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«THESIS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Multi-scale characterisation of pasta Effects of raw materials on water absorption, water distribution, ...»

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THESIS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

Multi-scale characterisation of pasta

Effects of raw materials on water absorption, water distribution, and microstructure

THOMAS STEGLICH

Department of Chemistry and Chemical Engineering

CHALMERS UNIVERSITY OF TECHNOLOGY

Gothenburg, Sweden 2015

Multi-scale characterisation of pasta Effects of raw materials on water absorption, water distribution, and microstructure

THOMAS STEGLICH

ISBN 978-91-7597-152-0 ©THOMAS STEGLICH. 2015 Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie Nr 3833 ISSN 0346-718X Department of Chemistry and Chemical Engineering Chalmers University of Technology SE-412 96 Gothenburg Sweden Telephone + 46 (0)31-772 1000

Cover:

Cross section of cooked spaghetti visualised by Magnetic Resonance Imaging and Light Microscopy. The figure illustrates radial changes in water-macromolecule interactions as well as in microstructure.

Dixa AB Gothenburg, Sweden 2015 Multi-scale characterisation of pasta Effects of raw materials on water absorption, water distribution, and microstructure

THOMAS STEGLICH

Department of Chemistry and Chemical Engineering Chalmers University of Technology ABSTRACT Pasta is a product with a long history, but is also still being developed today. Producers want to use new raw materials to make pasta more nutritious, less allergenic, and less dependent on durum wheat. All have in common that new raw materials shall not compromise the desired texture properties of cooked pasta such as the “al dente” feeling.

To facilitate the development of new pasta products, understanding the microstructure of pasta can be a tool. Water transforms and interacts with the microstructure during cooking and the outcome determines the texture. The main objective of this work was to analyse the interplay of microstructure and water, and how this is affected by the choice of raw materials.

We combined light microscopy and Magnetic resonance imaging (MRI) to study the microstructure and water distribution of pasta. We improved the resolution of MRI to yield data in 3D and were able to link MRI data to microstructure components such as fibre particles and the extent of starch gelatinisation.

Monitoring microstructure transformations during cooking and warm-holding of pasta revealed that some transformations are not dependent on the raw materials used. Water ingress towards the core is regulated by starch gelatinisation, which holds true both during cooking and warm-holding. The extent of the continuous starch and protein transformation from core to surface in cooked pasta is mainly governed by the product geometry. Also, texture changes during warm-holding depended mainly on the amount of available water within and around the pasta after cooking.

Nevertheless, raw materials are of importance: A higher protein content limited the degree of starch swelling within the gelatinised region in cooked pasta. Fibre particles can hinder water migration locally due to their perpendicular alignment against the direction of water ingress. Bran particles in particular do not absorb water during cooking, but redistribute it around the particles and create a strong variation in the degree of starch swelling. The severity of this effect correlated with bran particle size.

This thesis provides a comprehensive overview over how local microstructure and raw material choice affect water distribution at different scales in the cooked product.

Keywords: pasta, light microscopy, magnetic resonance imaging, microstructure, water distribution, texture properties, starch, gluten, bran, dietary fibre

This thesis is based on the work contained in the following papers, referred to by Roman numerals in the text:

I Microstructure and water distribution of commercial pasta studied by microscopy and 3D magnetic resonance imaging T. Steglich, D. Bernin, M. Röding, M. Nydén, A. Moldin, D. Topgaard, M. Langton Food Research International, 62, 2014, pp 644–652.

II Multi-scale characterization of pasta during cooking using microscopy and real-time magnetic resonance imaging D. Bernin1, T. Steglich1, M. Röding, A. Moldin, D. Topgaard, M. Langton Authors contributed equally Food Research International, 66, 2014, pp 132–139.

