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Old Wednesday, July 01, 2015
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Default My Notes on General Science..

Food Deterioration and its Control
All foods undergo varying degrees of deterioration during storage. Deterioration may include losses in organoleptic desirability, nutritional value, safety, and aesthetic appeal. Foods may change in color, texture, flavor, or another quality attribute as discussed in Chapter 6. Food is subject to physical, chemical, and biological deterioration. The highly sensitive organic and inorganic compounds which make up food and the balance between these compounds, and the uniquely organized structures and dispersions that contribute to texture and consistency of unprocessed and manufactured products are affected by nearly every variable in the environment. Heat, cold, light and other radiation, oxygen, moisture, dryness, natural food enzymes, microorganisms and macro organisms, industrial contaminants, some foods in the presence of others, and time-all can adversely affect foods. This range of potentially destructive factors and the great diversity of natural and processed foods is why so many variations of several basic food preservation methods find application in modern food technology. The rapidity with which foods spoil if proper measures are not taken is indicated in Table 7.1, which lists the useful storage life of typical plant and animal tissues at 21C. Meat, fish, and poultry can become inedible in less than a day at room temperature. This is also true for several fruits and leafy vegetables, raw milk, and many other products. Room temperature or field temperature can be much higher than 21C during much of the year in many parts of the world. Typically, slower rates of deterioration occur with foods that are low in moisture, high in sugar, salt, or acid, or modified in other ways. Nevertheless, even in our modern and efficient warehouses and supermarkets, shelf stable, refrigerated, and frozen foods undergo continuous change, necessitating stock rotation and product removal at definite intervals, which may be days, weeks, or months for such products as baked goods, soft cheeses, and frozen specialties, respectively. Rapid spoilage has significance in less developed areas as well as in the most highly advanced and organized societies. In less developed areas starvation because of spoilage has been known to occur in villages only 20-30 km from locations of a lush harvest. In highly advanced societies food production generally is centralized in areas where food can be most efficiently grown or processed. These areas in the United States can be half a continent or more distant from a population center where the food will be consumed. Unless the deteriorative factors are controlled, there would be no food for these population centers and, indeed, there could be no highly advanced society. History has been made and wars won or lost over food deterioration and its control. Wars, and the need to provide food for armies in regions remote from areas of food production, have always focused attention on the problems of food deterioration. this is still very true today. It is interesting to note that some of the most important advances in preventing food deterioration have been made in time of war. At the close of the eighteenth century France was at war and Napoleon's armies were doing poorly on inadequate rations that frequently included spoiled meat and other unwholesome or unpalatable items. Similar problems, including elimination of scurvy, were facing the navy and merchant shipping. Prizes were offered as incentive to encourage development of useful methods of preserving food. From this came the discovery by Nicolas Appert that if food was sufficiently heated in a sealed container and the container not opened, the food would be preserved. Appert was awarded 12,000 francs and honored in 1809, and the world gained the art of food canning. It was not until the work of Pasteur some 50 years later, that growth of microorganisms was shown to be a major cause of food spoilage; this provided an explanation for Appert's method of preservation. One of the most important aspects of food science is an understanding of food deteriorative factors and their control. Commonly, various forms of preservation were developed long before an understanding of the principals involved were known; and many of the foods we prize today developed out of attempts to prevent deterioration and prolong storage life. One might not ordinarily think of butter as a means of preserving food, but long ago it was discovered that while milk deteriorated in a day or two, clumps of butter fat that formed when milk was agitated could be removed from the milk and would store for weeks or months. Similarly, cheese, smoked fish, dried fruits, and many fermented foods had their beginnings in attempts to slow down deteriorative processes.

