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

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natural ingredients with similar properties, if such are

available. This trend goes hand in hand with the push

towards more natural, more fresh-like products, evi-

dent from reviews of new product introductions on

the UK market. The most active market segment is the

chilled, ready-to-eat or ready-to-heat foods; typically

in single-serving packages, designed to meet the needs

of convenience and simplicity. But many of these

products have short shelf-lives, typically a few days

only, limiting the geographical area where they can be

marketed. Technologies that allow for a two- or

threefold prolongation of the shelf-life are much

sought after, and increasingly applied.

0007 It is not only the extended time of distribution and

storage that is attractive. Added product safety is

much in demand by producers and distributors.

0008 The fresh-like products are highly perishable, and

actions that add to the safety factors are important,

e.g., in the light of the new legislation on product

liability, and due diligence.

0009 On the other side of the shelf-life spectrum, trad-

itional long-life ambient storage products are also

affected by the consumer trends. Long shelf-life is

no longer an important selling point, with market

trends towards more fresh-like products. Therefore,

preservation technologies that prolong shelf-life but

do not act detrimentally on the product quality attri-

butes are in favor on the market. The push towards

shorter shelf-lives for ambient shelf-life products is

also supported by the demand for more rapid turn-

over of stock. This is due to the need to decrease

capital costs and to the more rapid market changes

pushed by improved communication between market

and production, through electronic data identifica-

tion (EDI) and computerized checkout counters.

0010 Finally, an important packaging trend is to down-

size material thickness and choose ‘mono’ material

packaging. With the resulting lower barriers to oxygen

of the packages, the product shelf-life is reduced.

0011 In conclusion, products with a very short shelf-life

will need preservation methods that will prolong

the shelf-life, while long-shelf-life products need

methods that give a shorter shelf-life but with im-

proved quality. These methods are to be found in

the group of technologies and methods called min-

imal processing.

Minimal Processing Technologies

0012 In minimally processed foods, storage, processing,

packaging, and distribution are accomplished in

highly integrated systems. Each step must be con-

sidered in conjunction with the other steps. Different

minimal processing methods can be applied at the

various steps in the food distribution chain, from

storage of agricultural products to packaging/pro-

cessing of the ready-to-eat product. In this introduc-

tion and overview, different minimal processing

technologies and methods will be presented following

the food distribution chain. Detailed accounts of

these methods can be found in the literature. A list

of the methods reviewed here is given in Table 1.

Modified Atmosphere Storage

0013By modifying the composition and sometimes the

overall pressure of the atmosphere in the storage

environment, quality-and safety-degrading biological

reactions can be slowed down or inhibited. For stor-

age of fruits and vegetables, increased carbon dioxide

concentration (up to 10%) and reduced oxygen con-

centration (3–5%) will retard respiration and prolong

the shelf-life. Relative humidity is also an important

factor, as is control of ethylene emission from the

respiring fruits and vegetables.

0014A reduced pressure during storage is also used,

often in combination with modified CO

and O

concentration levels. This method is often called

hypobaric storage, using pressures of some hundreds

of mmHg.

Postharvest Treatments

0015In order to add convenience to vegetables and other

agricultural products, centralized cleaning, peeling,

and cutting is common. The resulting products are

often less stable after the treatment, due to enzymatic

activity of cut cell walls and bacteriological con-

tamination from the handling. Various postharvest

treatment methods are employed to add biological

stability and extend shelf-life. Chlorinated cleaning

water is used. Soaking in solutions of reducing agents

such as ascorbic acid or sulfite or preservatives such

as sorbate or benzoate are used. Also, divalent ions,

2þ

, are used to strengthen the texture. In all these

treatments low temperature and good processing

hygiene are essential to achieve the desired shelf-life,

as treatments such as cutting instead reduce the

shelf-life.

Clean-Room Technologies

0016The objective in clean-room technology is to elimin-

ate microbiological contamination from humans or

from the environment. The handling of the food is

automated as much as possible. Equipment and the

processing environment are presterilized before pro-

duction. Air curtain and a positive air pressure are

maintained in the processing line using sterile-filtered

air. Production personnel use extensively protective

clothing to reduce air- and human-borne micro-

organisms. Clean-room technologies are primarily

4024 MINIMALLY PROCESSED FOODS

developed for fresh prepared food and for dairy prod-

ucts. Clean-room technology is often an expensive

method, which often means that the application is

limited to high-value-added products and to a limited

part of the production line. Also the hygiene classes

used in the food industry are higher than in other

industries using clean-room technology.

