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Food Science and Technology

Food Science and Technology is a multidisciplinary linkage of the sciences of microbiology, chemistry, engineering and nutrition to ensure the production of safe and nutritious food of adequate quantity and quality to feed the world’s population. Various publications refine and expand this definition, sometimes keeping science and technology together, sometimes separating them. Definitions can be as short as ‘the application of scientific principles to create and maintain a wholesome food supply’ (UCDavis) to the separate definitions such as those below (Institute of Food Technologists):

  • Food Science: Food science draws from many disciplines such as biology, chemical engineering, and biochemistry in an attempt to better understand food processes and ultimately improve food products for the general public. As the stewards of the field, food scientists study the physical, microbiological, and chemical food properties. By applying their findings, they are responsible for developing the safe, nutritious foods and innovative packaging that line supermarket shelves everywhere.
  • Food Technology: The food you consume on a daily basis is the result of extensive food research, a systematic investigation into a variety of foods’ properties and compositions. After the initial stages of research and development comes the mass production of food products using principles of food technology. All of these interrelated fields contribute to the food industry.

This wiki will not attempt to keep food science and food technology separate as they must almost always be considered together to give the reader the complete picture. The purpose of this wiki is to outline the main aspects of the combination of all the disparate but interlinked sciences that contribute to the understanding of our food and food products and to understand the technology that allows their production in adequate and sustainable quantities. As a web based document, there is of course no restriction on readership but it is not written for food scientists or food technologists. Instead it is written for consumer scientists and others from the various human sciences disciplines who often provide the linkage between the physical scientists and the end user. Consequently, no section will include any of the mathematical theory underlying that section but will be written in a style that it is hoped will be of use to the target audience.

2. Food Preservation

2.1 The historical drivers for the development of food processing

Major drivers for process development:

  • Prehistory: Accidental discovery, for example, Cheese making often quoted as the earliest process and was discovered accidely. Most consider that cheese was first made in the Middle East. The earliest type was a form of sour milk. Legend has it that cheese was 'discovered' by an Arab nomad who filled a saddlebag with milk. After several hours riding he discovered that the milk had separated what we now know as curds and whey. The saddlebag, made from the stomach of a young animal, contained the enzyme, rennin, with coagulation caused by the rennin, the hot sun and the motions of the horse.
  • Until the late 20th century: Military needs (a secure supply of preserved foods in suitable form for the army). For example, canning was developed in 1809 by Nicolas Appert (1749-1841) who won the prize of 12,000FFr offered by Napoleon for developing a practical method of food preservation. The House of Appert became the first commercial cannery in the world, many years before Louis Pasteur proved that heat killed bacteria. His work was probably the first driven by ‘applied’ research interests when in the 1860s Pasteur was asked to help resolve some of the problems of the French wine industry, particularly that of spoilage. He found that heating the wine gently (120°F) to kill the lactic acid bacteria and let the wine age. He also suggested that greater cleanliness was needed to eliminate bacteria; this could be done with heat. Military needs also drove the development of food drying. While sun drying has been known for 4000 years, industrial drying much more recent. In 1917 the US Dept. of Agriculture produced a booklet on drying foods in the home using methods that would nowadays be regarded as probably unsafe. However, during World War 1 & 2, military research led to the development of many foods dried for military use. In the same period, spray drying and freeze drying was developed. More recently, military research led to leading to the development of packaging such as retortable pouches and trays for use in military field kitchens but now seen on supermarket shelves.
  • In the mid to late 20th century space exploration became a major driver. This was mainly towards dehydration methods but also rapid heating methods. More emphasis was placed on food weight, preservation and rapid heating with methods such as ohmic and pulsed electric field developed in the 1960/70s but this was too early for commercial uptake.
  • Simultaneously, some consumer demands started to evolve, especially in more technologically developed societies. For example, industrial freezing was developed by Clarence Birdseye in 1923 with an investment of $7. He later sold the patents in 1929 for $22 million. The first quick-frozen vegetables, fruits, seafoods and meat were sold to the public in 1930 in Springfield, Massachusetts. Currently consumer demands for convenience, low cost and safety are driving food technology developments. A major scientific problem is the inherently conflicting demands of the consumer where more convenience (often unadmitted), less processing, fresher foods, safer foods, healthier diets, greater food functionality are demanded. The conflict arises in that greater convenience and safety normally implies a greater processing need and takes the products further away from the fresh product. In addition, greater knowledge and communication is demanded by the consumer rather than just by marketing needs as had hitherto been the case.