III Bran particle size influence on pasta microstructure, water distribution, and sensory properties T. Steglich, D. Bernin, A. Moldin, D. Topgaard, M. Langton Submitted to Cereal Chemistry IV Texture of pasta during warm-holding T. Steglich, A. Moldin, M. Langton Manuscript, intended for publication in Journal of Texture Studies I Planned, performed and evaluated the experiments (except for MRI parameter estimation), primarily responsible for writing and revising the manuscript II Planned, performed and evaluated the experiments, primarily responsible for writing and revising the manuscript III Planned, performed and evaluated the experiments, and wrote major parts of the manuscript IV Planned, performed and evaluated the experiments, and wrote the manuscript * MRI experiments in paper I-III were planned, developed, performed and evaluated in close collaboration with Diana Bernin.





All other types of experiments were performed by the author BFLM Bright-field light microscopy MRI Magnetic Resonance Imaging OCT Optimal cooking time PLM Polarized light microscopy Industrial-scale spaghetti DS Durum semolina DS+FB Durum semolina and wheat fibre DS+WG Durum semolina and durum whole-wheat flour DS+SW Durum semolina and soft wheat flour Laboratory-scale spaghetti S40D60 40% starch powder, 60% durum wheat flour D100 100% durum wheat flour G20D80 20% gluten powder, 80% durum wheat flour G40D60 40% gluten powder, 60% durum wheat flour WW0 0% durum whole-wheat flour, 100% durum wheat flour WW50 50% durum whole-wheat flour, 50% durum wheat flour WW100 100% durum whole-wheat flour, 0% durum wheat flour S440 10% bran fraction (median particle size 440 µm), 90% durum wheat flour S370 10% bran fraction (median particle size 370 µm), 90% durum wheat flour S160 10% bran fraction (median particle size 160 µm), 90% durum wheat flour S90 10% bran fraction (median particle size 090 µm), 90% durum wheat flour D100 and WW0 were produced with different batches of the same flour type Introduction

Objectives

Background

Durum wheat components

Starch

Protein/Gluten

Bran/non-starch polysaccharides

Pasta processing

Durum wheat pre-processing

Mixing

Extrusion

Drying

Transformations during cooking

Water transport models

Influence of starch and gluten properties on pasta quality

Influence of bran on pasta quality

Transformations after cooking: Storing pasta

Materials and Methods

Spaghetti preparation

Cooking and Warm-holding

Water absorption

Light microscopy

Magnetic resonance imaging

Cooking quality

Statistical analysis

Magnetic resonance imaging of pasta: Challenges and method development

Basic principles of MRI

Challenges in MRI for pasta research

MRI in this work

MRI of cooked pasta

Real-time MRI during cooking of pasta

Combining MRI and light microscopy

Results and Discussions

General structural transformations

Cooking: Water absorption and structure transformations

Microstructure of cooked spaghetti

Post-cooking: Warm-holding

Post-cooking: Storing at ambient temperatures

Effects of raw materials on texture, microstructure, and water distribution in pasta......27 Starch and protein content

Bran addition and bran particle size

Conclusions

Future research

Acknowledgements

References

Pasta is a universal food mainly made from wheat, but also from rice and other cereals. It has a century-old history with roots in China and Italy. However, the industrial production did not start before the 1950s (De Vita, 2009). Even in Italy the consumption of pasta rose first after this time, from being limited to feast days to everyday use. Today, Italians consume about 25 kg per capita and year. For comparison: Swedes consume 9 kg per capita and year (International Pasta Organisation, 2012).

Pasta and noodles are offered in manifold forms and ways - as fresh pasta, instant pasta and, most importantly, dried pasta. However, it is still not completely understood how the textural properties of cooked pasta are formed and influenced by every step of the production chain - starting from the choice of raw materials over production itself to end with how to keep the product warm after cooking.

A better understanding of these processes can aid to develop and improve pasta products.

The challenges are plenty: nowadays, cooked pasta is often stored before consumption; e.g. by holding it warm in a canteen kitchen or keeping it cold in ready-to-eat meals and salads. Other pasta products are produced with high amounts of dietary fibre to improve their nutritional profile, but this affects texture properties.