Protective Microbiological Treatment

0017 It is well known that many microorganisms produce

antimicrobial agents. Some lactic acid-producing

bacteria produce bacteriocins that are efficient in

stopping the growth of, in particular, Gram-positive

spoilage bacteria. By adding selected lactic acid-

producing strains to the surface of foods, controlled

growth can create an antimicrobial condition at the

food surface. The application of these methods is still

in development. (See Microbiology: Classification of

Microorganisms.)

Combination Methods: Hurdle Technology

0018 Reducing the levels of salt, sugar, or acid in foods

in order to improve consumers’ acceptance often

means increased perishability of the food. To attain

sufficient shelf-life, a combination of preservation

methods is often applied. The ‘hurdle concept’ is a

simplified illustration of the principle for combin-

ation processing. By fine-tuning a preservation system

with a number of methods, more knowledge is built

into the product – an important step in product

development.

Nonthermal Processing

0019High-pressure By applying pressures in the range of

some thousand atmospheres on biological material,

some enzymes and vegetative microorganisms can be

inactivated. Cell membranes are broken. Very high

pressures in combination with elevated temperatures

are needed for the inactivation of bacterial spores.

High-pressure treatment also changes the texture of

food, e.g., coagulate proteins and swell starches. The

method is commercialized in Japan and Europe for

treatment to prolong the shelf-life of low-pH fruit

products. In Spain, ham products are treated, with a

resulting prolonged shelf-life. In the USA, avocado

paste is treated at very high pressure. Extensive

tbl0001 Table 1 Applications of minimal processing methods

Process Applications Mechanisms

Controlled-atmosphere storage Bulk-stored fresh fruit and vegetables Antimicrobial effect (inhibition of aerobic

and anaerobic organisms); altered

respiration rates

Postharvest treatments Fresh vegetables Antimicrobial effect

Chlorinated water soaking

Reducing agents Oxidation prevention

Preservatives Antimicrobial effect

Divalent ions Improved texture

Clean-room technologies Fresh meat and fish Reduced levels of environmental

microorganisms

Protective microbes

(lactic acid bacteria)

Dairy products; sausages Release by the protective microbes of

bacteriocins that reduce pH and

microorganisms

Nonthermal processing methods

High-pressure treatment Many products, currently especially fruit

products

Microorganisms ruptured under high

pressure

Gamma irradiation Many products, particularly fresh fruits,

poultry, and spices

Ability of microorganisms to reproduce

eliminated

High-electric-field pulses Many products, particularly fruits Microbial cell rupture due to uneven

distribution of electrical charge across

cell

Thermosonication Drinks Synergistic effect of ultrasound and heat

New thermal processing methods Many products, especially finished meals Optimized heating regime reduces levels

of microorganisms while minimizing

thermally induced quality losses

(e.g., impaired flavor)

Ohmic heating

High-frequency heating

Microwave heating

Sous-vide technology

New packaging technologies

Modified-atmosphere packaging

and active packaging

Fresh meat and fish, prepared foods, baked

goods, fresh fruit, and vegetables

Antimicrobial effect (inhibition of aerobic or

anaerobic microorganisms); altered

respiration rates in fruits and vegetables

Edible films Dry, frozen, and semimoist foods Protection against oxygen ingress, moisture

loss, and flavor loss

MINIMALLY PROCESSED FOODS 4025

research and development is now taking place to inves-

tigate the possibilities and limitations of the method.

0020 Irradiation Very extensive research into the method

in the 1950s and 1960s, particularly the wholesome-

ness of the irradiated products, demonstrated that the

method is an efficient and safe preservation method.

In spite of this, the commercial use has been very

limited, due to consumer scepticism coupled with

legislative limitation. Use of the method is gradually

increasing in Europe and the USA, with applications

for fresh fruits, poultry, and spices. The method is

expensive, with typical processing costs of 0.1–0.2

Euro kg

1

product. Of course, this limits the applica-

tion to high-value products. (See Irradiation of Foods:

Applications.)

0021 High-electric-field pulses When cells are subjected

to electrical pulses with field strength of 15–35

kV cm

1

, cell membranes are broken due to an

uneven distribution of electrical changes on both

sides of the cell membranes. The broken cell walls

cause inactivation of microorganisms. The method is

more efficient the larger the cell, e.g., for yeast. Thus,

the most interesting future application is for products

where yeast growth is limiting the shelf-life, e.g.,

fruit products and other drinks. A number of active

research programs are found in Europe and the USA,

but the method is not commercial yet. (See Heat

Treatment: Electrical Process Heating.)

