Pasteurization

Pasteurization 

Pasteurization is a relatively mild heat treatment, in which food is heated to below 100ºC. In low acid foods (pH > 4.5, for example, milk) it is used to minimise possible health hazards from pathogenic micro-organisms and to extend the shelf life of foods for several days. In acidic foods (pH 4.5, for example, bottled fruit) it is used to extend the shelf life for several months by the destruction of spoilage micro-organisms (yeasts or moulds) and/or enzyme inactivation. In both types of food, minimal changes are caused to sensory characteristics or nutritive value. Processing containers of food, either which have a naturally low pH (for example fruit pieces) or in which the pH is artificially lowered (for example pickles) is similar to canning. It is often termed pasteurization to indicate the mild heat treatment employed.

The process was named after the French microbiologist, Louis Pasteur, whose research in the 1880s demonstrated that thermal processing would inactivate unwanted microorganisms in wine. Spoilage enzymes are also inactivated during pasteurization. Today, pasteurization is used widely in the dairy industry and other food processing industries to achieve food preservation and food safety.

Purpose of pasteurisation for different foods

 Pasteurization process

Pasteurization is a mild heat treatment of liquid foods (both packaged and unpackaged) where products are typically heated to below 100 °C. The heat treatment and cooling process are designed to inhibit a phase change of the product. The acidity of the food determines the parameters (time and temperature) of the heat treatment as well as the duration of shelf life. Parameters also take into account nutritional and sensory qualities that are sensitive to heat.

Equipments-

Food can be pasteurized in two ways: either before or after being packaged into containers.

Batch method

The batch method uses a vat pasteurizer which consists of a jacketed vat surrounded by either circulating water with added steam or heating coils of hot water or direct steam. In the vat the milk is heated and held throughout the holding period while being agitated. The milk may be cooled in the vat or removed hot after the holding time is completed for every particle. As a modification, the milk may be partially heated in tubular or plate heater before entering the vat. This method has very little use for milk but some use for milk by-products (e.g. creams, chocolate) and special batches. The vat pasteurizer is used extensively in the ice cream industry as it allows for dissolution and blending of ingredients during the heating stage.

Continuous Method

Continuous process method has several advantages over the vat method, the most important being time and energy saving. For most continuous processing, a high temperature short time (HTST) pasteurizer is used. The heat treatment is accomplished using a plate heat exchanger. This piece of equipment consists of a stack of corrugated stainless steel plates clamped together in a frame. There are several flow patterns that can be used. Gaskets are used to define the boundaries of the channels and to prevent leakage. The heating medium can be vacuum steam or hot water.

Temperature	Time	Pasteurization Type
63ºC (145ºF)*	30 minutes	Vat Pasteurization
72ºC (161ºF)*	15 seconds	High temperature short time Pasteurization (HTST)
89ºC (191ºF)	1.0 second	Higher-Heat Shorter Time (HHST)
90ºC (194ºF)	0.5 seconds	Higher-Heat Shorter Time (HHST)
94ºC (201ºF)	0.1 seconds	Higher-Heat Shorter Time (HHST)
96ºC (204ºF)	0.05 seconds	Higher-Heat Shorter Time (HHST)
100ºC (212ºF)	0.01seconds	Higher-Heat Shorter Time (HHST)
138ºC (280ºF)	2.0 seconds	Ultra Pasteurization (UP)

Effect on foods

Pasteurisation is a relatively mild heat treatment and even when combined with other unit operations there are only minor changes to the nutritional and sensory characteristics of most foods. However, the shelf life of pasteurised foods is usually only extended by a few days or weeks compared with many months with the more severe heat sterilisation. Minimising postprocessing contamination is essential to ensure adequate shelf life.

Colour, flavour and aroma

In fruit juices, the main cause of colour deterioration is enzymic browning by polyphenol oxidase. This is promoted by the presence of oxygen, and fruit juices are therefore routinely deaerated prior to pasteurisation. The difference between the whiteness of raw milk and that of pasteurised milk is due to homogenisation, and pasteurisation alone has no measurable effect. Other pigments in plant and animal products are also mostly unaffected by pasteurisation. A small loss of volatile aroma compounds during pasteurisation of juices causes a reduction in quality and may also unmask other ‘cooked’ flavours. Volatile recovery may be used to produce high-quality juices but this is not routinely used, due to the high cost. Loss of volatiles from raw milk removes a hay-like aroma and produces a blander product.

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Ohmic Heating

 Ohmic Heating 

Also termed ‘resistance heating’ or ‘electro-heating’, this is a more recent development in which an alternating electric current is passed through food, and the electrical resistance of the food causes the power to be translated directly into heat. As the food is an electrical component of the heater, it is essential that its electrical properties (its resistance) are matched to the capacity of the heater.

