Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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surroundings for a year or more. (See Extrusion

Cooking: Chemical and Nutritional Changes; Sens-

ory Evaluation: Texture.)

Effects of Drying on the Microflora of

Foodstuffs

0023 Dehydration is a method of food processing which

relies for its effectiveness on the prevention of micro-

bial proliferation by making the food environment

too water-retentive for microorganisms to abstract it

for their metabolic requirements (Table 1).

0024 Lowering the a

of many fresh foods from 1.00 to

inhibit the growth of only some of the bacteria re-

quires that around 90% of the water content of the

raw material be removed unless drying is accompan-

ied by the infusion of solutes as in salting.

0025 The lowering of a

from 0.95 to 0.60 (enough to

suspend growth of all microorganisms) requires only

a relatively small reduction in water content but, as

drying merely suspends microbial proliferation,

rather than destroying the organisms, outgrowth

quickly recurs in dried foods as they absorb small

amounts of water from a humid environment. Certain

species of xerophilic yeasts and osmophilic molds are

able to proliferate on material which is still appar-

ently dry. The mold Wallemia sebi, associated with

rotting wood, has often caused problems of surface

spotting, ‘dun’ spoilage, in dried salt-cured fish. Halo-

philic bacteria, like Halinobacterium salinaria, can

cause more severe damage, proteolysis, and putrefac-

tion in salt-cured fish during the ‘water reduction/

salt-infusion’ process if the ambient processing tem-

perature is not kept below 10



C. The characteristic

coloration of the colonies of such organisms, which

eventually pervade the entire product surfaces, gives

rise to the description ‘pink’ spoilage. The microbial

flora so far described cause spoilage of dried foods at

intermediate water contents whether during desorp-

tion, in processing, or resorption during subsequent

storage in too humid ambients. There are, however,

pathogenic species which can affect the safety of dried

food products, particularly if atmospheric moisture

condenses upon the surfaces.

0026Certain molds produce mycotoxins whilst growing

upon such surfaces. The most dangerous of these are

the highly carcinogenic aflatoxins produced by the

black mold Aspergillus flavus. Of the pathogenic bac-

teria, Staphylococcus aureus is the most tolerant to-

wards low water availability, being capable of growth

down to around a

0.8, although it is doubtful

whether it would produce its exotoxin at these levels.

Perhaps of greatest concern, though, is the ability of

much of a food’s contaminating microflora to survive

and remain dormant in the dried food and then pro-

liferate as the reconstituted food is held. Cases of

Salmonella infection stemming from the use of dried

infant formula foods contaminated with such organ-

isms have emphasized this. (See Mycotoxins: Occur-

rence and Determination; Staphylococcus: Properties

and Occurrence; Spoilage: Bacterial Spoilage; Molds

in Spoilage; Yeasts in Spoilage.)

0027It seems to be generally assumed that dried foods

once rehydrated are consumed more or less immedi-

ately. Since this is not always the case, and because

the heat exposure involved in most drying processes is

insufficient to destroy pathogens the starting material

may contain, careful selection of raw materials and

hygienic processing conditions may not be sufficient

to guarantee the safety of the product under all pos-

sible consumer usage circumstances. Heat or irradi-

ation pasteurization of the starting material may have

tbl0001 Table 1 The water activity (a

) of foods and microbial inhibition

Food examples Microorganismsinhibited

1.00 Most fresh, high-water-content foods None

0.95 40% sucrose or 7% salt (NaCl) solutions, breadcrumb, cooked

sausage, canned ham, sponge cake, immature cheese

Gram-negative rods like Escherichia coli,

spores of Bacillaceae

0.91 55% sucrose or 12% salt solutions, Parma ham, mature cheese,

reduced-sugar jams

Most cocci and lactobacilli vegetative cells

of Bacillaceae

0.88 65% sucrose or 15% salt solutions, salami sausage, Parmesan

cheese, fishmeal

Most yeasts

0.82 Wheat flour, dry cereal grains and pulses, fruit cake, intermediate-

moisture pet foods, traditional fruit conserves, dry smoked

sausages, orange juice concentrates

Most molds and Staphylococcus aureus

0.75 26% salt (i.e., saturated NaCl) solution, honey fondants, rolled oats,

rich fruit cakes, glace

cherries, salt-cured cod (prior to final

drying), marzipan

Most halophilic bacteria

0.65 Dried dates, prunes, and figs, toffees and caramels, soft brown

sugar, stockfish, fishmeal dried to 5% water content

Xerophilic molds

0.60 Licorice, fruit gums, salt-cured cod (after final drying) Osmophilic yeasts

1946 DRYING/Physical and Structural Changes

to be considered as an essential part of the process.

For example, spray-dried egg should be produced

from pasteurized liquid egg. Alternatively, the dry

product itself may be treated. Some dried herbs and

spices are irradiated to destroy a large proportion of

their contaminating flora, especially potential patho-

gens. (See Irradiation of Foods: Basic Principles; Pas-

teurization: Principles.)

See also: Browning: Nonenzymatic; Coffee: Instant;

Extrusion Cooking: Chemical and Nutritional Changes;

Freeze-drying: The Basic Process; Freezing: Structural

and Flavor (Flavour) Changes; Irradiation of Foods:

Basic Principles; Mycotoxins: Occurrence and

Determination; Oxidation of Food Components;

Pasteurization: Principles; Preservation of Food;

Sensory Evaluation: Texture; Aroma; Spoilage:

Bacterial Spoilage; Water Activity: Effect on Food

Stability

Further Reading

Christian JHB (1963) Water activity and growth of micro-

organisms. In: Hawthorn J and Leitch JM (eds) Recent

Advances in Food Science, vol. 3. London: Butterworths.

