The Milk Microbiology section contains information relating to microbial concerns in milk. A brief overview of dairy microbiology is presented below as an introduction this section.
The topics covered are:. Milk is virtually sterile when it is synthesized in a healthy cow's udder mammary gland. Cows, like humans, are natural reservoirs of bacteria. Many of these bacteria are not harmful to humans, but some may be harmful to humans even though the cows are not affected and appear healthy. Milk may become contaminated with bacteria during or after milking. All butter contains some micro-organisms. However, proper control at every stage of the process can minimise the harmful effects of these organisms.
An adjustment of the fat content of cream is required, or if the fat content of whole milk must be reduced to a given level, skim milk must be added. This process is known as standardisation. Tests are available to know the milk microbiological quality.
Bacteria, coliform and somatic cell counts are frequently used. Coliforms are generally only present in food that has been fecally or environmentally contaminated.
Somatic cells are blood cells that fight infection and occur naturally in milk. The presence of mastitis an infection of the mammary gland in the cow will increase the somatic cell count. The somatic cell count can be determined by direct microscopic examination or by electronic instruments designed to count somatic cells. Antibiotics are used to treat mastitis infections.
Cows under antibiotic treatment for mastitis infections may have antibiotic residues in their milk, therefore, milk from treated cows is either discarded or collected into a separate tank. Milk containing antibiotic residues is not used for human consumption. The legal standard, as defined by the Food and Drug Administration FDA , requires that milk contain no detectable antibiotics when analysed using approved test methods.
Regulatory action is taken against the farm with the positive antibiotic test. Some features of this module might be unavailable. Milk composition and microbiology. Mode: Lesson. Milk composition. Milk microbiology. Factors affecting milk composition. Milk microbiology In addition to being a nutritious food for humans, milk provides a favorable environment for the growth of microorganisms.
Table 3. Bacterial types commonly associated with milk. Microbial growth Microbial growth can be controlled by cooling the milk. Milk pasteurisation Pasteurisation is the process used to destroy bacteria in milk.
Effects of pasteurisation on milk Pasteurisation reduces the cream layer, since some of the fat globule membrane constituents are denatured. Milk sterilisation In pasteurisation, milk receives mild heat treatment to reduce the number of bacteria present. Microbiology of butter Butter is made as a means of extracting and preserving milk fat.
Equipment In smallholder buttermaking, bacterial contamination can come from unclean surfaces, the butter maker and wash water. Wash water Wash water can be a source of contamination with both coliform bacteria and bacteria associated with defects in butter. Air Contamination from the air can introduce spoilage organisms: mould spores, bacteria and yeasts can fall on the butter if it is left exposed to the air.
Packaging Care is required in the storage and preparation of packaging material. Personnel A high standard of personal hygiene is required from people engaged in buttermaking. Control of microorganisms in butter Salting effectively controls bacterial growth in butter. Standardisation of milk and cream An adjustment of the fat content of cream is required, or if the fat content of whole milk must be reduced to a given level, skim milk must be added. Microbial tests for raw and pasteurised milk Tests are available to know the milk microbiological quality.
Somatic Cell Count Somatic cells are blood cells that fight infection and occur naturally in milk. Antibiotics in milk Antibiotics are used to treat mastitis infections. Bacteria have since been shown to preferentially locate at the fat—protein interface and sometimes within whey pockets in dairy products. The growth of bacteria within fermenting food products is of extreme importance as this growth rate determines final cell numbers, acidification rates and thus the intensity of the fermentation process.
Starter bacteria are also responsible for releasing intracellular enzymes upon death and subsequent lysis of the cell membrane. These enzymes catalyze a wide range of metabolic pathways lipolysis, proteolysis, and glycolysis which result in the formation of flavor compounds adding to flavor development in many cases of fermented foods Wilkinson et al.
Each bacterial cell is believed to grow and form a colony within the food matrix after inoculation and in certain cases immobilization within the matrix Jeanson et al. It is the activity, location, and environment of these colonies which are of interest in this review. To date a lot of research has been undertaken with relation to bacterial growth in various conditions in dairy foods. This review will focus on colony growth, location, and influence on the surrounding environment of starter, non-starter LAB, spoilage and pathogenic bacteria in dairy products such as cheeses, sour creams and yogurts using microscopy to accurately quantify and visualize the bacteria in the food matrices.
Methods for analysis of food can be separated into separate categories such as microbiological, microscopy, sensory, physical, and physico-chemical. These methods can be destructive or non-destructive in their procedure. For this review we will focus on the microscopy methods associated with dairy food analysis. Examples of non-destructive microscopic methods include confocal laser scanning microscopy CLSM , cryo- and regular scanning electron microscopy SEM , and transmission electron microscopy TEM which have helped to map the location and distribution of bacterial colonies in dairy foods.
The 16S rRNA gene is universal amongst bacteria and large databases exist for specific species of most food related bacteria. They have been used to determine the presence and quantity of undesirable bacteria such as C. However, the issue with these methods of bacterial detection is the lack of information we obtain relating to bacterial colony location and distribution within dairy foods. For these reasons, this review focuses on microscopy methods allowing for the visualization of bacterial colony location and distribution.
The use of CLSM in food analysis has been at the forefront in recent years due to its ability to image individual components within a food matrix via the use of various fluorescent dyes Auty et al. This method is favored as a result of its ability to visualize thin optical sections below the surface of a sample due to the laser scanning function.