0022 Thermosonication By combining ultrasound treat-

ment at 20–40 kHz with heat treatment at moderate

temperatures, the inactivation of microorganisms can

be strongly enhanced. It will thus enable pasteuriza-

tion of drinks at lower temperatures with less ther-

mally induced quality changes. Thermosonication is

also efficient for enhancing enzyme inactivation. The

method is not used commercially in the food industry.

(See Preservation of Food.)

Thermal Processing

0023 Mild heating methods There is much interest in

mild heat treatment methods, which avoid excessive

temperatures, resulting in thermally induced quality

losses. Often a reduction of only a few degrees can

have a dramatic influence on liquid losses of meat or

fish. Yet the heat treatment needs to give microbio-

logical safety to the product. With the help of modern

process optimization and control methods, mild heat

treatments that combine these objectives have been

developed. Among these, most interest is found in

direct heating methods such as microwave heating,

that can be used to raise the temperature quickly

and shorten the processing time. For industrial

applications, the microwave equipment is designed

by computerized methods to control the heating uni-

formity, by controlling the overheating of the edges

and corners of the foods. The direct electric heating

methods are typically two to five times more expen-

sive than traditional heating methods. Thus their use

is limited to processes with benefits in terms of better

production yield or product quality.

0024Sous-vide cooking In sous-vide cooking, fresh food

is vacuum (sous-vide)-packed, under hygienic condi-

tions. The packed products are cooked at fairly low

temperatures in water to internal temperatures deter-

mined by culinary objectives. The long cooking times

give some tenderization effects. Excessive tempera-

tures are avoided, allowing for high moisture reten-

tion and juiciness. As the microbiological safety of the

sous-vide cooking can give rise to some concerns,

depending on the processing temperature, the shelf-

life is often limited to 6–20 days at temperatures

of þ3



Packaging

0025Modified-atmosphere The methods described

above for controlled-atmosphere storage are also

applied for individually packed food products. Fresh

meat and fish, prepared foods, and baked foods are

packed in modified atmosphere, with high concen-

trations of CO

. Permeability of the packaging

material to CO

is important to control.

0026Carbon dioxide is bacteriostatic and fungistatic.

Increased CO

concentration will thus inhibit the

growth of microorganisms as long as a sufficient

concentration of dissolved CO

is maintained in the

surface of the food. Refrigerated storage is required

for CO

to be effective. A wide range of fresh and

prepared foods are distributed and stored in the

atmosphere with high CO

concentration (50–10%).

Often O

concentration is reduced or essentially nil,

except for fresh meat, where it is held at 20% oxygen

in order to maintain the red meat color.

0027For vegetables, where the respiration is continuing

and the CO

concentration would increase to levels

that adversely affect the quality, microperforations in

the packaging film allow the respired CO

gradually

to escape from the package. (See Packaging: Pack-

aging of Solids.)

0028Active packaging This term covers packaging

methods and agents that actively influence the shelf-

life of the food during storage. The best-known

example is the oxygen absorbers or scavengers,

which reduce the head space and permeating oxygen

levels. The scavengers come as small sachets or tablets

to be introduced into the package. It is predominantly

4026 MINIMALLY PROCESSED FOODS

2 þ

ions that are used. The reduced level of oxygen

in the head space prevents development of rancidity,

and is also effective in reducing growth of certain

types of microorganisms, e.g., molds.

002 9 Other types of active packaging systems are etha-

nol vapor generators. The ethanol absorbed on silicon

dioxide powder and contained in paper sachets pre-

vents the growth of molds. They are mainly used for

bakery products.

Edible Coatings

0030 The rapid development of biodegradable films for

food packaging has helped to strengthen the devel-

opment of edible coating applied directly on food.

Coatings are made from films of proteins, starches,

or waxes. The coating will protect against oxygen,

aroma components, and moisture to the product,

reducing the requirements on packaging. Most films

are sensitive to moisture, which limits their applica-

tion to dry, frozen, and semimoist foods.