Davies R, Birch GG and Parker KJ (1976) Intermediate

Moisture Foods. London: Applied Science.

Duckworth RB (1975) Water Relations of Food. London:

Academic Press.

Gilbert SG (1986) New concepts on water activity and

storage stability. In: Charalambous G (ed.) The Shelf

Life of Foods and Beverages. Amsterdam: Elsevier.

Hamm R (1963) The water imbibing power of foods. In:

Hawthorn J and Leitch JM (eds) Recent Advances in

Food Science, vol. 3. London: Butterworths.

Horner WFA (1992) Preservation of fish by curing (drying,

salting and smoking). In: Hall GM (ed.) Fish Processing

Technology. Glasgow: Blackie.

Johnston K (1985) Water-sorbent-solute relationships in

dry food model systems. Paper given at a Royal Society

of Chemistry Discussion Conference, Water Activity: A

Credible Measure of Technological Performance and

Physiological Viability. Cambridge: Girton College.

Labuza TP, Tannenbaum SR and Karel M (1970) Water

content and stability of low moisture and intermediate

moisture foods. Food Technology 24: 543–550.

Mathlouthi M (1986) Food Packaging and Preservation:

Theory and Practice. London: Elsevier.

Mossel DAA and Westerdijk J (1949) The physiology of

microbial spoilage of foods. Antonie van Leeuwenhoek

15: 190–202.

Scott WJ (1957) Water relations of food spoilage micro-

organisms. In: Mark EM and Stewart GF (eds) Advances

in Food Research, vol. 7. London: Academic Press.

Van den Berg IC and Bruin S (1981) Water activity and its

estimation in food systems: theoretical aspects. In: Rock-

land LB and Stewart GF (eds) Water Activity: Influences

on Food Quality. London: Academic Press.

Chemical Changes

N Shilton, University College Dublin, Dublin, Ireland

Background

0001Drying of food products can be carried out by using a

variety of different processes, ranging from solar

drying, air-drying to the more expensive freeze

drying. All of these processes influence the properties

of the resulting product in some way, not least in the

way that components of the food will undergo chem-

ical reactions during the drying process. Usually, these

reactions will be detrimental to product quality, but

this is not always the case. For example, drying usu-

ally results in a reduction in volatiles due to vaporiza-

tion, but in certain cases, the volatiles can increase as

a result of drying; for example, it has been found that

drying of basil leaves in a vacuum-microwave dryer

actually increases the amount of volatiles present in

samples when compared with fresh samples, as a

result of chemical reactions.

0002There are many different methods of drying food

materials. The main drying processes that have been

identified as being studied with respect to chemical

changes are solar drying, air convective drying, micro-

wave drying, and freeze-drying. The effects of these

drying processes on the constituents of food products

will be examined in the context of the chemical reac-

tions occurring during the drying process. The chem-

ical changes occurring during the drying process

will be examined, and not those that occur during

storage.

Reaction Kinetics/effect of Water Activity

0003As water is removed from the product being dried, the

water activity of the product decreases. As the water

activity decreases, so too will the reaction rates of

some chemical reactions, such as oxidation of free

fatty acids. However, for a number of chemical reac-

tions, such as Maillard browning and anthocyanin

degradation, the reaction rate increases in the water

activity range 0.4–0.8, before decreasing again,

whereas other reactions such as autoxidation under-

go an exponential type increase in reaction rate as the

water activity is reduced to below 0.2. This can be

related to an increase in the concentration of react-

ants as the moisture content falls, which can lead to

an increase in reaction rates.

0004An examination of the chemical kinetics of drying

shows that as the temperature of the product in-

creases, so will the rate constant of the decomposition

of food components, so it is recommended that foods

DRYING/Chemical Changes 1947

be dried at low temperatures or under vacuum. The

chemical changes described in this section will show

the types of chemical reactions that can occur if the

drying process is not controlled carefully.

Effect on Proteins

0005 The main effect of a drying process on proteins is that

of denaturation. This is not a chemical reaction in

itself (in the initial stages) and usually involves a

change in the tertiary or secondary structure of the

protein. However, severe denaturation will involve a

chemical reaction in the formation or dissociation

of disulfide bonds and coagulation of the proteins.

The drying temperature will play an important part

in determining the changes undergone by proteins

during drying. The lower the drying temperature,

the less thermal, and so potential chemical damage

will be done to the proteins. This has the effect of

extending the drying time of the product. An exten-

sion in drying time will also increase the risk of

degradative reactions as the water activity passes

through the critical zones described earlier.

0006 During the spray drying of skim milk, milk proteins

can undergo lactolyzation. High drying and outlet

temperatures can cause extensive lactolyzation, but

reducing the inlet and outlet temperatures that the

degree of lactolyzation can minimize this reaction.

Amino Acids

0007 Specific amino acids can be lost as a result of chemical

reactions. For example, losses of lysine are higher

with drum-dried milk than with spray-dried milk

powder. This is most probably as a result of the direct

contact with the hot drum, which causes thermal

cross-linking between the disulfide bonds in the

lysine. One of the most important reactions involving

amino acids is the Maillard reaction; this is described

in the section on Maillard browning.