The major factors affecting food deterioration include the following:
1. growth and activities of microorganisms, principally bacteria, yeasts, and molds;
2. Activities of food enzymes and other chemical reactions within food itself;
3. Infestation by insects, parasites, and rodents;
4. Inappropriate temperatures for a given food;
5. Either the gain or loss of moisture;
6. Reaction with oxygen;
7. light;
8. physical stress or abuse; and
9. Time.
• These factors can be divided into biological, chemical, and physical factors. Often these factors do not operate in isolation. Bacteria, insects, and light,
For example, can all be operating simultaneously to spoil food in the field or in a warehouse?
• Similarly, heat, moisture, and air simultaneously affect the multiplication and activities of bacteria, as well as the chemical activities of food enzymes. At any one time, many forms of deterioration may take place, depending on the food and environmental conditions.
Effective preservation must eliminate or minimize all of these factors in a given food.
• For example, in the case of canned meats the meat is sealed in a metal can, which protects it from insects and rodents as well as from light, which could affect its color and possibly its nutritive value. The can also protects the meat from drying out. Vacuum is applied or the can is flushed with nitrogen to remove oxygen before sealing. The sealed can is then heated to kill microorganisms and to destroy meat enzymes. The processed cans are stored in a cool room and the length of time the cans are held in supermarkets and in our homes is limited. In this case the preservation method takes into account all of the major factors in food deterioration. It is well to consider these factors individually.
Bacteria, Yeasts, and Molds
 There are thousands of genera and species of microorganisms. Several hundred are associated in one way or another with food products. Not all cause disease or food spoilage, and the growth of several types is actually desirable because they are used to make and preserve foods.
1. For Instance: The lactic acid-producing organisms used to make cheese, sauerkraut, and certain types of sausage are examples
2. Others are used for alcohol production in making wine or beer, or for flavor production in other foods.
3. Microorganism multiplication on or in foods frequently is the major cause of food deterioration.
Microorganisms capable of spoiling food are found everywhere-in the;
 soil,
 water, and
 air;
 on the skins of cattle
 the feathers of poultry;
 In the intestines and other cavities of the animal body.
They are found in following areas
 on the skins
 peels of fruits and vegetables
 and on the hulls of grain and the shells of nuts.
 food processing equipment that has not been sanitized
 on the hands, skin,
 clothing of food-handling personnel.
How Microorganism is threat for Human race…
1. microorganisms generally are not found within healthy living tissue-such as within the flesh of animals, or the flesh or juice of plants
2. But they are always present to invade the flesh of plants or animals through a break in the skin, or if the skin is weakened by disease or death.
3. In this case they may digest the skin and penetrate through it to the tissue below
4. In nearly all cases, the presence of spoilage organisms in foods is a result of contamination (Polluting).
How to reduce food spoilage?
1. One of the major strategies in reducing food spoilage due to microorganisms is to reduce contamination by ensuring good sanitation practices.
2. Milk from a healthy cow is sterile as secreted, but becomes contaminated as it passes through the teat canals, which are body cavities. Milk becomes further contaminated from dirt on the cow's hide, from the air, from dirty utensils and containers, and so on.
3. Beef becomes contaminated when the animal is slaughtered and the protective skin is broken, especially during cutting.
4. Fruits, vegetables, grains, and nuts become contaminated when the skins or shells are broken or weakened.
5. This is also true of healthy eggs. The inside of a healthy egg is sterile, but the shell of the egg can be highly contaminated from passage through the chicken's body cavity at time oflaying.
Bacteria are single-celled organisms, many of which can be classified into one of three types based on the shape of individual cells.
1. These are the spherical shapes represented by several forms of cocci, the rod shape of the bacilli, and spiral forms possessed by the spirilla and vibrios.