The concept of direct heating in this way is not new, but it has been developed into a commercial process during the last 15 years by the APV Baker company, using a licensed design by EA Technology. The process can be used for UHT sterilization of foods, and especially those that contain large particles (up to 2.5 cm) that are difficult to sterilize by other means. It is now in commercial use in Europe, the USA, and Japan for: • aseptic processing of high added-value ready meals, stored at ambient temperature for

 • pasteurization of particulate foods for hot filling 

 • pre-heating products before canning 

 • high added-value prepared meals, distributed at chill temperatures (Fryer, 1995). 

Ohmic heating is more efficient than microwave heating because nearly all of the energy enters the food as heat. Another important difference is that microwave and radio frequency heating has a finite depth of penetration into a food whereas ohmic heating has no such limitation. However, microwave heating requires no contact with the food, whereas ohmic heating requires electrodes to be in good contact. In practice, the food should be liquid or have sufficient liquid with particulate foods to allow good contact and to pump the product through the heater. 

Principles

The principles of ohmic heating are very simple as illustrated in Figure. Ohmic heating is based on the passage of alternating electrical current (AC) through a body such as a liquid-particulate food system which serves as an electrical resistance in which heat is generated. AC voltage is applied to the electrodes at both ends of the product body. The rate of heating is directly proportional to the square of the electric field strength, E, and electrical conductivity. The electric field strength can be varied by adjusting the electrode gap or the applied voltage. However, the most important factor is the electrical conductivity of the product and its temperature dependence. If the product has more than one phase such as in the case of a mixture of liquid and particulates, the electrical conductivity of all the phases has to be considered. The electrical conductivity increases with rising temperature, suggesting that ohmic heating becomes more effective as temperature increases, which could theoretically result in runaway heating. A difference in the electrical resistance and its temperature dependence between the two phases can make the heating characteristics of the system very complicated. Since electrical conductivity is influenced by ionic content, it is possible to adjust the electrical conductivity of the product (both phases) with ion (e.g. salts) levels to achieve effective ohmic heating. 

In ohmic heating, microbes are thought to be thermally inactivated. Other contributions to the kill mechanism have also been suggested. A mild electroporation mechanism may occur during ohmic heating operating at low frequency (50–60 Hz) which allows electrical charges to build up and form pores across cell walls.

Advantages

The advantages of ohmic heating are as follows: 

• The food is heated rapidly (1ºC s-¹ ) at the same rate throughout and the absence of temperature gradients result in even heating of solids and liquids if their resistances are the same 
• Heat transfer coefficients do not limit the rate of heating 
• Temperatures sufficient for UHT processing can be achieved 
• There are no hot surfaces for heat transfer, as in conventional heating, and therefore no risk of surface    fouling or burning of the product which results in reduced frequency of cleaning 
• Heat-sensitive foods or food components are not damaged by localized overheating 
• Liquids containing particles can be processed and are not subject to shearing forces that are found in,      for example, scraped surface heat exchangers  
• It is suitable for viscous liquids because heating is uniform and does not have the problems associated     with poor convection in these materials 
• Energy conversion efficiencies are very high (>90%) 
• Lower capital cost than microwave heating 
• Suitable for continuous processing. 
• Heating food materials by internal heat generation without the limitation of conventional heat transfer    and some of the non-uniformity commonly associated with microwave heating due to limited dielectric penetration. Heating takes place volumetrically and the product does not experience a large temperature gradient within itself as it heats. 
• The higher temperature in particulates than liquid can be achieved, which is impossible for conventional heating. 
• Reducing risks of fouling on heat transfer surface and burning of the food product, resulting in minimal mechanical damage and better nutrients and vitamin retention. 
• High energy efficiency because 90% of the electrical energy is converted into heat. 
•  Optimization of capital investment and product safety as a result of high solids loading capacity. 
• Ease of process control with instant switch-on and shut-down. 
• Reducing maintenance cost (no moving parts). 
• Ambient-temperature storage and distribution when combined with an aseptic filling system. 
• A quiet environmentally friendly system.

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Hurdle Technology

Hurdle Technolgy in Food Preservation

In traditionally preserved foods, such as smoked fish or meat, jams, and other preserves, there is a combination of factors that ensure microbiological safety and stability of the food, and thus enable it to be preserved. In smoked products, for example, this combination includes heat, reduced moisture content, and anti-microbial chemicals deposited from the smoke onto the surface of the food. Some smoked foods may also be dipped or soaked in brine or rubbed with salt before smoking, to impregnate the flesh with salt and thus add a further preservative mechanism. Smoked products may also be chilled or packed in modified atmospheres to further extend the shelf life. In jams and other fruit preserves, the combined factors are heat, a high solids content (reduced water activity), and high acidity. These preservative factors also strongly influence the sensory characteristics of the product and contribute to important differences in flavor, texture, or color between different products.