Effect on Carbohydrates

0008 For starchy products, such as potatoes, if the drying

temperature is too high, there is a risk of gelatiniza-

tion of the starch granules within the cells. This is

often not desired. Increasing the drying temperature

above 65



C has also been found to increase the rate

of glucose and fructose degradation during the

drying of products such as onions. One of the most

important reactions involving carbohydrates is the

Maillard reaction; this is described in the section on

Maillard browning. This reaction can also modify the

resulting aroma profile of the dried product.

0009 During the drying of prunes, three phases involving

chemical changes to constituent carbohydrates can be

identified; these correspond to the decreasing mois-

ture content. As the moisture content is initially re-

duced, there is a hydrolysis of sucrose to give fructose

and glucose, and at intermediate moisture contents,

there is a rapid loss of this fructose and glucose as a

result of the Maillard reaction. As the moisture con-

tent is reduced further, caramelization of the sugars

will occur.

Maillard Browning

0010Nonenzymatic browning is the major chemical reac-

tion that can take place during the drying of all types

of foods. The most prevalent form of nonenzy-

matic browning that can occur is that of Maillard

browning. This is a complex series of reactions that

is initiated by the reaction of amino acids and a redu-

cing sugar, in the so-called Amadori rearrangement,

that leads, via a series of intermediate compounds,

to give melanoidins, insoluble compounds with a

characteristic brown color. The reaction is very heat-

sensitive, and so during drying, the use of higher tem-

peratures, in high-temperature short-time processing,

can cause an increase in the reaction rate. Formation

of the intermediate compounds takes some time, so it

is possible for a dried product to be produced that has

no visible signs of browning, but has in fact undergone

substantial chemical reactions. This could have an

impact on the nutritive quality of the product. The

degree of Maillard browning can be controlled by

the use of sulfite treatment, through the use of either

a metabisulfite solution or sulfur dioxide.

0011Of potential concern is the possible production of

carcinogenic compounds during the Maillard reac-

tion, either as intermediate compounds or as com-

pounds during the final color development. Dry

heating also produces aminocarbolines, and the for-

mation of these seems to be favored by free-radical

reactions and a low water activity. However, the

Maillard reaction can also produce antioxidative

components, and these have been found to prevent

oxidation in products such as oven-dried peanuts.

0012It has been found that harsher, higher drying tem-

peratures will cause an increase in Maillard

browning, but this can be controlled by monitor-

ing the free amino acid levels in the product and

adjusting the drying temperature accordingly. In

pasta, a combination of high temperature and a high

water activity during the initial stages of drying will

contribute to cause nonenzymatic browning by the

Maillard reaction. With pasta dried at high and

low temperatures (50 and 110



C), the higher process-

ing temperature causes a large increase in furosine

over the lower processing temperature. Furosine is

an important precursor in the Maillard reaction.

1948 DRYING/Chemical Changes

0013 The Maillard reaction can also result in the degrad-

ation of amino acids by the so-called Strecker

degradation. This reaction produces aldehydes, am-

monia, and carbon dioxide. The aldehydes contribute

to the development of aroma during the Maillard

reaction, with individual amino acids producing

different aldehydes and so different aromas.

Effect on Fats

0014 Oxidation of fats and lipids is most likely more of a

problem during long-term storage of dried products.

However, high-fat products or those containing un-

saturated fatty acids are more likely to undergo chem-

ical reactions during the actual drying process. Dried

fish products are an example of a product type that is

particularly susceptible to oxidative deterioration of

fats and lipids. As oxidation and chemical reactions

proceed, the relative proportions of saturated and

monounsaturated fatty acids will increase, and the

proportions of polyunsaturated fatty acids will

decrease. Impingement drying, a high-temperature

short-time process, of pressed fish cake can help

minimize the oxidation of polyunsaturated fatty

acids. However, the formation of free fatty acids

during drying can also be a problem, and these will

contribute to overall product degradation during

subsequent storage. Polyunsaturated fat and caroten-

oid oxidation can be a problem in products such

as dried egg powder. Low spray-drying temperatures

are recommended in combination with the use of

antioxidants.

Effect on Nutrients

0015 Reactions involving minor nutrients during drying

are of particular concern in considering the chemical

changes during drying. For example, dried baby foods

and infant formulae are products where nutrient de-

struction could be potentially harmful to the health of

the consumer. The type of drying process chosen can

have a major impact on the potential for nutrient

degradation; for example, very few chemical changes

occur during the spray drying of infant formulae,

whereas roller drying can cause degradative reac-

tions, such as nonenzymatic browning, by the Mail-

lard reaction in the final product. In some cases, the

dried product may have improved nutrient availabil-

ity over more traditional processes; for example, it

has been found that the bioavailability of nutrients in

powdered formula is better than that in sterilized

liquid formula. This is most likely because the

powdered formula had a less severe heat treatment

during drying than the sterilization step in the pro-

duction of the liquid formula.

Effect on Vitamins

0016The water-soluble vitamins are sensitive to chemical

degradation as a result of drying; for example, loss

of vitamin C is dependent on the presence of heavy

metals such as copper and iron, light, water

activity, and the temperature of drying. Losses can

typically run between 10 and 50%. Loss of thiamin

during food processing has been extensively re-

searched, and losses of up to 89% have been reported

with certain combinations of moisture content,

drying temperature, and drying time. In general,

losses of water-soluble vitamins as a result of conven-

tional drying processes are less than 20%, and for

vegetables, these losses are generally less than 5%,

except for vitamin C.