Minimal Processing in the Future

0031 The minimal processing technologies represent a

means of meeting the well-established, long-term

trends in consumer demands for convenience, variety,

and fresh-like quality. Such technology will allow

the food industry the possibility of producing high-

quality, high-value-added products to meet future

consumer demands. The technologies require varying

degrees of capital investment. But, most importantly,

the technologies require investment into product and

process know-how, not only of the minimal process-

ing technology itself, but also of the integrated chain

of food distribution, from agricultural production to

the consumer.

003 2 An important future area is the understanding of

the antimicrobial effects of enzymes and other bio-

chemical agents. In addition, nonthermal processing

methods need to be further investigated in terms of

the mechanisms of preservation as well as engineering

aspects of the industrialization of the methods.

003 3 Finally, future research into shelf-life treatments

must include integrated approaches involving many

different aspects along the production chain, such as

hygienic design, and automated handling of products

and packaging.

Seealso: Convenience Foods; Cooking:DomesticUse

ofMicrowaveOvens; Heat Treatment:Ultra-high

Temperature(UHT)Treatments;ElectricalProcess

Heating; Irradiation of Foods:BasicPrinciples;

Applications;ProcessingTechnology;Legaland

ConsumerAspects; Packaging:PackagingofLiquids;

PackagingofSolids; Storage Stability:Parameters

AffectingStorageStability

Further Reading

Ahvenainen R, Mattila-Sandholm T and Ohlsson T (eds)

(1994) Minimal Processing. SIK and VTT Symposium in

Majvik, Kirkkonummi, Finland, April 14–15, VTT

Symposium Series no. 142. VII Information Service,

PO Box 2000, FI-02044 VII, Finland.

Ahvenainen R, Mattila-Sandholm T and Ohlsson T (eds)

(1995) New Shelf Life Technologies and Safety Assess-

ment. SIK and VTT Symposium in Helsinki, Finland,

May 18–19. VTT Symposium Series no. 148. VII Infor-

mation Service, PO Box 2000, FI-02044 VII, Finland.

Francis GA, Thomas C and O’Beirne D (1999) The micro-

biological safety of minimally processed vegetables.

International Journal of Food Science and Technology

34(1): 1–22.

Leistner L and Gorris LGM (1995) Food preservation by

Hurdle technology. Trends in Food Science and Technol-

ogy 6(2): 41–46.

Martens T and Luchetti A (1999) Harmonization of Safety

Criteria for Minimally Processed Foods. Rational and

Harmonization Report. FAIR Concerted Action CT

96–1020. Copies can be obtained from www.euro-

pe.eu.int.

Minimal Processing of Food: A Challenge to Quality and

Safety. EFFoST conference in Cologne, Germany,

November 6–9, 1997. Wageningen, The Netherlands:

EFFoST, c/o ATO.

Ohlsson T, Ahvenainen R and Mattila-Sandholm (eds)

(1996) Minimal Processing and Ready Made Foods.

Symposium in Go

teborg, Sweden. April 18–19. SIK

Document 114. SIK, PO Box 5401, S-40229, Gothen-

burg, Sweeden.

Oliveira FAR (ed.) (1995, 1996 and 1997) Process

optimisation and minimal processing of foods.EU

Copernicus project. Proceedings of first, second and

third meeting. Copies can be obtained from www.euro-

pe.eu.int.

Singh RP and Oliveira FAR (1994) Minimal Processing of

Foods and Process Optimization. Boca Raton, FI: CRC

Press.

Wiley RC (ed.) (1994) Minimally Processed Refrigerated

Fruits and Vegetables. London: Chapman and Hall.

Mites See Insect Pests: Insects and Related Pests; Problems Caused by Insects and Mites

MINIMALLY PROCESSED FOODS 4027

MIXING OF POWDERS

N Harnby, University of Bradford, Bradford,

West Yorkshire, UK

Introduction

0001 The mixing of dry or near-dry powders is a very

common operation in the food industry. Powder

mixing may be required prior to dispersing the

powder into a dough, paste, or dispersion form, but

an increasing number of products are now packaged

and marketed directly to the customer in the dry

form. In either case a poor understanding of the

mixing process can result in a failure to mix consist-

ently and effectively.

0002 Many of the problems associated with powder

mixing arise because it is assumed that powders

behave in a similar way to the more familiar liquid

and gaseous mixing processes. They do not! The

major differences are that, in comparison to liquid

and gaseous systems, the powder system has poor

mobility and large primary particles. The poor mobil-

ity or flow leads to processing and packaging prob-

lems, while large particles lead to poor mixture

quality. (See Agitation.)

