5.5 Preservation with sugar

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The principle of this technology is to add sugar in a quantity that is necessary to augment the osmotic pressure of the product's liquid phase at a level which will prevent microorganism development.

From a practical point of view, however, it is usual to partially remove water (by boiling) from the product to be preserved, with the objective of obtaining a higher sugar concentration. In concentrations of 60% in the finished products, the sugar generally assures food preservation.

It is important to know the ratio between the total sugar quantity in the finished product and the total sugar concentration in the liquid phase because this determines, in practice, the sugar preserving action. The percent composition of a product preserved with sugar, for example marmalade, can be expressed as follows: [i + S + s + n + w] = 100;

i = insoluble substance;
s = sugar from fruits;
S = added sucrose;
n = soluble "non sugar"
w = water.

In this case, total sugar concentration, in the liquid phase, of the finished product is:

X = 100 (S + s) / 100 - (n + i) [%]

Therefore, in the case of a standard marmalade with 55 % sugar added (calculated on the finished product basis), the real concentration in the liquid phase is for example:

X = 100 (55 + 8) / 100 - (5 + 3) = 68.5%

In the food preservation with sugar, the water activity cannot be reduced below 0.845; this value is sufficient for bacteria and neosmophile yeast inhibition but does not prevent mould attack. For this reason, various means are used to avoid mould development:

It is very important from a practical point of view to avoid any product contamination after boiling and to assure an hygienic operation of the whole technological process (this will contribute to the prevention of product moulding or fermentation). Storage of the finished products in good conditions can only be achieved by ensuring the above level of water activity.

5.6 Heat preservation/heat processing

5.6.1 Various degrees of preservation

There are various degrees of preservation by heating; a few terms have to be identified and understood.

  1. Sterilisation. By sterilisation we mean complete destruction of micro-organisms. Because of the resistance of certain bacterial spores to heat, this frequently means a treatment of at least 121 C (250 F) of wet heat for 15 minutes 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 sterilised, then immersing it into a 121 C pressure cooker or retort for the 15 minutes will not be sufficient because of relatively slow rate of heat transfer through the food in the can to the most distant point.
  2. "Commercially sterile". Term describes the condition that exists in most of canned or bottled products manufactured under Good Manufacturing Practices procedures and methods; these products generally have a shelf-life of two years or more.
  3. Pasteurized means a comparatively low order of heat treatment, generally at a temperature below the boiling point of water. The more general objective of pasteurization is to extend product shelf-life from a microbial and enzymatic point of view; this is the objective when fruit or vegetable juices and certain other foods are pasteurized.
    Pasteurization is frequently combined with another means of preservation - concentration, chemical, acidification, etc.
  4. Blanching is a type of pasteurization usually applied to vegetables mainly to inactivate natural food enzymes. Depending on its severity, blanching will also destroy some microorganisms.


5.6.2 Determining heat treatment/thermal processing steps

Since heat sufficient to destroy micro-organisms and food enzymes also usually has adverse effects on other properties of foods, in practice the minimum possible heat treatment should be used which can guarantee freedom from pathogens and toxins and give the desired storage life; these aims will determine the choice of heat treatment.

In order to safely preserve foods using heat treatment, the following must be known:

a) what time-temperature combination is required to inactivate the most heat resistant pathogens and spoilage organisms in one particular food?

b) what are the heat penetration characteristics in one particular food, including the can or container of choice if it is packaged?

Preservation processes must provide the heat treatment which will ensure that the remotest particle of food in a batch or within a container will reach a sufficient temperature, for a sufficient time, to inactivate both the most resistant pathogen and the most resistant spoilage organisms if it is to achieve sterility or "commercial sterility", and to inactivate the most heat resistant pathogen if pasteurization for public health purposes is the goal.

Different foods will support growth of different pathogens and different spoilage organisms so the target will vary depending upon the food to be heated.

Food acidity/pH value has a tremendous impact on the target in heat preservation/ processing. Table 5.6.1 lists various types of fruit and vegetables and their pH value, together with the heat processing requirements.

TABLE 5.6.1 Heat processing requirements - dependence on product acidity

Acidity class pH value Food item Heat and processing requirements
Low acid 6.0 Peas, carrots, beets, potatoes, asparagus High temperature processing 116-121C (240-250F)
5.0 Tomato soup  
Medium acid 4.5 Tomatoes, pears, apricots, peaches Boiling water processing 100C (212F)
Acid 3.7 Sauerkraut, apple,  
High acid 3.0 Pickles  


Source: Desrosier and Desrosier (1977)


5.6.3 Sequence of operations employed in heat preservation of foods (fruit and vegetables, etc.)

In a simplified manner, the main operations employed in heat preservation can be described as follows:

Food preparation: Preparation procedures will vary with the type of food. For fruit, washing, sorting, grading, peeling, cutting to size, pre-cooking and pulping operations may be employed.
Can/receptacle This may be carried out manually or by using sophisticated filling machinery. The ratio of liquid to solid in the can must be carefully controlled and the can must not be overfilled. A headspace of 6-9 mm depth (6-8% of the container volume) above the level of food in the can is usual.
Vacuum production This can be achieved by filling the heated product into the can, by heating the can and contents after filling, by evacuating the headspace gas in a vacuum chamber, or by injecting superheated steam into the headspace. In each case the can end is seamed on immediately afterwards.
Thermal processing The filled sealed can must be heated to a high temperature for a sufficient length of time to ensure the destruction of spoilage micro-organisms. This is usually carried out in an autoclave or retort, in an environment of steam under pressure.
Cooling The processed cans must be cooled in chlorinated water to a temperature of 37C. At this temperature the heat remaining is sufficient to allow the water droplets on the can to evaporate before labelling and packing.
Labelling and packing Labels are applied to the can body, and the cans are then packed into cases.

In principle, all these operations can also be carried out at the farm/community level using the appropriate small scale equipment, preferably only glass jars (e.g. no metal cans).


5.6.4 Technological principles of pasteurization Physical and chemical factors which influence pasteurization process are the following:

  1. temperature and time;
  2. acidity of the products;
  3. air remaining in containers. Pasteurization processes. In pasteurising certain acid juices for example, there are two categories of processes:

a) Low pasteurization where pasteurization time is in the order of minutes and related to the temperature used; two typical temperature/time combinations are as following:

63 C to 65 C over 30 minutes or
75 C over 8 to 10 minutes.

Pasteurization temperature and time will vary according to:

In this first category of pasteurization processes it is possible to define three phases:

b) Rapid, high or flash pasteurization is characterized by a pasteurization time in the order of seconds and temperatures of about 85 to 90 C or more, depending on holding time. Typical temperature/time combinations are as follows:

88 C (190 F) for 1 minute;
100 C for 12 seconds;
121C for 2 seconds.

While bacterial destruction is very nearly equivalent in low and in high pasteurization processes, the 121 C/2 seconds treatment give the best quality products in respect of flavour and vitamin retention. Such short holding times, however, require special equipment which is more difficult to design and generally is more expensive than the 63-65 C/30 minutes type of processing equipment.

In flash pasteurization the product is heated up rapidly to pasteurization temperature, maintained at this temperature for the required time, then rapidly cooled down to the temperature for filling, which will be performed in aseptic conditions in sterile receptacles. Taking into account the short time and rapid performance of this operation, flash pasteurization can only be achieved in continuous process, using heat exchangers.

Industrial applications of pasteurization process are mainly used as a means of preservation for fruits and vegetable juices and specially for tomato juice. Thermopenetration. The thermopenetration problem is extremely important, especially in the case of the pasteurization of products packed in glass containers because it is the determining factor for the success of the whole operation.

During pasteurization it is necessary that a sufficient heat quantity is transferred through the receptacle walls; this is in order that the product temperature rises sufficiently to be lethal to micro-organisms throughout the product mass.

The most suitable and practical method to speed up thermopenetration is the movement of receptacles during the pasteurization process. Rapid rotation of receptacles around their axis is an efficient means to accelerate heat transfer, because this has the effect, among others of rapidly mixing the contents.

The critical speed of for this movement is generally about 70 rotations per minute (RPM). This enables a more uniform heating of products, reducing heating time and organoleptic degradation. Heating may precede or follow packaging. These principles of different temperature time combinations very largely determine the design parameters for heat preservation equipment and commercial practices.

The food processor will employ no less than that heat treatment which gives the necessary degree of micro-organism destruction. This is further ensured by periodic inspection by local sanitary authorities or by the importing countries sanitary services. However, the food processor also will want to use the mildest effective heat treatment to ensure highest food quality.

It is convenient to separate heat preservation practices into two broad categories: one involves heating of foods in their final containers, the other employs heat prior to packaging.

The latter category includes methods that are inherently less damaging to food quality, where the food can be readily subdivided (such as liquids) for rapid heat exchange. However, these methods then require packaging under aseptic or nearly aseptic conditions to prevent or at least minimise recontamination.

On the other hand, heating within the package frequently is less costly and produces quite acceptable quality with the majority of foods and most of our present canned food supply is heated in the package.

In practice, therefore, most of the canned food produced locally in developing countries should be heated within the package.

Fig. 5.6.1 (see Figure 5.6.1 Simplified and illustrated flow-sheet of the operation cycle for fruit and vegetables heat preservation) shows a simplified and illustrated flow-sheet of the operation cycle for fruit and vegetable heat preservation. Fig. 5.6.2 (see Figure 5.6.2 Illustration of technological steps for preservation of fruits in glass jars: Peaches and Figure 5.6.2 (continued)) is an illustration of technological steps for preservation of fruits in glass jars.