0017The type of drying process again has a major influ-

ence on vitamin retention. Freeze-drying preserves the

vitamin C content more efficiently than processes

such as oven drying or solar drying. This can be

attributed to the high drying temperatures of the

oven dryer and the slow drying rate of the solar

drying process, and many researchers have reported

a relationship between increased drying temperature

and increased vitamin C degradation. Degradation of

vitamin C has been found to follow first-order reac-

tion kinetics in different products, such as guava and

potatoes. In general, vitamin C is better preserved

when rapid drying rates are used rather than with

slower drying rates.

0018Losses of fat-soluble vitamins can occur as a result

of oxidation and, again, can be linked with the drying

temperature. Losses of vitamins A, D, and E are often

reported to be negligible; the vitamin most prone to

loss would appear to be b-carotene, with other studies

reporting significant losses of carotenoids and toco-

pherols. It is often difficult to compare losses of vita-

mins, as retention will be dependent on a number of

factors, such as the food material, type and extent of

pretreatments, and drying conditions.

Effect on Dietary Fiber

0019Dietary fiber has also been shown to be modified

during processing. Thermal processing of wheat

bran, including by drum drying, causes an increase in

soluble dietary fiber and a decrease in insoluble diet-

ary fiber. The total dietary fiber content is unaffected.

Color Degradation

0020Color is often degraded during drying, especially

during the drying of fruits and vegetables. Again,

this can be correlated most often with drying tem-

perature; for example, a more rapid drying process

improves the retention of color as a result of reduced

degradation of carotenoids and chlorophyls in many

DRYING/Chemical Changes 1949

vegetable products. Degradation of carotenoids is

most often a result of oxidation, particularly as car-

otenoids have a high degree of unsaturation in their

molecular structure.

0021 Pretreatments can also have an effect on the carot-

enoid content of the final dried product; for example,

blanching of carrots in water causes more carotenoid

degradation during drying than steam blanching.

However, pretreatments is beneficial, as unblanched

controls will suffer the greatest degree of carotenoid

losses. The use of additives such as citric acid, salt,

and sodium metabisulfite can help retain carotenoid

during convection drying. Treatment of carrots has

been shown to have a pronounced effect on improv-

ing the carotenoid retention in carrots. Chlorophyl

has been shown to be fairly stable at low moisture

contents. However, degradation does occur at low

pHs, when it is converted into pheophytin.

0022 Again, the type of drying process used will affect

the overall color retention. For example, in a com-

parison between hot-air drying and microwave

drying, it was found that hot-air drying at 60



Chad

the least effect on color degradation of kiwi fruit,

whereas microwave drying caused the highest degree

of browning.

See also: Browning: Nonenzymatic; Carotenoids:

Physiology; Chlorophyl; Drying: Physical and Structural

Changes; Infant Foods: Milk Formulas; Powdered Milk:

Characteristics of Milk Powders

Further Reading

Borquez R, Wolf W, Koller WD and Spiess WEL (1999)

Impinging jet drying of pressed fish cake. Journal of

Food Engineering 40(1/2): 113–120.

Damodaran S (1996) Amino acids, peptides and

proteins. In: Fennema O (ed.) Food Chemistry, 3rd

edn. pp. 321–429. New York: Marcel Dekker.

Hwang JK, Kim CT, Cho SJ and Kim CJ (1995) Effects of

various thermal treatments on physicochemical proper-

ties of wheat bran. Korean Journal of Food Science and

Technology 27(3): 394–403.

Jayaraman KS and Das Gupta DK (1995) Drying of fruits

and vegetables. In: Mujumdar AS (ed.) Handbook of

Industrial Drying, pp. 643–713. New York: Marcel

Dekker.

Sarria B, Lopez-Fandino R and Pilar-Vaquero M (2001)

Does processing of a powder or in-bottle-sterilized liquid

infant formula affect calcium bioavailability? Nutrition

17(4): 326–331.

Sokhansanj S and Jayas DS (1995) Drying of foodstuffs. In:

Mujumdar AS (ed.) Handbook of Industrial Drying,

pp. 589–625. New York: Marcel Dekker.

Wilford LG, Saberez H and Price WE (1997) Kinetics of

carbohydrate change during dehydration of d’Agen

prunes. Food Chemistry 59(1): 149–155.

Hygiene

M N Ramesh and A Chakkaravarthi, Central Food

Technological Research Institute, Mysore, India

Introduction

0001Food hygiene is a subject of wide scope. It aims to

study methods for the production, preparation, and

presentation of food, which is safe, and shelf-stable. It

covers the proper handling of every variety of food-

stuff and drink, and the equipment and apparatus

used in their preparation, service, and consumption.

Good hygiene is a necessity in all food industries. The

consumer has the right to demand both good quality

and good shelf-life. The safe and hygienic processing

of food products is becoming even more important as

public awareness of food safety continues to increase.

Hygienic design of any food-processing equipment is

essential if a microbiologically safe product is to be

produced. Food should be nutritious and attractive. It

must be visibly clean and free from noxious materials

which may lead to food poisoning or gastroenteritis.

Foods heavily contaminated with food-poisoning

germs may be normal in appearance, odor, and flavor.

Microorganisms may be introduced directly from

infected raw materials, from workers, or the environ-

ment during the preparation of foods. Poisonous sub-

stances may be produced by the growth of bacteria

and molds in food.

0002The food-processing plant is made up of four basic

components:

0003factory layout, structure, fittings, and perimeter

0004equipment

0005personnel

0006handling of foodstuffs from raw material to the

final finished product

Plant for manufacture of food products should be laid

out, preferably in straight lines of product flow. Final

or intermediate processes must not be in a position

where there could be cross-contamination by raw

materials. Appropriate walls should physically separ-

ate the preparation areas for raw materials and dried

materials.