The Influence of Powder Flow on the

Mixing Process

Powder Flow Types

0003 For all process fluids a high mobility is generally a

desirable property. This speeds up the rate-controlling

processes and facilitates both process control and the

product-packaging process.

0004 Visual examination of a variety of powders leads to

a broad classification into free-flowing and cohesive

categories. Free-flowing powders, such as granulated

sugar, exhibit a smooth flow, an attractive nondusty

appearance, and little adhesion to the container. Co-

hesive powders, such as flour, have an erratic stick–

slip flow, are often very dusty, and stick to the walls of

the container. Particle size is evidently an important

determinant of flow type. If the mean particle size of a

powder is greater than about 50 mm, it will tend to be

free-flowing, whilst below that size it will tend to

be cohesive. This boundary between flow types is

dependent on more characteristics than particle

size, but it is a very significant boundary in that it

determines the philosophy of processing the powder

and the type of mixer to be used.

0005Particularly for the large-tonnage industries, the

attractions of a free-flowing powder from both a

processing and a marketing point of view tend to

outweigh the quality advantages of the cohesive

powder. The attractions of free flow are often so

strong that significant process costs will be incurred

in order to aggregate an otherwise cohesive powder to

achieve a free-flowing characteristic. Only when

product quality becomes dominant does the finer

texture of the cohesive powder become attractive.

This is often the case for high-value-added applica-

tions, such as in the pharmaceutical and ceramics

industries.

Free-flowing Powders

0006A single particle is the ultimate mobile element of a

powder mixture and, in contrast to the uniform mo-

lecular nature of the gaseous and liquid systems, each

of these elemental particles will have unique proper-

ties of size, shape, roughness, density, porosity, etc.

0007It is the uniqueness of individual particles that gives

rise to the extremely dangerous process characteristic

of segregation. If particles are subject to motion they

will not randomize as in a molecular system, but will

instead take preferred or segregating paths depending

on their individual characteristics. This can lead to a

gross loss of quality. If the constituent powder par-

ticles are chemically identical, then the segregation

will only lead to physical variation between product

packages. If there is also a chemical variation in a

powder mixture then segregation can be much more

dangerous, as the product will then have chemical as

well as physical variation between packages.

0008Free-flowing powders are especially prone to seg-

regation as individual particles are not subjected to

any structuring restraints and have considerable free-

dom of movement.

0009All powder motion can potentially cause segre-

gation, but the following are some of the more

dangerous:

0010Projection. Coarser particles are generally pro-

jected farther than finer particles if projected at

a constant velocity. Sources of projection in

powder processing can be identified in pneumatic

conveying, conveyor belt transport, chute flow,

and higher-speed mixers.

0011Percolation. Smaller, denser particles have a

greater ability and mobility to percolate through

a loose mass of particles. One of the severest

segregating mechanisms takes place when par-

ticles tumble or roll down an inclined plane.

4028 MIXING OF POWDERS

When particles are poured into a heap the coarser

particles roll to the edge of the heap and the fine

particles percolate through the mobile face of the

heap to form a central core. A rolling plane can

also often be observed as hoppers, drums, sacks,

and packages are being filled or emptied, or when

a drum of powder is rolled. Another form of per-

colating segregation occurs when a loose powder

is vibrated. Coarse particles tend to rise to the

surface as fine particles percolate to the base of

the container. Most product packages are vibrated

in the transport operation between manufacture

and receipt by the customer, and the product

homogeneity can be destroyed in the process.

0012 Elutriation. If a gas is passed through a loose

powder mixture, the smaller particles will tend to

be blown clear or elutriated from the mixture. This

process commonly occurs when containers are

filled. Powder displaces air as filling occurs, and

the air elutriates out the finer particles, which are

either blown clear or settle back on to the surface

of the container.

0013 As even small differences in particle size, shape,

and density can produce significant segregation, the

best way of eliminating or significantly reducing seg-

regation is to build a structuring mechanism into the

powder flow in some way. This can be done by redu-

cing the mean particle size of the mixture, adding a

fibrous or flake-like material, or adding some mois-

ture to the system. This is to say, making the powder

more cohesive. If this is not possible, then segregation

has to be minimized by careful choice of mixer and

mixture handling.

