Extrusion Cooking of Pulses

Aleksandar Yovchev | Michael Nickerson*

Photos courtesy Saskatchewan Food Industry Development Centre

In today’s marketplace, the nutritional quality of food products is of the upmost importance to the consumer. Pulses are a protein, carbohydrate, vitamin and mineral rich raw material that are underutilized in the food industry. One of the reasons for this is that in general the cooking of pulses tends to be time consuming. In order to continue to deliver healthy and convenient food products using ever changing ingredients, industry needs to look to available and emerging technologies. Extrusion cooking is a technology capable of both cooking and modifying the pulse ingredients to produce nutritious creative products with a wide range of textures. This review will introduce the principles behind extrusion cooking, and its potential for developing pulse-based ingredients and products.

WHAT IS AN EXTRUSION COOKING?
Extrusion cooking is a short time process in which food (feed) material is cooked by a combination of temperature, moisture, mechanical shear and pressure, as it is forced through a die.¹ As products exit the die, the release of pressure causes them to expand in different shapes, based on the die geometry. It is a relatively new food processing technique, compared to other traditional cooking methods.² Extrusion cooking is widely employed in the food industry because it is a continuous and energy efficient process that can be used to alter flavour, texture, and functionality, while offering superior control of processing parameters and greater versatility in ingredient selection as compared to conventional hydrothermal processing techniques.^3,4 Extrusion cooking has been used to produce a wide variety of food products, not only pasta and ready to eat breakfast cereals, but also baby foods, snack foods, dried soups and many more, as it improves digestibility and nutrient bioavailability.⁵ The use of extrusion as a food processing technology is gaining more and more interest, being one of the fastest growing trends in the food industry.

TYPE OF EXTRUDERS AND FACTORS TO CONTROL
Screws are the main elements of an extruder, they carry out the mixing, kneading, shearing and conveying operations within a single unit, and based on their number extruders are classified as single- or twin-screw. Single-screw extruders were developed in the 1940s to produce puffed cereals, whereas twin-screw extruders became popular in the 1970s. Some of the advantages of twin-screw extruders in comparison to single-screw include better handling of the raw materials, higher process consistency and productivity.² In general, the processing section of an extruder (barrel section) is divided into three zones: feed (mixing) zone, cooking zone and high pressure (forming) zone.² During the extrusion, in each section different processes occur (mixing, hydration, kneading, melting, expansion), as the raw ingredients undergo many order-disorder transitions, such as starch gelatinization and dextrinization, protein denaturation, and complex formation between lipids and amylose.^6,7 The conditions within the barrel can have both a positive and negative impact on the nutritional profile of the extrudates. The extrudates’ physical characteristics are strongly related to the physicochemical properties of the raw materials and extrusion processing variables. Materials low in protein (high in starch) tend to form low-density puffed-type products, whereas materials high in protein (low in starch) tend to form denser texturized products.⁸ For preparing expanded cereals, a minimum starch concentration of 60-70% is recommended.⁹ Depending on the moisture content, barrel temperature, screw configuration (gentle or assertive) and speed, die size and geometry, expanded products (e.g., puffed cereals) or low-moisture formed products (e.g., pellets) can be fabricated. The residence time for extrusion cooking can vary between 20 and 90 s, with a die temperature between 100 and 200°C and under pressure between 4 and 15 MPa.²

EXTRUSION OF PULSES
There is a variety of food products prepared by extrusion cooking. Several categories can be distinguished: ready-to-eat (RTE) breakfast cereals (puffed and flaked cereals, high-fiber strands), snacks and confectionary products (puffed snacks, pellets, licorice, chocolate), proteins and meat analogues (e.g., soy meat analogues), and infant foods (biscuits, weaning cereals, instant soups). The production of many snacks and RTE foods are in the form of expanded products, as expansion provides a desirable mouthfeel, which is related to the texture of the product.² Expansion is a critical part of the process which usually occurs with high-temperature and low-moisture extrusion as a result of a number of events, such as biopolymer structural transformations, nucleation, extrudate swell, bubble growth and collapse, with bubble dynamics contributing to expansion the most.^7,10 Conventional food extrudates contain significant concentrations of starch from corn, wheat, or rice in order to promote expansion of the product.⁸ As the protein content increases during blending of flours, for instance by adding pulses, the level of expansion in the product declines creating denser structures. Today, pulses are being extruded alone or in-combination with cereal flours for their nutritional properties¹¹ and for developing novel food structures with unique texture profiles. For instance, durum wheat semolina (containing vital wheat gluten) was fortified with 10% navy bean flour and pinto protein concentrates to produce spaghetti.¹² Sensory evaluation of the fortified spaghetti showed acceptable scores with good shelf stability. Faba bean protein concentrates produced by air classification have also been used for the partial replacement of wheat flour in noodle formulations without compromising the color and texture.¹³ In another instance, lentil-corn flour blends were used to create puff snacks with acceptable sensory properties to consumers.¹⁴ The level of expansion and porosity in these lentil-corn puffs could be controlled by altering the temperature, resident time within the barrel and moisture levels.¹⁵

HEALTH BENEFITS OF PULSE EXTRUDATES
Extrusion cooking can have both a positive and negative effect on the nutritional profile of the final product. For instance, extrusion causes pulse proteins to denature and unfold to improve their digestibility⁸, and also reduces the levels of enzymes (e.g., trypsin and chymotrypsin inhibitors) that inhibit protein digestion.^8,16 During the extrusion process, the concentration of dietary fiber and resistant starches in the final product increases to help reduce the risk of a colon cancer and improve product quality.^2,17-19 And the heating conditions used during processes acts to reduce or inactivate some of the anti-nutritional compounds present, such as lectins, enzyme inhibitors (trypsin, chymotrypsin and alpha-amylase inhibitors), phytates, etc. that interfere with their digestibility and mineral absorption.^2,20 However, it is unknown if it is desirable to remove some of these compounds. Recently, lectins and protease inhibitors have also been shown to help reduce the risk of various cancers, obesity and hypertension.²¹ Pulses are also rich in phenolic compounds which are known to be potent anti-oxidants however depending on the heating conditions (time and temperature), phenolic compounds can polymerize and cross link proteins to limit their digestibility.² Preservation of vitamins during extrusion also depends on the stability of vitamins present and the processing conditions. Thiamine (B1) and pyridoxine (B6) were shown to be more sensitive to heat in comparison to riboflavin (B2).⁵ During extrusion cooking the exposure to high temperatures can influence the stability of vitamin A and vitamin E.²² A decrease in α- and γ-tocopherol levels with an increase in extrusion cooking temperature, as well a decrease in γ-tocopherol with an increase in moisture during extrusion of grass pea seeds (Lathyrus sativus L.) has been previously reported.²³

Summary
In summary, the cooking and modification of pulse flours using extrusion technology will play a significant role in the utilization of pulses as companies start integrating pulse flours into traditional cereal based extrudate materials and producing gluten-free pulse based products to meet market demand for healthy and convenient foods.

 

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Aleksandar Yovchev and Michael Nickerson*

Department of Food and Bioproduct Sciences, University of Saskatchewan

 

(*Corresponding author email: Michael.Nickerson@usask.ca)