In vegetable fermentation, the desired product quality, and microbial stability are achieved by a sequence of factors that arise at different stages in the fermentation process: the addition of salt selects the initial microbial population which uses up the available oxygen in the brine. This reduces the redox potential and inhibits the growth of aerobic spoilage micro-organisms and favors the selection of lactic acid bacteria. These then acidify the product and stabilize it. Further treatments may include pasteurization and packaging to extend the shelf life and facilitate distribution. The demand by consumers for high-quality foods having ‘fresh’ or ‘natural’ characteristics, that require a minimum amount of preparation has led to the development of ready-to-eat and convenience foods that are preserved using mild technologies. 

The main preservation technique is refrigeration, but because of the difficulty in maintaining sufficiently low temperatures throughout the production, distribution, and storage chain, additional barriers (or ‘hurdles’) are required to control the growth of spoilage or pathogenic micro-organisms. The concept of combining several factors to preserve foods has been developed by Leistner (1995) and others into the Hurdle effect (each factor is a hurdle that micro-organisms must overcome). This in turn has led to the application of Hurdle Technology, where an understanding of the complex interactions of temperature, water activity, pH, chemical preservatives, etc. are used to design a series of hurdles that ensure the microbiological safety of processed foods. 

The hurdles are also used to improve the quality of foods and the economic properties (for example, the weight of water that can be added to a food, consistent with its microbial stability). To be successful, the hurdles must take into account the initial numbers and types of micro-organisms that are likely to be present in the food. 

The hurdles that are selected should be ‘high enough’ so that the anticipated numbers of these microorganisms cannot overcome them. However, the same hurdles that satisfactorily preserve food when it is properly prepared (Fig.a), is overcome by a larger initial population of micro-organisms (Fig. b) when for example raw materials are not adequately cleaned. In this example, the main hurdles are low water activity and chemical preservatives in the product, with storage temperature, pH, and redox potential having a smaller effect. Blanching vegetables or fruits has a similar effect in reducing initial numbers of micro-organisms before freezing or drying. If in given fig., the same hurdles are used with a different product that is richer in nutrients that can support microbial growth (Fig.c), again the hurdles may be inadequate to preserve it and a different combination may be needed or the height of the hurdles increased. It should be noted that although the hurdles in Fig. are represented as a sequence, in practice the different factors may operate simultaneously, synergistically, or sequentially. The combination of hurdle technology and HACCP in process design is described by Leistner (1994). By combining hurdles, the intensity of individual preservation techniques can be kept comparatively low to minimize loss of product quality, while overall there is a high impact on controlling microbial growth. 

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Effect of blanching on foods

 

As we know that blanching is the most common and easiest way to preserve the fruits and vegetables for short shelf life and also it is done prior to any processing. Throughout college, it is one of my favorite topics. But do we know that what are the after-effects of blanching on foods? Let me explain about it further - 

In general, the time-temperature combination used for blanching is a compromise which ensures adequate enzyme inactivation but prevents excessive softening and loss of flavor in the food.

Nutrients

Some minerals, water-soluble vitamins, and other water-soluble components are lost during blanching. Losses of vitamins are mostly due to leaching, thermal destruction, and, to a lesser extent, oxidation. The extent of vitamin loss depends on a number of factors including: 

• the maturity of the food and variety 

• methods used in the preparation of the food, particularly the extent of cutting, slicing or dicing

the surface-area-to-volume ratio of the pieces of food 

• method of blanching 

• time and temperature of blanching (lower vitamin losses at higher temperatures for shorter times)

• the method of cooling • the ratio of water to food (in both water blanching and cooling).  

Colour and flavor

Blanching brightens the color of some foods by removing air and dust on the surface and thus altering the wavelength of reflected light. The time and temperature of blanching also influence the change in food pigments according to their D value. Sodium carbonate (0.125% w/w) or calcium oxide are often added to blancher water to protect chlorophyll and to retain the color of green vegetables, although the increase in pH may increase the losses of ascorbic acid. Enzymic browning of cut apples and potatoes is prevented by holding the food in dilute (2% w/w) brine prior to blanching. When correctly blanched, most foods have no significant changes to flavor or aroma, but under-blanching can lead to the development of off-flavors during the storage of dried or frozen foods. Changes in color and flavor are described in more detail by Selman (1987).