2. Many bacteria can move by means of whiplike flagella
3. Some bacteria, yeast, and all molds produce spores, which are seedlike packets and which under proper conditions can germinate into full-sized cells called "vegetative cells.
“Spores (a small usually single celled asexual reproductive body produced by many nonflowering plants fungi and some other bacteria that are capable of developing into a new individual without sexual fusion) are remarkably resistant to heat, chemicals, and other adverse conditions.
1. Bacterial spores are far more resistant than yeast or mold spores, and more resistant to most processing conditions than vegetative cells.
2. Sterilization processes are designed specifically to inactivate these are highly resistant bacterial spores.
3. All bacteria associated with foods are small.
4. Most are of the order of one to a few micrometers (fJ.m) in length and somewhat smaller than this in diameter.
Yeasts are somewhat larger, of the order of 20 j.Lm or so in individual cell length and about a third this size in diameter. Most yeasts are spherical or ellipsoidal.
Molds are still larger and more complex in structure. They grow by a network of hairlike fibers called mycelia and send up fruiting bodies that produce mold spores referred to as conidia.
Signs of Molds presence..
1. The blackness of bread mold
2. the blue-colored veins of blue cheese are due to conidia;
3. beneath the fruiting heads,
4. the hair-like mycelia anchor the mold to the food
Mycelia are 1 jelly or so in thickness and, like bacteria, can penetrate the smallest opening; in the case of a weakened fruit skin or egg shell, they can digest the skin and make their own route of penetration. Bacteria, yeasts, and molds can attack virtually all food constituents. Some ferment sugars and hydrolyze starches and cellulose; others hydrolyze fats and produce rancidity; some digest proteins and produce putrid and ammonia like odors. Other types form acid and make food sour, produce gas and make food foamy, and form pigments and discolor foods.
A few produce toxins and give rise to food poisoning. When food is contaminated under natural conditions, several types of organisms will be present together and contribute to a complex of simultaneous or sequential changes, which may include production of
1. acid and gas
2. putrefaction,
3. And discoloration.
How bacteria multiply their quantity
1. Most bacteria multiply best at temperatures between 16C and 38C; these are termed mesophilic.
2. Some will grow at temperatures down to the freezing point of water and are called psychrotrophic or psychrophilic.
3. Others will grow at temperatures as high as 82C, and we call these thermophilic.
4. The spores of many bacteria will survive prolonged exposure to boiling water and then multiply when the temperature is lowered.
5. Some bacteria and all molds require oxygen for growth and are called aerobic.
6. Other bacteria will not grow unless all free oxygen is absent and are designated anaerobic.
7. Still others can grow under either aerobic or anaerobic conditions and are called facultative.
8. Most important is the tremendous rate at which bacteria and other microorganisms can multiply. Bacteria multiply by cell division. One cell becomes two, two become four, and so on in exponential fashion. Under favorable conditions, bacteria can double their numbers every 30 min. Under such conditions milk with an initial bacterial count of 100,000 or so per milliliter, which is not uncommon before pasteurization, if left standing at room temperature can reach a bacterial population of about 25 million in 24 h, and over 5 billion per milliliter in 96 h.
Food-Borne Disease
 A special kind of food deterioration that may or may not alter a food's organoleptic properties has to do with food-borne disease.
 Food-borne diseases are commonly classified as food infections or food intoxications.
 Whereas the distinction is sometimes imperfect, food infections involve microorganisms present in the food at time of consumption which then grow in the host and cause illness and disease.
 Food intoxications involve toxic substances produced in foods as by-products of microorganisms prior to consumption and cause disease upon ingestion. Where the toxin producer is a microorganism, it need not grow in the host to produce disease or even be present in the food.
 Staphylococcus aureus and Clostridium botulinum produce bacterial food poisoning by intoxication through the production of specific bacterial toxins.