Another advantage is its ability to analyze various components simultaneously via the use of fluorescent labels and stains, allowing for fat, protein, and bacterial colony location to be identified from one sample Ong et al.
Thus, viable bacterial cells fluoresce green and those with a damaged or non-viable membrane fluoresce red Auty et al. Bacterial location alone can be measured using Acridine orange which is a fluorescent dye which stains the DNA of bacteria Lopez et al. This methodology has also been utilized recently to identify and track pathogenic bacterial growth in dairy foods Fleurot et al. Conventional SEM involves the generation of an electron beam which interacts with a given sample resulting in the emission of multiple secondary electrons.
The image obtained is based on the electrons which scatter back when the electron beam strikes the surface of the sample McMullan, In order to obtain a high number of secondary electrons and therefore give a clear image, a conducting layer is often placed over the sample surface to prevent charging El-Bakry and Sheehan, The dehydration of the sample using a series of ethanol concentrations is necessary prior to examination, which must also be carried out under vacuum.
This method has been used to study food microstructure for many years, offering a clear concise image of a samples surface showing fat, protein, and bacterial location Pitino et al. The limitations associated with this methodology are the labor intensive sample preparation and the ability to only view the topographical area of a sample in addition to artifacts Tunick et al. Cryo-SEM comprises conventional SEM with a cryo-chamber attached allowing for the microscopic examination of dairy foods high in moisture, fat or air, i.
The advantages of this method over conventional SEM include a substantially reduced sample preparation time and a greater ability to view fat components which can become distorted due to the dehydration and defatting steps associated with conventional SEM. The use of this method has increased in recent years, in conjunction with techniques such as CLSM, for the study of dairy food microstructure and microbial population Hassan et al.
Transmission electron microscopy is similar to its scanning counterpart in that a beam of electrons are used but in this case the electrons pass through transmit the sample and the image generated is based on the scatter of these electrons, therefore samples need to be very thin 0. In relation to dairy analysis, replica type TEM is most commonly used.
Advantages include the best resolution of all electron based microscopy techniques allowing for greater examination of a samples ultra-structure Laloy et al. Disadvantages consist of high cost, labor intensive sample preparation and possible presence of artifacts due to the use of osmium tetraoxide, which can cause fat and proteins to be misinterpreted due to inadequate fixing of the structures during sample preparation Reis and Malcata, ; Auty et al. The use of these microscopy techniques in relation to bacterial location, survival and distribution in cheeses, yogurts and soured creams are discussed below in relation to the various studies which have been conducted on this important topic.
Location and distribution of various types of starter bacteria, NSLAB, contamination bacteria, and spoilage bacterial strains in a number of cheese varieties has been widely studied but rarely visualized using microscopic methods. Hannon et al. The ability to visualize the location of these bacteria within the developing protein matrix is of huge importance in relation to food quality, consistency and safety.
Jeanson et al. Bacterial colonies were shown to be randomly distributed which fit the proposed Poisson model. Colonies can consist of bacterial cells in various physiological states of growth and McKay et al. This hypothesis allows for the assumption that the larger the interfacial area, the greater the bacterial activity on the food matrix which will in turn influence ripening.
Studies have been conducted into the possible location of bacterial micro-colonies within the cheese matrix. Laloy et al. They found bacterial populations to be directly related to the fat content of the cheese.
Bacteria were found to be located in direct contact with the milk fat globular membrane MFGM or located at the casein—fat interface.
Transmission electron microscopy TEM images ultrathin sections of starter bacteria Lactococcus lactis subsp. The Bar represents nm. Reprinted with permission from Laloy et al. Copyright Elsevier. Pitino et al. They showed the interaction between Lb. Scanning electron microscopy images of naturally occurring microflora arrows and strains of Lb.
Reprinted with permission from Pitino et al. Romeih et al. As in the case of many others, the images appear to show preferential location of bacteria at the protein—fat interface. Images produced of yogurt show the entrapment of bacterial cells amongst the acidified casein-based matrix. Scanning electron microscopy left image of low-fat cheddar cheese matrix showing colonies of starter cultures L.
Reprinted with permission from Romeih et al. Auty, unpublished results. Auty et al. Reprinted with permission from Auty et al. Confocal laser scanning microscopy images of bacteria in Emmenal cheese after 1 day of ripening, showing location of bacterial colonies light color in whey pockets A , black areas and at the interface B between fat red and protein gray. Adapted from Lopez et al. Fleurot et al. They found S. The cheese sample structure is visualized by the reflection of the nm laser diode in a grayscale image.
Reprinted adapted with permission from Fleurot et al. Copyright ASM. The application of microscopic techniques to investigate bacteria localization in dairy foods has been limited and in cases where it has been used, bacterial location is often observed as a result of investigating components such as fat content.
Studies rarely focus on bacterial location and distribution specifically Lopez et al. Despite many of the aforementioned studies confirming that bacteria in dairy foods invariably locate on or in close proximity to the fat—protein interface or in contact with whey pockets, the effect this localization has on micro-gradients pH, water activity , flavor development and overall product quality is still relatively unknown.
This review provides an overview on current information regarding use of microscopic techniques to investigate the growth and localization of bacteria within dairy based fermented foods.
Such studies on bacterial colony location and distribution have not yet addressed the relationship between colony location and on product quality, consistency and on ripening parameters. However, the use of microscopy has made the visualization of bacteria in food matrices possible and allows for the enumeration, location, and distribution of starter LAB, NSLAB, spoilage, and pathogenic bacteria via non-destructive methods.
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