0014 For a free-flowing mixture, the philosophy of

mixer choice should be to minimize the opportunity

of individual particles to choose their own path of

motion and to impose a displacement on the particles

by the mixer. A rolling, tumbling mixer gives individ-

ual particles freedom and is undesirable, whilst a

ribbon or paddle mixer pushes and relocates groups

of particles and is to be preferred (Figure 1). For this

type of mixer, considerable thought should also be

given to the method of emptying the mixer, as the

emptying process can segregate an otherwise accept-

able mixture as the mixture discharges.

0015 Ideally, a free-flowing mixture should be packaged

immediately on discharge from the mixer as any inter-

mediate handling between discharge and packaging

can potentially destroy the mixture quality.

Cohesive Powders

0016 Cohesive powders have many unattractive properties.

They flow badly and can disrupt a packaging line.

They cannot easily be metered into the mixing

process. They can be dusty and potentially hazardous.

They are a potential source of contamination between

batches. They have an unattractive appearance.

0017From a mixing point of view, however, cohesive

powders have two very strong advantages. The cohe-

sive structure inhibits segregation and the relatively

small constituent particles provide a fine texture.

Both these advantages can be overriding if the mix-

ture quality and texture are the dominant require-

ments of the product.

0018Cohesive powders have poor mobility because in-

dividual particles are held in a structure with their

neighbors and cannot easily move independently. If

particles cannot move independently, they cannot

segregate in the gross manner associated with free-

flowing mixtures and the overall mixture quality is

enhanced. Structuring is also promoted when the in-

dividual particles are small and lack the weight to fall

free from the structure. Small particles also give a

better statistical chance of achieving a high mixture

quality.

0019A general comment would be that it is much more

difficult to produce a bad mixture with a cohesive

powder than with a free-flowing powder. With the

reduced mobility associated with cohesivity, mixing

will be a slower process but, unlike the free-flowing

mixing process, it is irreversible.

0020The two main problems associated with cohesive

powder mixing both relate to the structuring capabil-

ities of the powder. It is very easy for cohesive

powders to be held up in dead spaces in the mixer

and be isolated from the mixing process, and efficient

scraping or sweeping of the entire mixing volume is

essential. The second problem concerns the strength

of the structure between individual particles. If the

mixer does not impose a breaking force larger than

the structuring force, then aggregates of an ingredient

can pass through the mixing process without being

dispersed. The aggregates might be efficiently distrib-

uted throughout the bulk of the mixture, but on a

localized scale they are not producing the same

texture as individually dispersed particles.

0021Whilst the philosophy of mixing free-flowing

powders was to suppress the free movement of indi-

vidual particles, the philosophy for a cohesive powder

must be to encourage individual particles repeatedly

to break free from their structuring neighbors.

0022For many cohesive powders, the gentle rolling

and breaking action of a tumbler mixer might be

sufficient to cause restructuring. For strongly cohe-

sive or aggregating systems it may be necessary to

increase significantly the energy input to the process

by the addition of high-speed impactors or intensi-

fiers, or by creating a high-shear mixing zone within

the mixer (Figure 1).

MIXING OF POWDERS 4029

(a)

(b)

(c)

(d)

fig0001 Figure 1 Typical powder mixers: (a) the tumbler mixer; (b) ribbon blender; (c) impactor; (d) high-shear rollers. Reproduced from

Mixing of Powders. Encyclopaedia of Food Science, Food Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds), 1993,

Academic Press.

4030 MIXING OF POWDERS

The Texture of Powder Mixtures

0023 What is well mixed? Very often a powder-mixing

process is designed and installed without properly

defining the quality requirements of the product.

Mixture quality can be assessed on the basis of the

number of letters of complaint received from disap-

pointed customers, but usually a better and earlier

quantitative assessment is required. To make this

assessment it is essential first to identify the weight

of mixture which will be judged for quality by the

customer, and then to assess both the possible and

attained statistical quality achieved in the mixing

process.

Scale of Scrutiny

0024 It is essential to identify the least weight of a powder

mixture used by a customer in a single ‘application.’

This weight is frequently referred to as the required

‘scale of scrutiny’ for the mixture. Mixture quality is

sensitive to the scale of scrutiny and the smaller the

scale, the greater will be the difficulty of obtaining a

satisfactory mixture quality.

0025 Identifying the scale of scrutiny is often quite

straightforward. Thus, if a sachet of dried soup

powder is completely emptied into a pan of boiling

water, the quality of the soup is determined by the

weight and quality of powder delivered in one sachet,

and this is then the scale of scrutiny. If marketing

requirements decree that a cup rather than a pan of

soup should be the package content requirement then

the scale of scrutiny has been reduced, and it is pos-

sible that the mixer will no longer be able to meet the

higher quality requirement.