2.2 Preservation by heat

2.2.1 Heat Exchangers

Since food is seldom exposed directly to the primary source of heat (except in gas-fired ovens or in wood burning bread and pizza ovens) some form of heat exchanger is required. In the food industry, the main sources of heat are steam (produced in boilers) and hot air (heated by electricity, or indirectly by heat exchangers from combustion gases). Heat exchangers were mainly developed for the chemical industry rather than the food industry but were adapted for the food industry, mainly by replacing the mild steel construction by stainless steel a material that is easier to clean and is more acceptable as a food contact material. The simplest type of heat exchanger is in the form of two concentric tubes with the material being heated flowing through the central tube and the heating material flowing through the annular space formed between the two tubes. Of course, heat exchangers are therefore limited to liquid foods since they must flow but have been developed into forms that can accommodate even very viscous liquids or liquids containing suspended solids. Capacity limitations ensured that the concentric tube system never move beyond a laboratory system and industrial versions were developed as the shell and tube system (where a large number of tubes through which the fluid could flow, were bundled into a single cylindrical shell through which the heating mediium could flow. That such a system never became popular in food factories is due more to hygiene and cleaning difficulties than to efficiency.

The cleaning problem was overcome y the development of the plate heat exchanger. In this system a large number of stainless steel rectangular plates are arranged vertically with a rubber gasket around the edge of each plate effectively forming a narrow space between each pair of plates. An ingenious system of gasket encased entrance and exit holds allows the food liquid to be pumper between every second pair of plates with the heating medium pumped between the intervening plates. This system is used almost universally for both pasteurisation and sterilization in the liquid food industry, especially for milk and fruit juices.

2.2.2 Sterilization

The initial form of sterilization as developed by Appert (see History), took place in a sealed metal can or glass jar. The container of food is heated to approximately 120C and held at that temperature for the time required to inactivate the most heat resistant bacterial spores. A holding time of several seconds at this temperature may be sufficient depending on the nature of the bacteria present. However, the holding time required is dependent on the temperature and increases dramatically as the temperature falls. At 100C, the holding time could be several hours. It is easy to see that in the early days of canning development when the only heating possibility was immersion of the cans in boiling water, the very long heating and holding times at the 100C boiling water temperature would result in severe thermal damage to the product and its internal structure. Boiling vegetables at 100C for a few hours is more likely to result in a disintegrated vegetable puree! It took the development of superheated steam boilers (a side-effect of railway developments) in the mid 1800’s to provide higer heating temperatures and more realistic heating times.

More recently, it has been possible to sterilize liquid foods using plate heat exchangers using two different mechanisms. In one, the indirect heating method, the liquid (very often milk) is heated to approximately 120C and the passes through a holding tube where the length of time it takes to traverse the tube (termed the holding time) is equal to or in excess of the time required to inactivate all the bacteria present at that temperature. The sterile liquid is then cooled by first using it as the heat source for the cold incoming raw material to the heat exchanger, hereby improving the energy efficiency of the process. Further heat exchangers using cold water and chilled water lower the temperature still further. Finally, the cold sterile liquid needs to be packaged aseptically to ensure it is not recontaminated during storage, distribution and sale.

The second method, the direct method, uses many of the features of the indirect method except that the final heating to approximately 20C is achieved not in a plate heat exchanger but by the direct injection of superheated culinary grade steam into the product. This instantaneously raises the food liquid to sterilization temperatures. Regulations in many countries demand that any steam that condenses in the food liquid and therefore dilutes it, must be subsequently removed. This is achieved by flash evaporation, a process where the hot sterile liquid food is sprayed into a vacuum chamber where it instantly boils and the previously condensed is evaporated.