Texture

One of the purposes of blanching is to soften the texture of vegetables to facilitate filling into containers prior to canning. However, when used for freezing or drying, the time– temperature conditions needed to achieve enzyme inactivation cause an excessive loss of texture in some types of food (for example certain varieties of potato) and in large pieces of food. Calcium chloride (1–2%) is therefore added to blancher water to form insoluble calcium pectate complexes and thus to maintain firmness in the tissues.

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Basic to advance in Blanching

 

Blanching

Blanching serves a variety of functions, one of the main ones being to destroy enzymic

activity in vegetables and some fruits, prior to further processing. As such, it is not

intended as a sole method of preservation but as a pre-treatment which is normally carried

out between the preparation of the raw material and later operations

(particularly heat sterilization, dehydration, and freezing).

Blanching is also combined with peeling and/or cleaning of food, to achieve

savings in energy consumption, space, and equipment costs.

A few processed vegetables, for example, onions and green peppers, do not require

blanching to prevent enzyme activity during storage, but the majority suffer considerably

loss in quality if the blanching is omitted or if they are under-blanched. To achieve adequate

enzyme inactivation, food is heated rapidly to a pre-set temperature, held for a pre-set

time and then cooled rapidly to near ambient temperatures. The factors which influence

blanching time is:

• type of fruit or vegetable

• size of the pieces of food

• blanching temperature

• method of heating

Theory

The theory of unsteady-state heat transfer by conduction and convection, which is used to

calculate blanching time.

The maximum processing temperature in freezing and dehydration is insufficient to

inactivate enzymes. If the food is not blanched, undesirable changes in sensory

characteristics and nutritional properties take place during storage. In canning, the time

taken to reach sterilizing temperatures, particularly in large cans, may be sufficient to

allow enzyme activity to take place. It is, therefore, necessary to blanch foods prior to

these preservation operations. Under-blanching may cause more damage to food than the

absence of blanching does, because heat, which is sufficient to disrupt tissues and release

enzymes, but not inactivate them, causes accelerated damage by mixing the enzymes and

substrates. In addition, only some enzymes may be destroyed which causes increased

activity of others and accelerated deterioration.

The heat resistance of enzymes is characterized by D and z values.

Enzymes which cause a loss of eating and nutritional qualities in vegetables and fruits

include lipoxygenase, polyphenol oxidase, polygalacturonase, and chlorophyllase. Two

heat-resistant enzymes that are found in most vegetables are catalase and peroxidase.

Although they do not cause deterioration during storage, they are used as marker enzymes

to determine the success of blanching. Peroxidase is the more heat resistant of the two, so

the absence of residual peroxidase activity would indicate that other less heat-resistant

enzymes are also destroyed. The factors that control the rate of heating at the center of the

product  and can be summarised as:

• the temperature of the heating medium

• the convective heat transfer coefficient

• the size and shape of the pieces of food

• the thermal conductivity of the food.

Blanching reduces the number of contaminating micro-organisms on the surface of

foods and hence assists in subsequent preservation operations. This is particularly

important in heat sterilization, as the time and temperature of processing are

designed to achieve a specified reduction in cell numbers. If blanching is inadequate, a

larger number of micro-organisms are present initially and this may result in a larger

number of spoiled containers after processing. Freezing and drying do not substantially

reduce the number of micro-organisms in unblanched foods and these are able to grow on

thawing or rehydration.

Blanching also softens vegetable tissues to facilitate filling into containers and

removes air from intercellular spaces which increases the density of food and assists in

the formation of a head-space vacuum in cans.  

Equipment  

The two most widespread commercial methods of blanching involve passing food

through an atmosphere of saturated steam or a bath of hot water. Both types of equipment

are relatively simple and inexpensive. Microwave blanching is not yet used commercially

on a large scale.  

Steam blanchers

In general, this is the preferred method for foods with a large area of cut surfaces

as leaching losses are much smaller than those found using hot-water blanchers.

At its simplest a steam blancher consists of a mesh conveyor belt that carries

food through a steam atmosphere in a tunnel. The residence time of the food is

controlled by the speed of the conveyor and the length of the tunnel. Typically a 

tunnel is 15 m long and 1–1.5 m wide. The efficiency of energy consumption is 

19% when water sprays are used at the inlet and outlet to condense escaping steam. 

Alternatively, food may enter and leave the blancher through rotary valves or hydrostatic

seals to reduce steam losses and increase energy efficiency to 27%, or steam

may be re-used by passing through Venturi valves. Energy efficiency

is improved to 31% using combined hydrostatic and Venturi devices (Scott et al., 1981).