0026 A significant problem arises if the weight of pack-

age delivered to the customer is larger than the

customer’s scale of scrutiny. It could be, for example,

that a customer makes two small pans of soup from a

single sachet or takes many bowls of muesli from a

single packet. The manufacturer fills the sachet or

packet with a satisfactory mixture but, in the subse-

quent transporting and pouring operations, the con-

tents may well segregate and the customer receives an

unsatisfactory mixture when the original quantity is

subdivided. It is evidently desirable to deliver the

mixture to the customer in package quantities identi-

cal to the customer’s scale of scrutiny.

0027As the scale of scrutiny is the weight of mixture

assessed by the customer to determine quality, it is

also the sample weight removed from the process

by the manufacturer for a statistical assessment of

quality.

The Statistical Assessment of Quality

0028If samples are withdrawn from a series of points in a

mixture and analyzed separately for composition,

then the variance, S

, of these samples can be esti-

mated. A large variance indicates a poor mixture. In

the form of the standard deviation, this experimental

statistical value can be used to predict the number

of product packages which would be outside the

required composition tolerance.

0029When related to mixing data states, the experimen-

tal variance can also be used to predict the minimum

mixing times or the presence of segregation. The pro-

cess in Figure 2 illustrates three possible mixing states

for a binary mixture of equal-sized particles. The

unmixed state has the largest variance value S

,and

in most circumstances the best attainable mixture will

be that of the randomized mixture, S

. If segregation

occurs, the variance S

will not be attained and the

product will have some higher variance value.

(a) (b) (c)

fig0002 Figure 2 Possible mixing states for a binary mixture of equal-sized particles: (a) unmixed state; (b) segregated; (c) randomly mixed.

Reproduced from Mixing of Powders. Encyclopaedia of Food Science, Food Technology and Nutrition, Macrae R, Robinson RK and Sadler

MJ (eds), 1993, Academic Press.

MIXING OF POWDERS 4031

0030 For the illustrated binary system of equal-sized

particles, the limiting variance values can be predicted

¼ pq ð1Þ

and

¼ pq =A ð2Þ

where p and q are the proportions of the two ingredi-

ents and A is the number of particles in a given sample

size or scale of scrutiny.

0031 Evidently, few ‘real’ mixtures fulfill the require-

ment of being binary systems of equal-sized particles,

but the ideal form of eqns (1) and (2) enables us to

derive some important mixing principles which will

be applicable to multicomponent mixtures of multi-

sized particles.

0032 Eqn (2) illustrates the importance of the scale of

scrutiny on mixture quality. A large scale of scrutiny

gives a large value of A and a low potential variance,

. If the weight or the scale of scrutiny is fixed, then

an alternative method of reducing the potential vari-

ance would be to reduce the particle size and hence

increase the value of A. This is a quantitative assess-

ment of the value of mixing cohesive particles as a

means of improving texture. Whilst there is a

definable relationship between the random mixing

state and sample size, this relationship is not valid

for other mixing states and, for completely unmixed

systems, the variance value S

is independent of

sample size. The inability to relate quality to sample

size for intermediate mixing states makes it very diffi-

cult to predict the effect on the resultant mixture

quality of scaling up or down a scale of scrutiny.

0033 Eqns (1) and (2) can be used in conjunction with

the experimentally determined variance S

, to give a

mixing index (M). A common index is

M ¼

ðS

 S

ðS

 S

How far the mixture has progressed

How far the mixture could progress

In this case M ¼ 0 for no mixing and M ¼1 when the

mixture is randomized. Evidently, a plot of M against

time would indicate the minimum mixing time required.

0034 Eqns (1) and (2) are available for multicomponent

and multisized particulate systems. The equations are

more complex but the principles are the same. When

numbers are inserted for real systems some general

conclusions become evident.

0035 A few large particles in an ingredient have a sig-

nificant effect on attainable mixture quality. This

is a particularly important effect for minor ingre-

dients and emphasizes the importance of keeping a

tight control on any prior processes which affect

the size distribution of mixture ingredients.

0036For a multicomponent mixture, the randomiza-

tion of one ingredient does not insure that other

ingredients will be randomly distributed. It is

necessary to carry out separate statistical analyses

for at least the key ingredients.

