Guy R. Extrusion Cooking - Technologies and Applications

Подождите немного. Документ загружается.

This page intentionally left blank

Part II

Specific extruded products

This page intentionally left blank

7.1 Introduction

Cooked wheat, barley or oats in the form of porridge have been common foods

for many hundreds of years, and are still very popul ar in several countries.

However, preprocessed, cereal-based products, like ready-to-eat cereals to be

eaten directly out o f the packaging, are relatively new. When one speaks of

ready-to-eat cereals, or breakfast cereals, the first thought that comes to mind is

of cornflakes for breakfast. Since they were first produced more than a century

ago by W. K. Kellogg in the United States, cornflakes have initiated a vigorous

development of the breakfast cereal industry, and stimulated creativity in

marketing and designing breakfast cereal products.

Today, the worldwide consumption of breakfast cereals amo unts to about 3

million tons. As shown in Table 7.1, this figure hides large dissimilarities

between countries and regions, depending upon their food culture and degree of

development. As a matter of fact, the per capita consumption is high in North

America and North Western Europe, but the volume growth of the market is

rather low (below 5%) . The growth mainly concerns breakfast cereals for adults,

through healthy diet specialities. In South America and South Western Europe,

the per capita consumption is still low (less than 1 kg), while the volume growth

ranges from 5 to 20%. Breakfast cereal manufacturers aim to supply products for

children and pay attention to taste (sweet flavourings), texture (crispness) and

nutrition (vitamins and minerals, in particular). In Eastern Europe, consumption

is very low (under 0.10 kg per capita), and breakfast cereals are eaten as bread

substitutes; Polan d is the largest and the most established market, while Slovakia

and the Czech Republic are the fastest growing markets. In Japan and Asia

Pacific, the per capita consumption is also very low (below 0.2 kg). It must be

Breakfast cereals

J-M. Bouvier, Clextral, Firminy

Table 7.1 Worldwide consumption of breakfast cereals and market trends

1–3

Country, region Consumption (1997) Yearly market growth Market trends (1995–98)

kg/capita Volume (1993–97) Sales value (1997–2002)

United Kingdom 7.6 3 to 4% < 1% Growth of healthy and diet specialties for adults

United States 4.5 3 to 4% < 1% (bran-rich products, crispy mueslis, etc.);

Germany 2.1 4 to 5% 3 to 4% steady or declining market of breakfast cereals for

France 1.5 6 to 8% 2 to 4% children.

Spain 0.75 6 to 8% < 1% Growth of consumption by children, mainly due to

Italy 0.5 15 to 25% 10 to 20% snacking in particular.

Eastern Europe < 0.10 20 to 25% – Bread substitute; exciting new market.

Latin America 0.25 to 1.0 15 to 20% – Growth of consumption by children in urban areas;

buying habits influenced by health benefits of

breakfast cereals (added minerals and vitamins).

Japan 0.18 3 to 4% 3 to 4% Health market-driven; overall growth limited by the

decline of the economy during that period.

Asia Pacific < 0.10 5 to 15 % < 0 Low consumption due to cultural issue; market

driven by young adults and urban professionals.

noted that the traditional model of ready-to-eat breakfast cereals with milk does

not apply, mainly for physiological reasons; lactose is not digested by Asian

children. Con sumption would only be in the form of snacks, and sweet snacks

are not very developed in these areas. Also, the economic problems in Asia

during the second part of the nineties depressed consumer markets in general,

and hence the consumption of breakfast cereals. However, some countries in this

region like South Korea, Taı¨wan and Singapore, do offer good opportunities for

breakfast cereal manufacturers, and the per capita consumption will probably

increase in the coming years, in urban areas in particular.

The popularity of breakfast cereals really stems from their nutritional content.

Such products can simultaneously provide energy (350–400 kcal/100 g),

nutrients (vitamins, minerals) and health-oriented components (dietary fibre,

for instance). Throughout the long history of breakfast cereals, advertising has

always underlined the health potential of the products, and consumers do

generally recognise such beneficial input. Today, the impact of nutrition on

health and disease is becoming a major concern of food designers, food

manufacturers and consumers. Thus, the healthy eating trend increasing ly

influences food choice, and breakfast cereals do not escape this development. Of

course, the health issue leads to innovation when designing breakfast cereals.