0007The risk of contamination of food products with

relevant microorganisms during open processing

increases with the opportunity to grow in poorly

designed driers as well as with the concentration of

the microorganisms in the environment. The risk of

airborne infection in open handling is much higher.

This means that, in open drying, environmental

1950 DRYING/Hygiene

conditions in addition to appropriate drier design

have an important influence on hygienic operations.

0008 This is often overlooked, in spite of the well-known

fact that microorganisms may be carried long dis-

tances in aerosols, which are easily produced during

cleaning of yards, floors, and the outsides of machin-

ery, not to mention water treatment. Another point of

importance in open food processing is the traffic of

people in process areas. As between the raw material

area and the process area, staff, facilities, and even air

should be kept strictly separated.

0009 In a closed process the function of the equipment,

the cleaning efficiency, and the control of process

parameters are most important for hygienic oper-

ation. There is a tendency to trust automation rather

than common sense for cleaning. Cleaning must be

considered as a process, and the product of cleaning

should be clean equipment. Inadequate cleaning may

be due to mechanical malfunction, the wrong type or

concentration of detergent, the wrong time and tem-

perature during cleaning, or wrong water quality

with regard to detergents used, or to several of these

in combination. Inadequate cleaning should be dis-

covered on process control and never be allowed to

cause loss of product through poor hygienic quality.

Inadequate cleaning, however, has many other eco-

nomic consequences, beside the contamination risk.

Poor product shelf-life, malfunction and corrosion

of equipment, overconsumption of detergents and

water, expensive waste water treatment, and energy

losses are just a few examples.

0010 Hygienic design of drier should be based on opti-

mum combination of mechanical, process, microbial,

and cost considerations. It is important that food

product and process are designed by competent food

scientists and technologists capable of understanding

the individual importance and interrelationships of

the physicochemical parameters. The water activity

) requirement of the various microbiological

agents of food spoilage and food poisoning should

be understood and, particularly, the limiting a

for

known pathogens should be known.

Processing Condition

0011 During conventional drying, hot air is used as a

heating medium. Air carries many microorganisms.

Concentrations of 100–10 000 microorganisms per

cubic meter are quite normal. The concentration

differs according to season and location. Therefore,

depending on the food produced or processed, con-

trol of airborne microorganisms may be necessary.

0012 Generally dry air with low dust content and higher

temperature has a low microbial level. The organ-

isms, which can be predominantly present in air, are

spores of Bacillus spp., Clostridium spp., molds and

some Gram-positive (e.g., Micrococcus spp., and Sar-

cina spp.) species as well as yeasts. If the surroundings

contain a source of pathogens (e.g., animal farms or

sewage treatment plant), different types of bacteria,

including pathogens and viruses (including bacterio-

phages), can be transmitted via the air. Ducts may

collect water and allow the growth of microorgan-

isms, which are subsequently carried by the flow of

air. It is important therefore to insure that air ducts

are fully self-draining or that other measures are

taken to prevent the accumulation of condensate.

0013The air used for drying must itself be of good

quality; thus removal of moisture, oil, and particles

from air is essential. Treatments such as inactivation

or removal of microorganisms present in the air can

be applied. Several methods may be used to reduce

the number of microorganisms in the air. These in-

clude physical treatments and chemical agents or a

combination of both. However, one of the most used

methods of producing sterile air with higher assur-

ance of sterility is filtration. Filtration is the removal

of particles, including microorganisms, from the air.

Air filtration has greatest particle potential of all the

separation methods. Air sterilization is the physical

removal of microorganisms from the air by filters of

appropriate retention efficiency. Depth filters are

made of cellulose, glass fiber mixtures with resin, or

acrylic binders. The fibrous sheet filters have a low

resistance to air flow and a large surface area. Such a

filter is required to provide air with an extremely low

microbiological load to aseptic areas.

Principles of Hygienic Design

0014The most important aim in the design of driers is to

avoid pockets and crevices. A pocket is relatively

large whereas a crevice is narrow and deep. Normally,

a pocket is nothing to be afraid of: it is more import-

ant to avoid crevices, which can cause trouble, be-

cause the detergent solution is not able to rinse them

clean. Sometimes hygienic design requirement may

conflict with functional design requirements. In such

cases, an acceptable compromise can be found.

Where this is not possible hygienic design should be

preferred and any deviation from the functionality of

the equipment has to be accepted, to insure the micro-

biological safety of foods. Hygienic design should be

based on an appropriate combination of mechanical,

process, and microbiological requirements. Comply-

ing with hygienic requirements may increase the life

expectancy of the equipment, and reduce mainten-

ance needs, especially when cleaning-in-place (CIP)

is built into the system, subsequently leading to lower

manufacturing costs.

DRYING/Hygiene 1951

Equipment Design

0015 Hygienic design of a very high standard is essential in

the modern plant to minimize down time for cleaning

and sterilizing. A drier used for drying food should

not be of such a nature that the food is in danger of

becoming unfit for human consumption, after drying.

The planning of a dehydration plant is just as import-

ant as the design of the drier. A good drier can be

dangerous if the planning is bad. Mechanization and

automation have made great progress in the cleaning

of the drier – both batch and continuous, previously

cleaned manually. Earlier, the machines were opened

every day, and if certain parts were not especially

hygienic they were given an extra cleaning. But

modern driers employ CIP with computer controls.

This will not affect the cleaning efficiency of individ-

ual components, but the computer helps to optimize

the total cleaning result.