2.2.3 Ohmic heating

Most food heating processes would not normally rank highly in any listing of green processes since the amount of energy needed to raise a food through a given temperature range is the same no matter what process is employed. However, the ‘greenness’ of ohmic heating comes from two sources. Firstly, it is a very rapid process and therefore the time available for heat losses from the product being heated is small and environmental losses are minimised. Secondly, and more importantly, it is a direct application of electrical energy to the product and consequently the significant energy loss implicit at each energy transformation or exchange step in the process is significantly reduced if not entirely eliminated. Consequently, it can justifiably be regarded as a green process and within heating processes will rank as one of the greenest.

While ohmic heating as applied to foods has developed significantly over the past two decades, it has been known for significantly longer than that. In the 1950s and 60s significant research was undertaken into the process, largely at the behest of electrical utility companies rather than the food industry, but those efforts did not result in significant industrial uptake, primarily because of problems of electrode design, electrode polarisation and fouling, difficulties in applying the electric current directly to the food and difficulties with food particles of differing conductivity to the main food matrix.

While the basic physics and mechanisms of ohmic heating are dealt with in more detail in section 2, it is worth noting in this introduction that the process involves the direct passage of an electric current through the food and the consequential heating of the food due to its electrical resistance. Simple in concept, it is anything but simple in application as is apparent from the later sections of this chapter. However, the absence of an intermediate step between the energy source (electricity) and the food such as is common in other processes (e.g. using the electricity to first heat a resistive element which in turn heats air or water with subsequent heat transfer to the food) brings with it the energy savings and efficiencies noted above. Finally, the advances in materials forming the electrical electrodes used to conduct the electrical energy to the food together with the deeper understanding of the heating process of complex foods due to improved modelling and computational techniques, give this ‘green’ process a very bright future.

Principles: Voltage, current and resistance are the primary characteristics of any electrical circuit. Voltage is the electrical driving force and can be supplied from a variety of sources such as the ac mains supply, battery, or a generator. This driving force causes a flow of electric current measured in amperes and the physical makeup of the circuit (wires, etc) contribute a resistance that opposes the flow and is measured in ohms. In ohmic heating, this resistance is provided by the food material through which the current is passed. For a material to be ohmically heated, it must be physically capable of conducting electricity. For a material to be classified as a conductor, electrical charges must be able to move from one point to another within it to complete an electrical circuit. While we are well used to the concept of metals being the best conductors of electricity (wires, etc.) and display metallic conduction due to the relatively free movement of electrons through metallic lattices, even solid foods are vastly different from metals. However, most foods contain high levels of water and dissolved salts and these solutions can conduct electricity through electrolytic conduction.

3. Food Properties

3.1 Food rheology

Food rheology is the study of deformation and flow of foods under well-defined conditions. There are many areas where rheological data are required by the food industry including:

  • plant design: pumps and pipe sizing and selection, heat and mass transfer calculations, filler designs and other process engineering calculations involving extruders, mixers, coaters and homogenisers
  • quality control: both of raw material and the product at different stages of the process (including ingredient functionality determination in product development and also shelf life testing)
  • evaluation of sensory attributes: quantitative measurement of consumer determined quality attributes by correlating rheology measurements with sensory data
  • assessment of food structure and conformation of molecular constituents.

Food rheology is often confined to the behaviour of liquid foodstuffs. However, there is an increasing tendency to consider the response of both solid and liquid materials to applied stresses and strains as being two extremes of the same science. There are in fact some foods that will exhibit either behaviour depending on the stress applied; molten chocolate, fat-based spreads, mashed potato and some salad dressings will exhibit a solid-like behaviour at low stresses and a liquid-like behaviour at high stresses. This tendency is increasing as more food products are developed that would be classed by the consumer as being semisolid or semi-liquid. A more exact definition would therefore be the study of both the elastic and the plastic properties of foods. It is proposed, however, to place greater emphasis here on classical liquid rheology measurements, although elastic and viscoelastic properties will also be discussed in the context of semi-liquid foods.