1. Produced by C. botulinum is one of the most toxic substances known.

 Certain molds produce mycotoxins, the best known being the allatoxins of Aspergillus flavus. Unlike the toxins of S. aureus and C. botulinum, which are highly toxic to man, alIatoxins may be more toxic to domestic animals than to man. However, their carcinogenic properties are cause for much concern since aflatoxins can be produced in a wide range of cereals, legumes, nuts, and other products allowed to become moldy. When such products occur in feeds, a1latoxins may subsequently be detected in the milk of animals consuming the feed and in cheese made from such milk.
 Many bacteria can transmit food-borne infections capable of causing human disease.
 Included
 Clostridium porringers
 numerous members of the genus Salmonella
 Shigella dysenteriae,
 Vibrio parahaemolyticus,
 Streptococcus pyogens
 Bacillus cereus,
 Campylobacter jejuni, and others.
A number of viral infections also may be contracted by man through contaminated food that has not been adequately processed or handled.
 Including
 infectious hepatitis
 poliomyelitis
 various respiratory and intestinal disorders
Microorganisms which cause disease in humans are known as pathogenic or pathogens. Scientists are still learning about food-borne diseases. Over the last decade or so, several bacteria that had not been thought to be transmitted by food and cause human disease have been found to do just that.
 Chief among these "newer" pathogens are
 Aeromonas hydrophila
 Yersinia enterocolitica
 Listeria monocytogenes
 Vibrio parahaemolyticus
 a particular type of Escherichia coli called 0157:H7.
Of particular importance is the recent discovery that some food-borne pathogenic bacteria can multiply at temperatures as low as 3.3C (38F). This means that temperatures which have been considered good for refrigerated storage may not always keep food from becoming a hazard. Considerable research is ongoing into ways to further protect foods from these psychrotrophic pathogens. Some of the causes of food intoxications and infections are listed in Table 7.2, along with the types of foods usually involved and general comments on corrective practices.

Insects, Parasites, and Rodents

Insects are particularly destructive to cereal grains and to fruits and vegetables. Both in the field and in storage it has been estimated that insects destroy 5-10% of the U.S. grain crop annually. In some parts of the world the figure may be in excess of 50%. The insect problem is not just one of how much an insect can eat, but when insects eat, they damage the food and open it to bacterial, yeast, and mold infection, causing further destruction.
Some measures for controlling Insects..
2. Insects have been controlled in stored grain, fruit, and spices by the use of pesticides, inert atmospheres, or cold temperatures.
3. The use of chemical pesticides on foods continues to raise questions of possible toxic effects and maximum safe levels, and there are currently active programs to increase plant resistance and other biological-based methods of insect control.
4. Genetic engineering, for example, offers the possibility of producing food plants which have the genes to make chemicals which are toxic to insects. When the pest eats the plant, the toxicant kills the insect. Many of these toxicants are only effective on insects and of little concern for humans or the environment.
5. Insect eggs may persist, or be laid, in food after processing, as for example in flour.
6. An interesting method of destroying insect eggs is to throw the flour with high impact against a hard surface as in a centrifuge-type machine known as an Entoleter. The impact destroys the eggs. They remain in the flour, but no further insect multiplication results.
Inspection of foods for insect contamination, which the Federal Food, Drug, and Cosmetic Act defines.” The life cycle of the common Drosophila fruit fly progresses through the egg, larval, pupal, and adult fly stages. It is virtually impossible to produce and transport grains and other food commodities completely devoid of insects and insect parts and so the Food and Drug Administration recognizes certain low levels of insect contamination as tolerable and takes action when these levels are exceeded. For example, the acceptable levels for Drosophila eggs and larvae in tomato products are indicated.”
 Commodities containing highly destructive insects are prohibited from import, export, and sometimes transport across state lines. In the United States, the Animal and Plant Health Inspection Service of the USDA is responsible for such regulations.
 Some states and many countries prohibit fresh fruits and vegetables from being imported in order to try and prevent spread of the insects.
 An important food-borne parasite is the trichinosis nematode, Trichinella spiralis, which can enter hogs eating uncooked food wastes. The nematode penetrates the hog's intestines and finds its way into the pork. If the meat is not thoroughly cooked, the live worm can infect man. It also is possible to destroy the nematode by frozen storage. All pork and pork products are government inspected, but as a further safeguard they should be thoroughly cooked before being consumed.