In conventional steam blanching, there is often poor uniformity of heating in the

multiple layers of food. The time-temperature combination required to ensure enzyme

inactivation at the center of the bed results in overheating of food at the edges and a

consequent loss of texture and other sensory characteristics. Individual quick blanching

(IQB) which involves blanching in two stages, was developed to overcome this problem

(Lazar et al., 1971). In the first stage, the food is heated in a single layer to a sufficiently

high temperature to inactivate enzymes. In the second stage (termed adiabatic holding) a

deep bed of food is held for sufficient time to allow the temperature at the center of each

piece to increase to that needed for enzyme inactivation. The reduced heating time (for

example 25 s for heating and 50 s for holding 1 cm diced carrot compared with 3 min for

conventional blanching), results in an improvement in the efficiency of energy

consumption to 86–91% (Cumming et al., 1984). The mass of product blanched per

kilogram of steam increases from 0.5 kg per kilogram of steam in conventional steam

blanchers to 6–7 kg per kilogram of steam, when small-particulate foods (for example

peas, sliced or diced carrots) are blanched.

Nutrient losses during steam blanching are reduced by exposing the food to warm air

(65ºC) in a short preliminary drying operation (termed ‘pre-conditioning’). Surface

moisture evaporates and the surfaces then absorb condensing steam during IQB. Weight

losses are reduced to 5% of those found using conventional steam blanching (Lazar et al.,

1971). Pre-conditioning and individual quick blanching are reported to reduce nutrient

losses by 81% for green beans, by 75% for Brussels sprouts, by 61% for peas, and by 53%

for lima beans and there is no reduction in the yield of blanched food (Bomben et al.,

1973).

The equipment for IQB steam blanching consists of a bucket elevator

that carries the food to a heating section. The elevator is located in a close-fitting tunnel

to reduce steam losses. A single layer of food is heated on a conveyor belt and then held

on a holding elevator before cooling. The cooling section employs a fog spray to saturate

the cold air with moisture. This reduces evaporative losses from the food and reduces the

amount of effluent produced. Typically the equipment processes up to 4500 kg h power -1 of

food. The complete inactivation of peroxidase is achieved with a minimum loss in

quality, indicated by the retention of 76–85% of ascorbic acid.

Batch fluidized-bed blanchers operate using a mixture of air and steam, moving at

approximately 4.5 m s power -1, which fluidizes and heats the product simultaneously. The

design of the blanching chamber promotes continuous and uniform circulation of the food

until it is adequately blanched. Although these blanchers have not yet been widely used at

a commercial scale, they are reported to overcome many of the problems associated with

both steam and hot-water methods (Gilbert et al., 1980). The advantages include:

• faster, more uniform heating

• good mixing of the product

• a substantial reduction in the volume of effluent

• shorter processing times and hence smaller losses of vitamins and other soluble heat

sensitive components of food.

A continuous fluidized-bed blancher is described by Philippon (1984).

 Hot-water blanchers

There are a number of different designs of blancher, each of which holds the food in hot

water at 70–100ºC for a specified time and then removes it to a dewatering-cooling

section. 

In the widely used reel blancher, food enters a slowly rotating cylindrical mesh drum

which is partly submerged in hot water. The food is moved through the drum by internal

flights. The speed of rotation and length control the heating time. Pipe blanchers consist

of a continuous insulated metal pipe fitted with feed and discharge ports. Hot water is

recirculated through the pipe and food is metered in. The residence time of food in the

blancher is determined by the length of the pipe and the velocity of the water. These

blanchers have the advantage of a large capacity while occupying small floor space. In

some applications, they may be used to transport food simultaneously through a factory.

Developments in hot-water blanchers, based on the IQB principle, reduce energy

consumption, and minimize the production of effluent. For example, the blancher-cooler

has three sections: a pre-heating stage, a blanching stage, and a cooling stage. The

food remains on a single conveyor belt throughout each stage and therefore does not

suffer the physical damage associated with the turbulence of conventional hot water

blanchers. The food is pre-heated with water that is recirculated through a heat

exchanger. After blanching, a second recirculation system cools the food. The two

systems pass water through the same heat exchanger, and this heats the pre-heat water

and simultaneously cools the cooling water. Up to 70% of the heat is recovered. A

recirculated water-steam mixture is used to blanch the food, and final cooling is by cold

air. Effluent production is negligible and water consumption is reduced to approximately

1 m3 per 10 t of product. The mass of product blanched is 16.7–20 kg per kilogram of

steam, compared with 0.25–0.5 kg per kilogram in conventional hot-water blanchers. 

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