There are many substantial reviews of basic rheology. However, before looking at rheological concepts, it is necessary briefly to consider here some of the fundamentals. It is also necessary to justify the need for measurement given the wealth of published data already available. The reason for this is as stated by Prins and Bloksma (1983): ‘Rheological measurements have to be made under the same conditions as those which exist in the system studied.’ In other words, there is little use in carrying out measurements on a product or extracting values from the literature, if the stresses used and their rates of application during the measurement differ from those in the process or calculation for which the measurement is required. (PRINS, A. and BLOKSMA, A.H. (1983), ‘Guidelines for the measurement of rheological properties and the use of existing data’, in Jowitt, R., Escher, F., Hallstrom, B., Meffert, H.F. Th. Spiess, W.E.L. and Vos, G., Physical Properties of Foods, arking, UK, Elsevier, 185–191.)

3.1.1 Types of rheological deformations and responses

  • Viscous flow: As has been stated, rheology is the study of deformation and flow of foods under well-defined conditions. These conditions could be defined in terms of their rate of deformation or in terms of the magnitude of the stress or the strain applied. Foods of differing internal structure and bonding will react in different manners to these applied conditions. We take as an example a system designed to apply a controlled rate of deformation to a fluid. In the simplest case the shear stress developed in the fluid is directly proportional to the rate of deformation or the rate of strain. In such cases, the liquid is said to be Newtonian
  • Elastic deformation: Certain types of solids, known as hookean solids, display ideal elastic (or hookean) behaviour. This particular behaviour occurs when a force is applied to a solid material and the resultant response gives a straight line relationship between stress and strain. This relationship is known as Hooke’s law and occurs in an ideal elastic solid (also called Hooke’s body).
  • Viscoelasticity:- Many complex structured foodstuffs display both viscous and elastic properties and are known as viscoelastic materials. The use of this term is often restricted to solids, with the term ‘elastico-viscous’ being used to describe liquids displaying similar characteristics. However, we will use the term viscoelastic to describe both, because it is often not possible to establish whether a material is behaving as a solid or as a liquid. Linear viscoelasticity is the simplest viscoelastic behaviour in which the ratio of stress to strain is a function of time alone and not of the strain or stress magnitude, while non-linear viscoelastic materials exhibit mechanical properties that are a function of time and the magnitude of stress used.

3.2 Sensory perception - The consumer’s perception

Ultimately the food product must be eaten, so sensory attributes become most important. However, en route from the farm to the mouth the product may have to be pumped, heated, stored or subjected to other processes, and must be amenable to flow when being placed in a container/package. Equally important is its ability to flow out of the container before consumption. Indeed, it is this ability (or the occasional lack of it) that first brings the consumer into a direct and sometimes frustrating contact with rheological principles. How often has the consumer experienced the dilemma of tomato ketchup refusing to flow from its bottle and found that the application of a sharp blow to the bottle base resulted in an excess amount being deposited on the plate? This provides an excellent example of a situation in which a product has a yield stress below which it will not flow, but flows perhaps too well once the consumer unknowingly provides the stimulus that exceeds it. Not only does this example illustrate yield stress, but it also shows the relationship between force and deformation and flow!

This simple example also gives emphasis to one of the basic rules of rheological measurements, namely that the product should be tested under a range of conditions of stress and shear rate that reflect those experienced during subsequent use, whether that use be tasting, pouring, shaking, stirring or any other action that requires movement of the material. Of course, rheological relevance does not stop when a food reaches the plate but influences the sensory perception or ‘mouthfeel’ of the product. Some authors have defined mouthfeel as the mingled experience deriving from the sensations of the skin of the mouth after ingestion of a food or beverage. It relates to density, viscosity, surface tension and other physical properties of the material being sampled. These relationships between rheology and mouthfeel have been the subject of extensive research, as reviewed in the author’s bibliography on food rheology (McKenna, B.M. 1990, The Liquid and Solid Properties of Foods – a Bibliography, London, Food Science Publishers).

start.1393616448.txt.gz · Last modified: 2015/02/18 17:01 (external edit)