0016 To produce microbiologically safe dehydrated

foods, a hygienically designed drier is needed. The

drier itself can be a main source of bacterial contam-

ination of foods, if the design and fabrication do not

conform to high levels of hygienic standards. Hygiene

in the drier is necessary to prevent food poisoning and

to reduce food spoilage. The main factors affecting

this are biological, but others, such as chemical and

physical factors, also contribute. A well-designed

drier is no safeguard against bacterial infection and

other contamination unless it is accompanied by

sound instructions for its installation, operation,

cleaning, and maintenance.

0017 To insure that it can be freed from relevant micro-

organisms, the drier must be properly cleaned before

the process begins. Equipment must be hygienically

designed. Where crevices and/or dead areas cannot be

avoided for functional reasons, equipment may have

to be dismantled for cleaning. It is easy to decontam-

inate a clean surface. The drier should be subjected

to inspection and treatment to control vermin and

infestation. Care must be taken to prevent the con-

tamination of foods with rodenticide and pesticide

residues.

0018 Five principles on which a minimum standard

could be established are:

0019 All surfaces in contact with food should be inert to

the food under the conditions of use, and constitu-

ents from their surfaces must not migrate into the

food or be absorbed by the food in quantities

which could endanger health.

0020 All surfaces in contact with food must be smooth

and nonporous so that tiny particles of food, bac-

teria, or insect eggs are not caught in microscopic

surface crevices and become difficult to dislodge,

thus becoming a potential source of contamination.

0021The drier should be so designed that all surfaces in

contact with food can be readily disassembled for

manual cleaning. In accordance with current prac-

tice, CIP techniques are preferred, but where this is

not practicable the design must allow for the ready

dismantling of the relevant parts for manual

cleaning.

0022The drier must be so designed as to protect the

contents from external contamination, including

accidental ingress of foreign matter, for example,

bolts, nuts, washers, and gaskets.

0023The exterior or nonproduct contact surfaces

should be arranged so to prevent harboring of

soils, bacteria, or pests in and on the equipment

itself as well as in its contact with other equip-

ment, and as far as possible with floors, walls, and

hanging supports.

Three basic factors that are important for hygienic

drier design are:

0024actual design, fabrication and finish

0025materials of construction

0026cleaning system adopted for the maintenance of

the drier

The drier should be designed to prevent the entry

of foreign materials, and the development of ‘out-

of-sight’ dead spots, especially within the operation

chamber and associated pipe work. Bolts and clips

should be attached externally to maintain all internal

product contact surfaces smooth and easily cleaned.

For operator safety, all equipment should be made

safe by electrical isolation during dismantling and

cleaning. During the design of continuous driers, me-

tallic belts are used to convey the material from inlet

to outlet through the drying chamber. Such driers

should be designed so that the whole of the belt is

accessible for cleaning. The belts should be made

endless, preferably by welding the two ends, instead

of riveting, which may be a potential spot for micro-

bial contamination. The transmission systems used to

drive the conveyor should be kept out of the chamber

and sealed to avoid any contamination from the

driving system, as shown in Figure 1.

0027The hygienic design approach for the food industry

differs from conventional design, as the hygienic re-

quirements have to be integrated with conventional

design to achieve good overall design results. The first

design requirement is ease of cleaning and maintain-

ing to insure that the drier will perform as expected

and to prevent any contamination during the drying

process. Under favorable conditions microorganisms

grow very rapidly. Consequently, dead legs, gaps, and

crevices, where microorganisms can be trapped and

multiply, must be avoided. When support structures

1952 DRYING/Hygiene

are attached to the floor or to walls, they must be

provided with legs of suitable height to provide a

minimum clearance for cleaning and inspection.

0028 Fabrication of the drier as per design is an import-

ant consideration. The surface finish of the product

contact parts should have an acceptable roughness

value (Ra) of 0.8 mm when measured following

ANSI/ASME B.46.1 methodology. Noncontact parts

must be smooth enough to insure ease of cleaning.

While welding joints, welds should be continuous and

free of imperfections. Postwelding operations like

grinding and polishing are mandatory.

Choice of Materials

0029 Materials used for construction must be nontoxic,

mechanically stable, inert, resistant to the product

being dried (at all stages of drying), and also to

cleaning and antimicrobial agents at the full range

of operating temperatures, The material should also

be smooth and with unpainted food contact surfaces.

Care is needed to insure that food materials do not

attack the metal; for example, acid reacts with mild

steel, and salt with aluminum. Equally, the construc-

tion materials must not attack the foodstuff, for

example, vegetable oil with copper. Appropriate ser-

vices for cleaning operations, such as hot and cold

water, drainage, compressed air, and vacuum lines

should be adjacent for cleaning operations. It is easy

to choose materials for machines used in the food

industry, as those parts in contact with the food prod-

uct or which are used as protection for the machines

should be stainless steel. The other parts come into

the category of normal machine construction.

0030 Material selection is generally done separately

for:

0031material not in contact, like frames, transmission,

etc.

0032materials in direct contact with feed material,

like trays, conveyors, feeding hoppers, scooping

arrangement, and feeding chutes

0033materials for components like seals, gaskets and

bearings, blowers, and ducting

Stainless steel is the universally accepted material for

contact parts. Normally only two grades of stainless

steel are used: AISI 304 and AISI 316 – a molyb-

denum alloy stainless-steel that is acid-proof. Both

are weldable and can be machined by normal ma-

chine tools. AISI 304 is often used externally, as it is

only needed to protect the machine from atmosphere,

water, and sometimes spilled liquids. A choice must

be made between AISI 304 and AISI 316 for parts

which are in contact with the product. AISI 304 tol-

erates the most common cleaning solutions and is

suitable for most purposes.