Over the last three decades, extrusion-cooking technology has played a very

important and decisive role in the innovation and development of breakfast

cereal products. The extrusion-cooking process is a new cooking concept –

thermomechanical HTST (high temperature short time) cooking – that makes

particular use of mechanical processing of the material and is therefore an

original alternative to classical hydrothermal cooking. T hen, i t allows

continuous cooking of a large range of recipes with various cereals to produce

different shapes and textures at satisfactory cost. Two types of extrusion-cooked

breakfast cereals can thus be found on the market:

• Directly expanded extrusion-cooked breakfast cereals. Cereal flours and/or

grits are cooked with ingredients and with a very low moisture content

(usually below 20%). The process may use single- or twin-screw extruders,

the configuration and operating characteristics of which generally lead to

highly mechanical cooking.

• Pellet-to-flakes extrusion-cooked breakfast cereals. Cereal flours and/or grits

are cooked with ingredients and at a moisture level in the range of 22–26%.

They are usually processed in twin-screw extruders, the configuration and

operating characteristics of whi ch lead to a lower mechanical component of

cooking, reinforcing the thermal component as opposed to the previous

processing conditions.

This chapt er begins with an overview of market segments, together with

breakfast cereal products which are on the market. Then, it focuses on those

products made by extrusion-cooking technology, describing and discussing

extensively the two generic extrusion-based processes. An important part of the

chapter is dedicated to describing and discussing the major unit operations and

Breakfast cereals 135

technologies of both processes: engineering analysis of process functions,

equipment design issues and their effect on product quality.

7.2 The range of products

The breakfast cereal section of supermarket shelves displays an impressive

range of products, which differ in their packaging design and sizes, as well as

product characteristics (nutritional value and sensory attributes, in particular).

The major manufacturers are competing to satisfy three important segments of

the population:

• children

• nutrition- and health-conscious consumers

• fitness-oriented consumers.

The market for children needs breakfast cereals which offer a large dive rsity

of tastes (honey, chocolate and malt), shapes (balls, cups, animals, puffed grains,

flakes, etc.), textures (from dense-hard to crispy-soft) and colours. These

products are generally packaged in colourful boxes displaying fancy animals and

characters. These requirements need to be balanced with those of the purchasing

parent for a healthy and nutritious product.

Adults show much more interest in functional breakfast cereals which

contribute to health preservation and fitness build-up. Typical products consist

of traditional and crispy mueslis, mixes of cereals with nuts and fruits, bran-rich

products, and so on. Breakfast cereals a re natural sources of complex

carbohydra tes, fibre and the B-group of water-soluble vitamins. Also, modern

manufacturing techniques like extrusion-based processes, make it possible to

complement the natural components of breakfast cereals with other groups of

vitamins (A, C, D, E), minerals (iron, calcium, phosphorous, magnesium, zinc)

and fibres (brans, soluble dietary fibre), to meet the expectations of the adult

segment of the market.

Market-driven factors and breakfast food cultures have both contributed to

promote three generic ready-to-eat breakfast cereals which are found on

supermarket shelves nowadays (Table 7.2):

• flaked cereals

• puffed cereals

• cereal mixes.

In view of their historical character, flaked cereals must b e mentioned first. In

fact, traditional cornflakes have good specific characteristics (crispy texture and

bubbled flat shape), and consequently a strong position on the market. Flaked

cereals (mainly corn and wheat flakes) traditionally use flaking grits, that are

raw pieces of grain endosperm. Flaking grits are hydrothermally cooked in a

batch steam cooker, then flaked and toasted to obtain the well-known texture

and shape. Flaked cereals can also be manufactured by extrusion-cooking.

136 Extrusion cooki ng

Table 7.2 Generic breakfast cereals. Process characteristics and product specificities

Breakfast cereals Main process characteristics

Main product specificities

Generic products Raw materials Cooking Texture Shape Taste Nutrition and Health Fitness

Flaked cereals Flaking grits Hydrothermal X X

(corn, wheat) (batch steam cooking)

Flours Thermomechanical X X X

(corn, wheat, oat) (continuous extrusion-cooking)

Flours and bran Thermomechanical X X X

(corn, wheat, oat) (continuous extrusion-cooking)

Puffed cereals Whole grains Hydrothermal X

(wheat, rice) (batch steam cooking)

Flours, grits and bran Thermomechanical X X X X

(any cereals) (continuous extrusion-cooking)

Cereal mixes Ground grains, nuts –

X X X

and dried fruits

Ground grains, nuts Thermal toasting X X X X

and dried fruits

Rather than specific flaking grits, any cereal flours including bran can be

thermomechanically cooked in a complete continuous process. Extrusion-

cooking technology then makes it possible to use cheaper raw materials as well

as to increase processing line productivity, and finally to decrease the price of

flaked cereals. Though the quality profile of the flakes produced from traditional

and extrusion-based processes is significantly different (textu re in particular),

owing to the different cooking characteristics of the processes, extrusion-

cooking technology has definitely helped promote basic flakes (corn and whe at

flakes) as well as to develop speciality flakes like nutrition- and health-oriented

flakes (bran flakes and oat-rich flakes, for example).