0037The derivatives of eqns (1) and (2) enable calcula-

tions to be carried out to insure that the mixing

process is not attempting the impossible. When a

product at its scale of scrutiny contains relatively

few particles of one ingredient (say, less than 500),

the spread of the number of particles per sample

can be calculated and checked with the marketing

requirement. Typical examples of this calculation

would be the number of chocolate chips on a

cookie, the number of sausages in a can of saus-

ages and beans, or the number of mushrooms in a

soup packet.

0038Given that a scale of scrutiny, the proportion by

weight of the ingredients, the particle shape, density,

and size distribution of the ingredients are known,

then the possible attainable quality of the mixture

can be calculated. This may not be reached because

of segregation but it enables the potential quality of a

range of formulations to be assessed and compared to

a marketing requirement without carrying out any

experimental testing.

The Testing of Mixers

0039If properly carried out, the testing of a mixer with a

particular powder formulation is a long and expen-

sive process and at best is to be avoided and at worst

to be carried out with the most effective use of time

and effort. One essential first step is to calculate the

possible value of S

for a mixture formulation at a

given scale of scrutiny, and check that the mixture

quality required is attainable. A second essential is to

test the quality of the mixture as it exits the mixer and

is about to be packaged rather than in the mixer itself.

This is especially important for free-flowing powder

formulations. All testing procedures require that

samples be taken from the mixture and subsequently

assessed for the experimental variance value S

or for

some running statistical control. Especially for free-

flowing powders, such sampling procedures are very

prone to bias and, if this occurs, much expensive

effort can be wasted.

Sampling of Mixtures

0040The quantity of sample removed from a point in the

discharge of a mixture should be that of the scale of

scrutiny. A larger sample will give an optimistic meas-

ure of quality and a smaller sample a pessimistic

measure. It is important to retain the identity of

4032 MIXING OF POWDERS

every point sample withdrawn from the mixture and

to carry out a bulk analysis on each of the point

samples taken. Many misunderstandings occur be-

tween production and quality assurance sections

when the quantity of material required for analysis

is less than the scale of scrutiny of the mixture. This

reduces the effective scale of scrutiny, increases the

variance between samples, and indicates a poor mix-

ture quality. Ideally, analysis should be carried out on

the entire point sample but, if this is not practically

possible, then the sample should be riffled down to

the required analytical quantity.

0041 Poor decisions on where and how to take samples

can result in significant bias of estimates of sample

composition and quality of the mixture. This is espe-

cially so for free-flowing powders, which are prone to

segregation.

0042 If careful instructions are not given to the process

operator on where to take a sample, then the most

convenient sample will be taken. Typically this will be

on the surface of a drum, adjacent to a valve, or

nearest to the control room, and will almost certainly

be nonrepresentative of the bulk material. A simple

random selection using random numbers is statistic-

ally the best way of locating representative samples

but, for a routine sampling procedure, it can be

inconvenient. A systematic sampling process which

locates samples at a fixed time interval is an easier

selection process to apply, but care must be taken not

to choose a sample frequency which coincides with a

natural frequency of the process.

0043 The thief or probe sampler is commonly used to

retrieve samples from a selected location in a bulk

powder. For a free-flowing powder, it can easily be

demonstrated that this is a heavily biased retrieval.

Rules to be followed to minimize the bias of sample

retrieval are:

0044 Retrieve the samples from a flowing stream and

not from a bulk of powder. The mixture should

ideally be sampled as it discharges from the mixer

and not in situ.

0045Sample the entire section of flowing powder to

allow for any segregation across the flowing stream.

0046Take as many samples at the required scale of

scrutiny as possible, as this will improve the statis-

tical precision of the quality estimates. The reader

is referred to a standard text to determine the pre-

cision reward-to-sampling-effort ratio but, for an

assessment of a batch mixture quality, there would

be little point in taking fewer than 10 samples.

0047When samples are taken in a time sequence, con-

siderable benefit can be obtained by plotting

composition against time to produce a ‘discharge

profile’ for the system. A mixture variance S

can

be calculated for the mixture but the discharge profile

often reveals cyclical variations in the product, which

can result in both process improvements and econ-

omy of sampling effort.

See also: Agitation; Bread: Dough Mixing and Testing

Operations; Cakes: Methods of Manufacture; Cereals:

Handling of Grain for Storage; Cleaning Procedures in

the Factory: Overall Approach; Marine Foods:

Production and Uses of Marine Algae; Packaging:

Packaging of Solids; Plant Design: Basic Principles;

Quality Assurance and Quality Control