Heat Preservation and Processing

Of the various means of preserving foods, the use of heat finds very wide application. The simple acts of cooking, frying, broiling, or otherwise heating foods prior to consumption are forms of food preservation. In addition to making foods more tender and palatable, cooking destroys a large proportion of the microorganisms and natural enzymes in foods; thus, cooked foods generally can be held longer than uncooked foods. However, cooking generally does not sterilize a product, so even if it is protected from recontamination, food will spoil in a comparatively short period of time. This time is prolonged if the cooked foods are refrigerated. These are common household practices. Another feature of cooking is that it is usually the last treatment food receives prior to being consumed. The toxin that can be formed by Clostridium botulinum is destroyed by a 10-min exposure to moist heat at 100C. Properly processed commercial foods will be free of this toxin. Cooking provides a final measure of protection in those unfortunate cases where a processing error does occur, or a faulty food container becomes contaminated. However, heat preservation of food generally refers to controlled processes that are performed commercially, such as blanching, pasteurizing, and canning.

It is important to recognize that there are various degrees of preservation by heating, and that commercial heat-preserved foods are not truly sterile. A few terms must be defined and understood.
 Sterilization
Sterilization refers to the complete destruction of microorganisms. Because of the resistance of certain bacterial spores to heat, this frequently requires a treatment of at least 121C of wet heat for 15 min or its equivalent. It also means that every particle of the food must receive this heat treatment. If a can of food is to be sterilized, then immersing it into a 121C pressure cooker or retort for 15 min will not be sufficient because of the relatively slow rate of heat transfer through the food into the can. Depending on the size of the can, the effective time to achieve true sterility may be several hours. During this time there can be many changes in the food which reduce its quality. Fortunately, many foods need not be completely sterile to be safe and have keeping quality.
 Commercially Sterile
The term commercially sterile or the word "sterile" (in quotes), sometimes seen in the literature, means that degree of sterilization at which all pathogenic and toxinforming organisms have been destroyed, as well as all other types of organisms which if present could grow in the product and produce spoilage under normal handling and storage conditions. Commercially sterilized foods may contain a small number of heatresistant bacterial spores, but these will not normally multiply in the food supply. However, if they were isolated from the food and given special environmental conditions, they could be shown to be alive. Most canned and bottled food products are commercially sterile and have a shelflife of 2 years or more. Even after longer periods, so-called deterioration is generally due to texture or flavor changes rather than to microorganism growth.
Pasteurization involves a comparatively low order of heat treatment, generally at temperature below the boiling point of water. Pasteurization treatments, depending on the food, have two different primary objectives. In the case of some products, notably milk and liquid eggs, pasteurization processes are specifically designed to destroy pathogenic organisms that may be associated with the food and could have public health significance. The second, more general, objective of pasteurization is to extend product shelf life from a microbial and enzymatic point of view. This is the objective when beer, wine, fruit juices, and certain other foods are pasteurized. In the latter case, these foods would not be expected to be a source of pathogens, or would be protected by some other means of control. Pasteurized products will still contain many living organisms capable of growth-of the order of thousands per milliliter or per gram-limiting the storage life compared to commercially sterile products. Pasteurization frequently is combined with another means of preservation, and many pasteurized foods must be stored under refrigeration. Pasteurized milk may be kept stored in a home refrigerator for a week or longer without developing significant off-flavors. Stored at room temperature, however, pasteurized milk may spoil in a day or two. Pasteurization is not limited to liquid foods. A newer application is the steaming of oysters in the shell to reduce bacterial counts.