Traditional puffed cereals are produced by gun puffing precooked whole

grains (hard wheat or durum, long-grain white rice or parboiled medium grain

rice). Gun puffing is a batch steam-induced expansion process, leading to crispy

products the shape and texture of which are defined by the type of raw material.

The process is in fact quite limited since texture and shape of final products no

longer vary. A direct expansion extrusion-cooking process makes it possible to

use various raw materials and recipes, and thus to modify product characteristics

extensively. The process is continuous and flexible, and offers real ways of

optimising product quality and process productivity. Table 7.3 shows the major

directly expanded, extrusion-cooked breakfast cereals which are found on the

market today. Such products provide energy (370–400 kcal/100 g) and have

different textures, shapes, colours and flavour s; they are particularly adapt ed to

the ‘children’ segment of the market. The direct expansion extrusion-cooking

process can also texturize nutritious bran-rich and oat-based breakfast cereals.

Traditional mueslis generally consist of a mixture of various natural

components: ground whole cereal grains (wheat, barley or oats ), oilseeds

Table 7.3 The major directly expanded, extrusion-cooked breakfast cereals on the market

Product Shape Cereals Texture Flavour

Ball

corn crispy, honey,

highly expanded chocolate

Rice shape

rice crispy, neutral,

low hardness chocolate

Loop

wheat, oat, crispy, neutral,

corn relatively hard honey

Cup

wheat hard, chocolate

dense structure

Cylinders

corn crispy honey

Bran stick

wheat and bran crunchy and hard, malt

dense structure

Heart shape

wheat and oat crunchy and hard, malt and honey

bran dense structure

138 Extrusion cooki ng

(sesame and sunflower seeds), nuts (almonds, coconut, walnut), dried fruits

(raisins, apple, banana, etc.). Such mixes are a good compromise between the

energy content (around 350 kcal/100 g) and the nutritional pote ntial (10 to 12%

proteins, 8 to 10% fibres, relatively low sugar content under 15%, balanced lipid

composition) of the food. Crunchy and crispy mueslis are having more and more

success with adult consumers, as they offer a wide diversity of products, from

high energy to healthy mixes. Such products are usually composed of toasted

ground cereal grains, together with nuts and dried fruits. Toasted ground cereal

grains may also be sugar coated and flavoured (chocolate, honey, malt,

cinnamon).

In addition to traditional ready-to-eat b reakfast cereals (flakes and cereal

muesli mixes) which offer low product diversity and flexibility, an impressive

range of new ready-to-eat breakfast cereal products have been developed over

the last two to three decades to respond to the consumer’s demand for

convenienc e, pleasure, nutrit ion and heal th. Advanced tech nology, lik e

extrusion-cooking, has made a considerable contribution to satisfying this

demand by offering continuous and flexible processes, allowing the breakfast

cereal manufacturers to process various recipes and develop a wide range of

products with diverse textures and shapes, and ultimately to reduce the cost of

final products.

7.3 Key process issues of the product range

The cooking characteristics of extrusion technology enable breakfast cereal

manufacturers to process any cereals and starch-based recipes, at high

productivity and product diversity. Compared with conventional, batch

hydrothermal cooking at 90–110ºC wit h 15–30 m inute s residence time,

continuous thermomechanical cooking as applied in extruders operates at

higher temperatures (140–180ºC) with a shorter residence time (0.5–1.5

minutes). This is crucial in making extrusion-cooking processes much more

productive, due to mechanical shear in particular. Also, although hydrothermal

and therm omechanical cooking processes both convert starch polymers from a

semi-crystalline to amorphous state, there are still substantial differences

between the two types of cooking. The plasticizing effect of extrusion leads to

specific macromolecular structures of starch polymers. Moreover, as the extent

of starch dextrinization is negligible in hydrothermal cooking, it is v ery sensitive

to the shearing effect present in extruders, which helps vary the distribution of

the molecular weights of starch macromolecules. Thus, by combining variations

in the plasticizing effect and the dextrinization extent in extrusion, it is possible

to obtain various polymer structures and molecu lar weight distributions, leading

to a broad diversity in the functional properties of extruded polymeric melts.

Finally, extrusion-cooking technology offers wide potential for varying the

functional properties of polymeric melts, and hence the characteristics of final

breakfast cereal products.

Breakfast cereals 139