importance of both protein and .B' -crystals in air stabilisation. The added importance of fat crystals is that they provide extra protein air/ water interface for the bubbles as they expand during cooking and, thereby, help to preserve the texture of the final product. The fat protein interactions observed at the air/ water interface of several different foams, provide a uni­ fying mechanism for the stabilisation of air in many di­ verse food systems.

Low temperature scanning electron microscopy in conjunction with X-ray microanalysis can be used to study not only the internal structure of intact bulk food systems but also the distribution of their constitutive chemical elements. However, the considerable practical problems that are usually encountered when performing X-ray analysis on frozen samples include: a) the controlled deposition of a good quality carbon film to prevent charging, b) producing digital elemental distribution maps of elements whose X-ray spectral energy peaks partially or completely overlap and c) controlling the plane of fracture through the specimen and ensuring that the resulting topography allows meaningful analysis to be performed.The quality and control of carbon film deposition is greatly improved by the inclusion of a turbomolecular pump in the vacuum system and by using thickness monitoring of the carbon film during deposition. However, the complications of energy peak overlap in digital X-ray mapping can only be overcome by using a procedure which produces maps corrected for all of the spectrum processing routines normally available in quantitative programmes. Another advantage of this approach is that statistics such as FIT index (see appendix) and standard deviation are available for each pixel.Problems associated with fracturing are avoided if the internal structure of materials is revealed using cryo-milling. This procedure uses a rotating diamond cutting tool to produce a very flat surface on the frozen specimen which is ideal for X-ray analysis and for image analysis of structural components. Quantitative X-ray mapping of milled cocoa beans shows that this preparation procedure does not cause smearing of the chemical components across the surface of the specimen.

The interfacial structure of air bubbles in normal and defective whipped creams were compared, using freeze fracturing and transmission electron microscopy, in an attempt to understand the underlying mechanism of the observed gross differences in their whipping times and overruns. In normal whipped creams sparsely distributed fat crystals were found to have penetrated some of the bubbles and were lying in the plane of the air/water interface.In defective whipped creams, large numbers of needle-like crystals had penetrated the air/water interface of every bubble and, as a consequence, reduced numbers of fat globules were found to have adsorbed. There was also morphological evidence that the crystals reached the interface before the milk fat globules during the whipping process. The presence of large masses of free fat in the aqueous phase of whipped cream, on whose surface arrays of very long fat crystals were found, suggested that the needle-like crystals were dislodged by the shear forces generated during whipping and were then free to adsorb to bubbles. The detection of such large amounts of free fat indicated a large scale damage to fat globules during processing with the consequent escape of both crystalline and liquid fat.Possible mechanisms to account for the low overrun and long whipping times in defective creams are discussed.

The bacterium Lactobacillus hilgardii (previously named L. brevis), isolated from sugary kefir grains, has been found to produce a gelling extracellular polysaccharide. Methylation analysis and 1H- and 13C-NMR analysis revealed a dextran-like structure for the polysaccharide. A high proportion of α-(1,6)-Glc and small proportions of α-(1,3,6)-Glc, α-(1,4,6)-Glc and terminal α-Glc were detected. The L. hilgardii polysaccharide differed from normal dextrans in containing a small proportion of α-(1,3)-Glc. The structure of the polysaccharide was found to be independent of harvesting time and unaffected by changes in the growth medium. The development of the gel structure has been followed by transmission electron microscopy.

x-ray microanalysis was used to compare the elemental composition of isolated cocoa solids with milk powder. Whereas mllk powder contained similar high levels of calcium and potassium, cocoa sol ids were rich in potassium but contained very small amounts of calcium . This difference in elemental composition allowed the two populations of particles to be

Summary A method of extracting cheese juice is described. Juice from a single one-month-old Cheddar cheese was used to investigate the relationship between the composition of the cheese juice and the occurrence of crystalline aggregates in the cheese. Two classes of crystalline aggregates were identified by light microscopy and also by electron microscopy in combination with X-ray microanalysis: the larger class contained both Ca and P whereas the smaller class contained Ca but not P. Concentrations of ions in the cheese and cheese juice were determined and corrected for the excluded volume of co-solutes and other phases present. When this was done, comparison of the concentrations of Na, K, Cl and lactate in the cheese and the cheese juice indicated that they were virtually entirely in solution, whereas appreciable amounts of the total Ca, P and P1 in the cheese were not extracted in the cheese juice. The amounts of the non-extractable salts were consistent with the volume fractions of the crystalline aggregates found by microscopy; calculations of the ion equilibria in the cheese juice showed that it was supersaturated with respect to various Ca phosphate salts and to tricalcium citrate. It is concluded that the physicochemical origin of the crystalline aggregates was the nucleation and growth of crystals from a supersaturated solution, though the sites of nucleation may have been formed by the microbial activity in the cheese. Of the free amino acids found in the juice, none was at a sufficiently high concentration to form a saturated solution, indicating that in this relatively young cheese the crystalline aggregates were not formed from amino acids.