Blanching is a kind of pasteurization generally applied to fruits and vegetables primarily to inactivate natural food enzymes. This is common practice when such products are to be frozen, since frozen storage in itself would not completely arrest enzyme activity. Blanching, depending on its severity, also will destroy some microorganisms, as pasteurization will inactivate some enzymes.
As with protein, the contents of other nutrients in foods determined by chemical or physical analysis may be quite misleading in terms of the nutrient status of a food. Apart from amount, what is important is whether the nutrient is in a form that can be utilized in metabolism; that is, whether the nutrient is bioavailable. For example, adding small iron pellets to cereals would increase their iron content, but the iron would not be very available to people eating the cereal and, therefore, be of little value. Many factors influence a nutrient's bioavailability, including the food's digestibility and the nutrient's absorbability from the intestinal tract, which are affected by nutrient binding to indigestible constituents and nutrient-nutrient interactions in food raw materials. Processing and cooking procedures also can influence nutrient bioavailability. Apart from the food itself, different animal species exhibit variations in bioavailability of specific nutrients from a particular food. The age, sex, physiological health, consumption of drugs, general nutritional status, combinations of foods eaten together, and other factors all influence the ability of an individual to make use of a particular nutrient. Bioavailability of carbohydrates, proteins, fats, vitamins, and minerals may be increased or decreased since all nutrients are reactive and generally present in varying amounts in food systems. There are many examples of how food composition, processing, and storage affect nutrient bioavailability. One example is the essential mineral iron. Under practical conditions its bioavailability from foods may be only 1-10% of its total level determined by chemical analysis. The recommended dietary allowances for nutrients in the United States and other countries attempt to take bioavailability into account. However, the many factors influencing nutrient bioavailability and the difficulties inherent in meaningful evaluation procedures leave much research in this area still to be done.


In addition to size, shape, and wholeness, pattern (e.g., the way olives are laid out in a jar or sardines in a can) can be an important appearance factor. Wholeness refers to degree of whole and broken pieces; the price of canned pineapple goes down from the whole rings, to chunks, to bits. Appearance also encompasses the positive and negative aspects of properly molded blue-veined cheeses, and the defect of moldy bread, as well as the quality attribute of ground vanilla bean specks in vanilla ice cream, and the defect of specks and sediment from extraneous matter. Although some ice cream manufacturers have added ground vanilla bean as a mark of highest quality, others have concluded that as often as not a less-sophisticated consumer misinterprets these specks and rejects the product.
Size and Shape

Size and shape are easily measured and are important factors in federal and state grade standards. Fruits and vegetables can be graded for size by the openings they will pass through. The simple devices shown in Fig. 6.2 were the forerunners of current high-speed automatic separating and grading machines, although they are still used to some extent in field grading and in laboratory work. Size also can be approximated by weight after rough grading, for example, determining the weight of a dozen eggs. Shape may have more than visual importance, and the grades of certain types of pickles include the degree of curvature (Fig. 6.3). Such curiosities can become quite important, especially in the design of machines to replace hand operations. When an engineer attempts to design a machine for automatically filling pickles into jars at high speeds, it must be recognized that all pickles are not shaped the same, and a machine that will dispense round objects like olives or cherries can be totally inadequate. Mechanized kitchen, restaurant, and vending systems for rapid mass feeding have become commonplace. Some of the most difficult engineering problems encountered in such facilities were in designing equipment that would dispense odd-shaped food pieces into moving dishes.
Color and Gloss