Cleaning Systems

0034Production operations require careful management

on a clean-as-you-go basis to maintain good house-

keeping, even during production at peak periods. A

structured approach is required to the formal

methods and the frequency of cleaning of all plant

and equipment, including instructions for reassembly.

Automatic cleaning, or CIP, should be introduced at

more and more points. For example, a conveyor is

used in continuous driers which can be cleaned auto-

matically after use, with the help of cleaning nozzles.

A great deal of manual work can be saved, as this type

of conveyor is common in the food industry. The

automatic wash system has been designed to achieve

the advanced cleaning required in today’s food-

processing environment. The high-pressure wash

system is designed to be housed over a conveyor belt

unit and employed in the periodic cleaning-down and

drying of the food-carrying belts at preset times. Ab-

solute hygiene is required for modern food-processing

plants and can be sized and fitted to incorporate any

part of the food-manufacturing process line, continu-

ally achieving 100% cleaning.

0035Certain driers often have to be completely disman-

tled for cleaning, but the dismantling could be made

simpler in many cases and involve fewer parts, with

quick-releasing systems which are easy to remove.

The spaces which are then to be cleaned should

be open and incline outwards so that all product

material and cleaning liquids can easily drain from

the machine. Much has already been done in this

field, but the food industry could encourage the

manufacturers of the machines to make their

Product

Geared motor

Covered and sealed transmission system

Conveyor

Belt

fig0001 Figure 1 Transmission system for conveyor driers. Repro-

duced from EHEDG Guideline No. 3 (1994) Hygienic design of

equipment for open processing. Trends in Food Science Technol-

ogy 6: 305–310, with permission.

DRYING/Hygiene 1953

machines easier to clean – and eventually automatic-

ally cleanable.

0036 During CIP, various flushing, cleaning, and sanitiz-

ing fluids are brought into contact with the wetted

parts of the plant. Cleaning may be done on a once-

through basis, or some of the cleaning agents may be

recirculated to reduce consumption of water, chem-

icals, and energy. Cleaning is achieved by the physical

action of high-velocity-flow jet sprays, agitation, and

the chemical action of cleaning agents enhanced

by heat. While mechanical forces are necessary to

remove soil and to insure adequate penetration of

cleaning solutions to all areas, most of the cleaning

action is provided by chemicals – surfactants, acids,

alkalis, and sanitizers.

0037 A CIP system consists of piping for distribution and

return of cleaning agents, tanks and reservoirs

for cleaning solutions, heat exchangers, spray heads,

flow management devices (supply-and-return pumps,

valves, sensors and gages, recording devices) and a

programmable control unit, as well as other items.

Although a CIP system requires a significant initial

capital investment, CIP is economical relative to

manual cleaning in most large-scale processes. CIP

greatly reduces labor. CIP methods often consume

less water and chemicals relative to manual cleaning,

especially if cleaning solutions are recycled. A prop-

erly designed, validated, and operated CIP system

insures consistent and reproducible cleaning. Because

there is little or no dismantling of equipment, the

down time is reduced and more time is available for

productive use of machinery. Consistent cleaning

eliminates product contamination and associated

rejection of batches. As another important consider-

ation, the CIP technology enhances safety by elimin-

ating or minimizing operator contact with hazardous

cleaning agents, bioactive products, and potentially

pathogenic biohazard agents. Other unsafe situations

are avoided because worker entry into equipment is

not required.

0038 A safety lock-out mechanism must prevent leakage

of detergent/disinfectant and foodstuffs during dehy-

dration of food. Modern industrial detergents and

disinfectants are often strongly acid or alkaline, and

if consumed in error in sufficient quantity, could give

rise to chemical poisoning. All CIP systems should be

supplemented periodically by the more conventional

strip-down methods of plant cleaning, with all coup-

lings open and gaskets removed. Chemical disinfect-

ants should be changed at intervals to prevent the

development of microbial resistance.

0039 In roller drying, used for manufacturing baby food

products, hygiene hazards will occur owing to partly

dried materials from the ends of the drier drum leav-

ing the drum and entering the main dry-product

collecting stream. This is more noticeable when ‘out-

board’ dam plates bear on the ends of the drum and

feed rolls, rather than inboard. To overcome this a

reciprocating doctor knife has been used to insure the

discharge of a continuous dried-product sheet, in

combination with short end and face scrape which

remove the partly dried material before it reaches the

doctor knife and the dried-product stream. The appli-

cator rolls are manufactured from SS316 stainless

steel, and the main drum is hard-chrome-plated

to insure that hygienic operating conditions are

maintained.

0040Where slurries are fed to the drier, steps should be

taken to prevent survival of microorganisms by

increasing the overall heat treatment so that,

apart from the heat exposure on the drum itself,

the slurries are preheated before transferring to the

applicator rolls. For thick slurries containing

starch, this can also be achieved by using a

scraped-surface heat exchanger or a steam injector.

0041Other slurries for roller drying, which are at

temperatures optimum for microbial growth, are

made up with ice at the milling stage to keep the

slurries at low temperatures in the holding tanks

before processing.