Textural attributes of Cheddar and Cheshire cheeses, falling within narrow compositional ranges, were assessed by sensory panels, and from force‐compression curves generated by compression between two plates, and, for Cheddar cheese only, by penetrometry. Individual sensory measurements did not relate well to any instrumental one, and were better at discriminating between cheeses. Samples of each cheese variety were fractured in different ways and the fracture surfaces were examined in a scanning electron microscope. Fracture surfaces were formed by cutting directly through the matrix, tearing of the matrix along planes high in fat or cracking at grain boundaries. It is suggested that consideration of fracture mechanism may aid the selection and development of useful instrumental methods for texture assessment of cheese.

Defects in cottage cheese curd were examined by electron microscopy and the structure of defective curd was compared with that of normal curd at cutting. The formation of the minor sludge frequently found as a thin layer at the bottom of vats was attributed to the very low pH produced by the sedimentation of agglutinated starter bacteria. In contrast, the ultrastructural appearance of curd from incidents of major sludging, in which whole vats were lost, suggested that a slowing down of acid production had caused the defect by allowing aggregated micelles to sediment. It is suggested that in such cases loss of acid production is the result of starter destruction by low initial numbers of bacteriophages.

The problem of cottage cheese sludge was investigated by attempting to recreate sludge in experimental vats and by retrospective examination of sludge as well as samples of starter and skimmed milk used in cases of commercial vat failures. Two independent types of sludge problem were identified. Minor sludge occurred regularly on the bottom of vats and was unconnected with major sludge which leads to total loss of vat contents. Major sludge was unrelated to composition of the skimmed milk and was probably caused by phage which infected the vat after development of acidity had begun.

SUMMARYInvestigation of the effect of pH on the formation of deposit from milk during ultra high temperature treatment using a plate-type plant showed that deposit formation was greatly increased when the pH of whole milk was reduced to 6·54, irrespective of whether the adjustment was made through the addition of HCl or lactic acid. Most of the increase in deposition took place in the higher temperature sections of the plant. Conversely, an increase in milk pH to 6·8 using NaOH resulted in considerably less deposit being formed during heat treatment. Reducing the pH of whole milk increased the deposition of both protein and fat, but reduced the deposition of minerals. Despite very high concentration of fat in the deposits, it is unlikely that fatper sewas responsible for increased deposit formation. Deposition also increased when the pH of skim milk was reduced to 6·51 before processing. Electron micrographs of the milks after heat treatment indicated that pH reductions caused the formation of large aggregates containing casein micelles during heating. Fat globules were also present in aggregates formed in whole milk with reduced pH. Slight reductions in the pH of milk before processing appear to enable the pH during heat treatment to fall below a critical value at which coagulation of milk takes place at the heated surfaces.

The formation of sludge on the bottom of vats during cottage cheese manufacture was investigated. The sludge layer contained 4.23‐8.11% of the casein originally present in the vat and was probably caused by agglutinins present in the skimmed milk. Homogenization of the skimmed milk before addition of starter reduced the agglutination of bacteria during the early stages of acid development and eliminated minor sludge formation. Curd yield was increased by up to 5% and average manufacturing time was reduced by 40 min following homogenization.

SUMMARY The pathology of hyperacute coliform mastitis was studied in 5 postparturient cows. In all infected quarters infiltration of neutrophils was negligible. In all except one case there was severe damage to the ductular and secretory system, involving most areas of the gland. Bacteria were dense in infected alveoli, and there was evidence of substantial phagocytosis of bacteria by the secretory epithelium. The exception showed a large lesion in the middle of the gland from which the spread was ductular; other infections were consistent with spread via the teat canal. The organisms were largely confined to the ductular/secretary lumen and there was little invasion of the parenchyma. The severity of the disease was considered due to the absence of the inflammatory response seen in mid lactation.