Food color not only helps to determine quality, it can tell us many things. Color is commonly an index of ripeness or spoilage. Potatoes darken in color as they are friedand we judge the endpoint of frying by color. The bleaching of dried tomato powder on storage can be indicative of too high an oxygen level in the headspace of the package, whereas the darkening of dried tomato can reflect too high a final moisture level in the powder. The color of a food foam or batter varies with its density and can indicate a change in mixing efficiency. The surface color of chocolate is a clue to its storage history. These and many other types of color changes can be accurately measured in the laboratory and in the plant-all influence or reflect food quality. If the food is a transparent liquid such as wine, beer, or grape juice, or if a colored extract can be obtained from the food, then various types of colorimeters or spectrophotometers can be used for color measurement. With these instruments, a tube of the liquid is placed in a slot and light of selected wavelength is passed through the tube. This light will be differentially absorbed depending on the color of the liquid and the intensity of this color. Two liquids of exactly the same color and intensity will transmit equal fractions of the light directed through them. If one of the liquids is a juice and the other is the same juice somewhat diluted with water, the latter sample will transmit a greater fraction of the incoming light and this will cause a proportionately greater response on the instrument. Such an instrument can also measure the clarity or cloudiness of a liquid depending on the amount of light the liquid lets pass. There are several other methods for measuring the color of liquids. If the food is liquid or a solid, we can measure its color by comparing the reflected color to defined colored tiles or chips. The quality control inspector changes tiles until the closest color match is made and then defines the color of the food as being identical to the matching tile or falling between the two nearest tiles. Working with tomato products, one would need to have only a few green and red disks to cover the usual range of tomato color. The grade standards for tomatoes have been based on such a method. Color measurement can be further quantified. Light reflected from a colored object can be divided into three components, which have been termed value, hue, and chroma. Value refers to the lightness or darkness of the color or the amount of white versus black; hue to the predominant wavelength reflected, which determines what the perceived color is (red, green, yellow, blue, etc); and chroma refers to the intensity strength ofthe color. The color of an object can be precisely defined in terms of numerical values of these three components. Another three-dimensional coordinate scale for describing color utilizes the attributes oflightness-darkness, yellowness-blueness, and rednessgreenness. These dimensions of color, used in tri-stimulus colorimetry, can be quantified by instruments such as the Hunterlab Color and Color Difference Mete. Food samples having the same three numbers have the same color. These numbers, as well as numbers representing value, hue, and chroma, vary with color in a systematic fashion that can be graphed to produce a chromaticity diagram (Fig. 6.5). The color chemist and quality controller can relate these numbers to color, and through changes in the numbers can follow gross or minute changes in products that may occur during ripening, processing, or storage. In similar fashion a quality controller can define the color of a product and relate this information to distant plants to be matched at any future date. This is particularly useful where the food color is so unstable as to make the forwarding of a standard sample unfeasible. As with color, there are light-measuring instruments that quantitatively define the shine, or gloss, of a food surface. Gloss is important to the attractiveness of gelatin desserts, buttered vegetables, and the like.

Although consistency may be considered a textural quality attribute, in many instances we can see consistency and so it also is another factor in food appearance. A chocolate syrup may be thin-bodied or thick and viscous; a tomato sauce can be thick or thin. Consistency of such foods is measured by their viscosity, higher viscosity products being of higher consistency and lower viscosity being lower consistency. The simplest method to determine consistency is to measure the time it takes for the food to run through a small hole of a known diameter; or one can measure the time it takes for more viscous foods to flow down an inclined plane using the Bostwick Consistometer (Fig. 6.6). This device might be used for ketchup, honey, or sugar syrup. These devices are called viscometers. There are several other types of viscometers using such principles as the resistance of the food to a falling weight such as a ball, and the time it takes the ball to travel a defined distance; and resistance to the rotation of a spindle, which can be measured by the power requirements of the motor or the amount of twist on a wire suspending the spindle. Viscometers range from the quite simple as shown above to highly sophisticated electronic instruments.