5.7 Food irradiation

5.7.1 Introduction

Food irradiation is one of the food processing technologies available to the food industry to control organisms that cause food-borne diseases and to reduce food losses due to spoilage and deterioration. Food irradiation technology offers some advantages over conventional processes. Each application should be evaluated on its own merit as to whether irradiation provides a technical and economical solution that is better than traditional processing methods.

TABLE 5.6.2 Possible causes of spoilage (real or apparent) in canned goods

Type of food: Acid and high acid foods (canned fruits)
Condition of can Action to be taken to identify cause
Insufficient vacuum or headspace Check vacuum and headspace in relation to storage temperature and altitude
"Springer" or "flipper" Cool can to 15C and check if still domed. Check can for denting, if possible measure headspace volume change brought about by dents, by comparing can volume with volume of a sound can
Hydrogen swell Check degree of detinning in can especially at the liquid level. Look for scratches or pinholes in lacquer or tin coating. Check if can is still domed on cooling to 15C.
"Hard" or "soft swell" Leaker spoilage. Check can for gross seam faults, perforation due to corrosion or damage to seams. Examine contents for signs of spoilage and can interior for detinning at air/product interface.

Source: FAO/WFP, 1970

5.7.2 Applications

For products where irradiation is permitted, commercial applications depend on a number of factors including the demand for the benefits provided, competitiveness with alternative processes and the willingness of consumers to buy irradiated food products. There are a number of applications of food irradiation. For each application it is important to determine the optimum dosage range required to achieve the desired effect. Too high a dosage can produce undesirable changes in texture, colour and taste of foods.

Shelf-life extension. Irradiation can extend the shelf-life of foods in a number of ways. By reducing the number of spoilage organisms (bacteria, mould, fungi), irradiation can lengthen the shelf life of fruits and vegetables.

Since ionising radiation interferes with cell division, it can be used as an alternative to chemicals to inhibit sprouting and thereby extend the shelf life of potatoes, onions and garlic. Exposure of fruits and vegetables to ionising radiation slows their rate of ripening. Strawberries, for example, have been found to be suitable for irradiation. Their shelf-life can be extended three-fold, from 5 to 15 days.

Disinfestation. Ionising radiation can also be used as an alternative to chemical fumigants for disinfestation of grains, spices, fruits and vegetables. Many countries prohibit the importation of products suspected of being contaminated with live insects to protect the importing country's agricultural base. With the banning of certain chemical fumigants, irradiation has the potential to facilitate the international shipment of food products.


5.7.3 Global developments

Consensus on wholesomeness.

In 1980, an FAD/IAEA/WHO Expert Committee reviewed in detail all the accumulated data on food irradiation from the past 40 years.

The Expert Committee concluded that irradiation to an overall dose of 10 kGy (kilograys) presents no toxicological hazard and introduces no special nutritional or microbiological problems, thus establishing the wholesomeness of irradiated foods up to an overall average absorbed dose of 10 kGy.

Data were insufficient to formulate conclusions on applications of food irradiation above 10 kGy. Data on radiation chemistry, nutritional and microbiological aspects of food treated above 10 kGy is currently being compiled.

In 1983, the Codex Alimentarius Commission, an international group that develops global food standards for the FAO and the WHO, incorporated the 1980 Expert Committee's conclusions regarding the wholesomeness of irradiated foods into the Codex General Standard for Irradiated Foods. This proposed international standard was submitted to member countries to accept or to modify according to individual country needs. Currently most countries that allow food irradiation approve its use on a case-by-case basis.

The Codex Alimentarius Commission has also adopted a Recommended International Code of Practice for the Operation of Radiation Facilities for the Treatment of Foods. It is intended to serve as a guide for irradiator operators and government regulators.

International Trade.

More than 30 countries have given clearances for the use of food irradiation to process some 40 food items and approximately 30 facilities world-wide treat food by irradiation processing. Approvals for additional items are being considered in many countries and many food irradiation facilities are being planned. It was anticipated in 1988 that by 1990 there could be approximately 50 commercial/demonstration irradiators in 25 countries.

Table 5.7.1 shows commercial applications of food irradiation to fruits and vegetables by country.

TABLE 5.7.1 International commercial applications of radiation for fruit and vegetables

Country Location (application date) Food Commodity
Argentina Buenos Aires (1986) Spinach
Belgium Fleurus (1981) Dehydrated vegetables
Brazil Sao Paulo (1985) Dehydrated vegetables
Chile Santiago (1983) Dehydrated vegetables onions, potatoes
China Shanghai (1985 Potatoes
Cuba Havana (1987) Potatoes, onions
German Dem. Rep Weideroda (1983) Onions, garlic
  Spickendorf (1986) Onions
Japan Hokkaido (1973) Potatoes
Korea Seoul (1985 Garlic powder
Netherlands Ede (1978) Dehydrated vegetables
South Africa Johannesburg (1981) Dehydrated vegetables
  Tzaneen (1981) Fruits, onions, potatoes
Thailand Bangkok (1971) Onions

Source: International Atomic Energy Agency (1989)

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