0042Dried foods are not always sterile. Some micro-

organisms and particularly bacterial spores will sur-

vive the dehydration process. There will be no growth

until the addition of water for reconstitution and

consumption. Spices, dehydrated soups, baby foods,

and other convenience-dried foods may all contain

spores, usually in small numbers. For example, the

addition of spices to meat will introduce spores and

add to the possibility of Bacillus cereus and Clostri-

dium perfringens food poisoning.

0043The principles of good hygiene practice in dehy-

drated food factories are basically the same as in any

other food-handling environment. The scale of food

manufacture, and the unique processes developed to

produce shelf-stable preserved foods for storage, dis-

tribution, and retail sale provide greater opportun-

ities for food-poisoning agents to become hazardous.

The most important factor for consideration is that

attention to the cleanliness of plant surfaces alone is

not enough; proper design and control of the com-

plete process are essential to insure the absence of

food-poisoning agents.

Food Hygiene in Dehydrated Food

Manufacture

0044Dehydration reduces the moisture content to levels

below which microorganisms cannot grow. Dehy-

drated foods, although not sterile, may be stored

1954 DRYING/Hygiene

indefinitely because of their low water content. The

bacteriological state of a product depends on

the degree of contamination before dehydration.

The growth and metabolism of microorganisms

require water in an available form.

0045 Dehydration is a method of food preservation that

arrests the activity of foodborne microorganisms and

retards chemical changes in the food. Microorgan-

isms require water in order to grow; however the

amount of moisture that permits growth varies with

different kinds of microbes. The term water activity is

used to designate the amount of moisture available in

foods. Drying decreases the a

of a food below the

limits required for microbial growth. The a

of any

food is in the range 0–1.0. The relationship between

and growth of organisms is shown in Table 1.

0046 Water activity in the range 0.995–0.980 is best for

most bacteria and spoilage is encouraged in meat,

fruit, and vegetables. At a

0.98–0.93 spoilage

by Gram-negative bacteria gives way to spoilage by

certain Gram-positive organisms; below 0.93–0.85

micrococci, yeasts, and molds can grow. Below 0.85–

0.60 certain fungi and yeasts predominate in spoilage

and below 0.60 there is no growth. The function of

preservatives, such as salt and sugar, is to reduce the

; heat may also remove water. The reduced a

important to prevent outgrowth of spores.

0047 It is very important to prevent the entrance into the

drying chamber of organisms which are capable of

producing toxins. This applies to the toxigenic forms

of C. botulinum and certain strains of Staphylococcus

aureus. Dehydrated foods should also be free from

certain intestinal microorganisms that are likely to be

pathogenic to humans, e.g., members of the genera

Salmonella and Shigella. The bacterial counts of dried

foods should be low to avoid development of undesir-

able flavors during the period of reconstitution.

Microorganisms respond to reduced a

levels. The

effect of reduced a

is characterized by an extension

of the lag phase, a suppression of the log phase, and a

reduction in the total number of viable microorgan-

isms. The minimal a

levels for growth and toxin

production of a number of foodborne pathogens are

shown in Table 2. Hence, the drying process should

be extended to reach the particular a

to eliminate

growth of such microorganisms. Heat applied during

a drying process causes a reduction in the total

number of microorganisms, but the effectiveness

varies with the kinds and numbers of organisms ori-

ginally present.

0048Usually, yeasts and most bacteria are destroyed but

spores of bacteria and molds survive, together with a

few vegetative cells of heat-resistant bacteria. More

organisms are killed by freezing than by dehydration

during the freeze-drying process. If the drying process

and storage conditions are adequate, there will be no

growth of microorganisms in the dried food. During

storage, there is a slow decrease in numbers of organ-

isms. The microorganisms that are resistant to drying

will survive. In particular, resistant spores of bacteria,

molds, and some of the micrococci survive. In con-

ventional drying of vegetables, the microbial flora

will be modified during drying. If the equipment is

not clean, an increase in counts may be anticipated.

Temperatures up to 90



C are used during the first

stage of drying. The rapid moisture loss from foods

during this period induces a cooling effect, and thus

the temperature of the product will be 40–50



Hence, reduction in microbial counts is small. In the

second stage of drying the product temperatures are

tbl0001 Table 1 Relationship between water activity and growth of organisms

Wateractivity Foods Predominant microorganisms

1.00–0.95 Most fresh foods, e.g., meat, fish, poultry, fruits, and vegetables Clostridium botulinum, Salmonella

Most normal bacteria

0.90–0.95 Many cured-meat products Staphylococcus aureus (anaerobic)

0.85–0.90 Most normal yeasts, S. aureus (aerobic)

0.80–0.85 Very high sugar or salt content Most normal molds

0.75–0.80 Halophilic bacteria

0.60–0.75 Dried foods Osmophilic yeasts and molds

Reproduced from Alur MD and Venugopal V (1999) Dried foods. In: Encyclopaedia of Food Microbiology, pp. 530–537. London: Academic Press, with

permission.

tbl0002Table 2 Effect of water activity (a

) on growth and toxin

production by some foodborne pathogens

Pathogenicmicroorganisms Minimum a

Growth Toxin production

Aspergillus flavus 0.82 0.78

Staphylococcous aureus 0.84 0.86

Clostridium botulinum type A and B 0.92 0.82

Listeria monocytogenes 0.92

Bacillus cereus 0.93

Vibrio parahaemolyticus 0.95

Salmonella spp. 0.96

Clostridium perfringens 0.96

Reproduced from Alur MD and Venugopal V (1999) Dried foods. In:

Encyclopaedia of Food Microbiology, pp. 530–537. London: Academic Press,

with permission.

DRYING/Hygiene 1955