Texture refers to those qualities of food that we can feel either with the fingers, the tongue, the palate, or the teeth. The range of textures in foods is very great, and a departure from an expected texture is a quality defect. We expect chewing gum to be chewy, crackers and potato chips to be crisp, and steak to be compressible and shearable between the teeth. The consumer squeezes melons and bread as a measure of texture which indicates the degree of ripeness and freshness. In the laboratory, more precise methods are available. However, the squeezing device in Fig. 6.7 gives only an approximation of freshness, since the reading also depends on the stiffness ofthe wrapping and the looseness with which the bread slices are packed.
Measuring Texture
Food texture can be reduced to measurements of resistance to force. Iffood is squeezed so that it remains as one piece, this is compression-as with the squeezing of bread. If a force is applied so that one part of the food slides past another, it is shearing-as in the chewing of gum. A force that goes through the food so as to divide it causes cutting-as in cutting an apple. A force applied away from the material results in tearing or pulling apart, which is a measure of the food's tensile strength-as in pulling apart a muffin. When we chew a steak, what we call toughness or tenderness is really the yielding of the meat to a composite of all of these different kinds of forces. There are instruments to measure each kind of force, many with appropriate descriptive names but none exactly duplicate what occurs in the mouth. Many specialized test instruments have been devised to measure some attribute of texture. For example, a succulometer (Fig. 6.8) uses compression to squeeze juice out of food as a measure of succulence. A tenderometer applies compression and shear to measure the tenderness of peas. A universal testing machine fitted with the appropriate devices can measure firmness and crispness and other textural parameters (Fig. 6.9). This and similar instruments frequently are connected to a moving recording chart. The time-force curve traced on the chart gives a graphic representation of the rheological properties of the food item. When an apple half is tested, the tracing would show an initial high degree of force required to break the skin, and then a change in force as the compressing-shearing element enters and passes through the apple pulp. Various forms of penetrometers are in use. These generally measure the force required to move a plunger a fixed distance through a food material. A particular penetrometer used to measure gel strength is the Bloom Gelometer. In this device, lead shot is automatically dropped into a cup attached to the plunger. The plunger positioned above the gel surface moves a fixed distance through the gel until it makes contact with a switch that cuts off the flow of lead shot. The weight of shot in grams, which is proportional to the firmness of the gel, is reported as degrees Bloom. This is one way of measuring the "strength" of gelatin and the consistency of gelatin desserts. Another kind of penetrometer, also referred to as a tenderometer, utilizes a multipleneedle probe that is pressed into the rib eye muscle of raw beef (Fig. 6.10). The force needed is sensed by a transducer and displayed on a meter. The carefully engineered needle probe was designed to give readings that correlate with the tenderness of the meat after cooking, while at the same time not altering the raw meat for further use. Several of the above methods for measuring texture alter the food sample being tested, so that it cannot be returned to a production batch. Since there are correlations between color and texture in some instances, there are applications where color may be used as an indication of acceptable texture. Under controlled conditions automatic color measurement may then be used as a nondestructive measure of texture; this is done in the evaluation of the ripeness of certain fruits and vegetables moving along conveyor belts. Another nondestructive indication of texture is obtained by the experienced cheesemaker who thumps the outside of a cheese and listens to the sound. This gives a rough indication of the degree of eye formation during ripening of Swiss cheese. One of the newer methods of nondestructive texture measurement makes use of sonic energy, which is absorbed to different extents depending on the firmness of an object.
Texture Changes

The texture of foods, like shape and color, does not remain constant. Water changes playa major role. Foods also can change texture on ageing. Texture of fresh fruit and vegetables becomes soggy as the cell walls break down and the cells lose water. This is referred to as loss of turgor. As more water is lost from the fruit, it becomes dried out, tough, and chewy. This is desirable in the case of dried apricots, prunes, and raisins. Bread and cake in the course of becoming stale lose some water and this is a quality defect. Steaming the bread refreshes it somewhat by softening the texture. Crackers, cookies, and pretzels must be protected against moisture pickup that would soften texture. Quite apart from changes in the texture of unprocessed foods, there are the textural aspects of processed foods. For example, lipids are softeners and lubricants that the baker blends into a cake formula to tenderize cake. Starch and numerous gums are thickeners; they increase viscosity. Protein in solution can be a thickener, but if the solution is heated and the protein coagulates, it can form a rigid structure as in the case of cooked egg white or coagulated gluten in baked bread. Sugar affects texture differently depending on its concentration. In dilute solution it adds body and mouthfeel to soft drinks. In concentrated solution it adds thickening and chewiness. In still higher concentrations it crystallizes and adds brittleness as in hard candies. The food manufacturer not only can blend food constituents into an endless number of mixtures but may use countless approved ingredients and chemicals to help modify texture.
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can u share other notes on general science portion?
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Please share source.?
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