Studying pasta microstructure and its interactions with water can be one tool to improve the understanding of the material. Already more than 30 years ago, Resmini and Pagani (1983)

noted:

“Pasta proves to be an interesting limited water-starch-protein system where starch/ protein competition for water, conformational changes and mutual interactions take place during processing and cooking. The understanding of these phenomena, which may parallel that of other cereal products, can be enhanced by the study of pasta fine structure”.

The focus of the current research was to combine microstructure analysis with a spatially resolved analysis of water distribution in pasta and relate the findings to texture properties.

The goal of this work was to better understand how raw materials affect in cooked pasta texture properties such as firmness and stickiness. The main objective was therefore to characterise the interplay of water and microstructure during and after cooking of pasta.

A schematic overview shows the focus of the individual papers (Figure 1).

The specific aims were:

- Improve the spatial resolution of MRI to determine water distribution in pasta after cooking in detail (I, III)

- Improve the temporal resolution of MRI to monitor water migration in pasta during cooking (II)

- Determine the interplay of water and microstructure of pasta

- During cooking (II)

- After cooking (I, III)

- After warm-holding (IV)

- Elucidate the effects of raw materials (determined by their variation in protein, starch and fibre/bran content) on water absorption, water distribution, and microstructure as well as texture properties (I-IV)

- Determine in detail the effects of bran particle size (III)

- Define the factors causing texture changes in warm-held pasta (IV) Overview of main analytical focus of each respective study The term pasta describes generally sheeted or extruded wheat dough products. They are often categorised into (Asian) noodles and pasta (Marchylo et al., 2004). While noodles are based on common wheat flour and are sheeted, pasta is mainly based on durum wheat flour and is extruded.

The ultimate goal for every pasta manufacturer is to produce pasta with the best texture properties possible. What defines the best cooking quality might be subjective and consumer preference can vary from country to country, but cooking quality is often linked to being high firmness, low stickiness as well as overcooking tolerance (Marti et al., 2014).

Recently, much research has been directed to changing the raw materials used while maintaining the cooking properties by adapting the production process. The research efforts can be

grouped into three trends (references refer to reviews):

Substitute durum wheat with cheaper and local crops (Fuad and Prabhasankar, 2010), Increase nutritional value such as content of dietary fibre (Rawat and Indrani, 2014;

Sissons and Fellows, 2014), and Decrease allergenicity (in particular replace gluten) (Hager et al., 2012; Marti et al., 2014; Petitot et al., 2009).

Durum wheat semolina is seen as the most suitable raw material for pasta production. As a starting point for any raw material modulation, it is useful to understand how the major durum wheat components starch and protein form the structure for desired texture properties during cooking. Texture properties are related to microstructural changes during cooking, which in turn are affected by water and temperature.

!

For pasta production, generally only parts of the wheat grain are used, while whole-wheat pasta requires using the whole grain (Manthey and Schorno, 2002). The main parts of a grain are the endosperm, bran and germ (Figure 2). The endosperm consists primarily of starch granules and protein bodies, which are grouped in a cellular structure surrounded by thin cell walls (Kill and Turnbull, 2001). The bran comprises several protective cell layers while germ is the embryo of the grain and thus is rich in vitamins, minerals, antioxidants and dietary fibre (Manthey and Schorno, 2002).

To achieve homogeneous flours, only the endosperm is used and thus bran and germ are often removed during milling. The remaining endosperm is milled to flour and for common and durum wheat these flours are called farina and semolina, respectively. The texture affecting wheat components starch, protein and bran will be described in the following section.

%!

Starch is composed of the two polysaccharides amylose and amylopectin, which both are based on glucose residues. Amylose consists of a linear chain with -1,4 linkages, while amylopectin is highly branched through additional -1,6 linkages. The ratio between amylose and amylopectin can vary, but is generally one to three in wheat (Delcour et al., 2010). Amylose and amylopectin are synthesized in granular form with alternating amorphous and semicrystalline growth rings. The semi-crystallinity makes starch granules birefringent and visible in polarised light (Delcour et al., 2010).



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