Strkalj, Lucija
(2025)
Fabricating functionally superior starch based products with fractionated Plantago seed mucilage.
PhD thesis, University of Nottingham.
Abstract
Starch plays a crucial role in nutrition and food production, serving as a primary source of energy in diets worldwide. Foods like potatoes, rice, and whole grains represent the base of a meal, serve as carbohydrate source, and contribute to overall health by promoting satiety and aiding digestion. Furthermore, starches are versatile in food production; they serve as thickening agents in cooking and because their behaviour varies widely in nature and they are conducive to further modifications, starches are integral to various processing applications beyond food, including pharmaceuticals and paper manufacturing. Due to its relatively low price, it is easily accessible around the globe, and its importance is studied from a culinary aspect, as well as health and metabolism. Despite its significance, starch consumption brings certain challenges. Firstly, starch and starch-based products are usually overprocessed and lack fibre which is essential for metabolic health. Fibre can reduce blood glucose spikes (a consequence of eating simple carbohydrates and starch), act as prebiotics to maintain gut microbiota, reduce cholesterol and associated risk of cardiovascular illness, and fibre contributes to satiety and healthy eating habits. The addition of fibre and hydrocolloids in starch has long been a subject of research, and improvement of functional properties has been observed by measuring physicochemical and functional properties, such as digestibility (in vitro and in vivo), texture analysis, thermal analysis, microstructure, rheology, pasting properties, and many more. Elucidating mechanisms of interaction between starch and fibre is extremely important as it helps to rationalize their use. Therefore, the aim of this thesis was to explore the addition of novel fibre materials to different types of starch, namely fractionated fibre from seeds of the Plantago genus. The most utilised and commercially relevant Plantago species is Plantago ovata, also known as psyllium. P. ovata is myxospermous plant, meaning that its seeds produce a gel-like coating called mucilage when wet. Importantly, in its dry form, P. ovata mucilage is essentially a layer of dried fibre that can be mechanically separated from the seed. Psyllium husk is often used as a fibre addition to many product categories but its functionality is fairly narrow which has led researchers to explore novel processing methods. Simple fractionations, where the mucilage polysaccharides are extracted with sequential steps usually involving temperature solubilisation and simple separation methods, have gained research interest as they can produce fibres with varying functional properties from the same starting material. Furthermore, the Plantago genus has over 200 species, many of them myxospermous, though underexplored, which prompted us to raise and answer certain research questions. Are there fibre fractions from other Plantago species which are comparable to P. ovata, and can we utilise their gel properties? How would their rheology differ? Would their addition in certain starches change the overall starch profile? Would those changes be transferable to food products? These questions helped shape topics and the flow of this thesis, which is presented in Fig. 1.1.
Chapter 2, a comprehensive literature review, brings an overview of nutritional and health benefits of P. ovata husk, as well as detailed summaries of extraction methods and rheological characterization. Challenges of psyllium husk application are described, which often coincide with the benefits of its high viscosity and proposed five solutions to said challenges with high level rationale.
Chapter 3 focuses on characterization of fibre fractions from four Plantago species (P. ovata, P. lanceolata, P. turrifera, P. drummondii), as one of the solutions proposed in Chapter 2 was to exploit natural variation in Plantago, and study the fractionation behaviour of underexplored species to discover additives with even broader functionality than could be gained from fractionating P. ovata alone. We compared corresponding fractions from each species and found that fractions between species shared key chemical similarities but differed in their rheological behaviour. Interestingly, P. turrifera and P. drummondii produced fractions with extremely high resilience under rheological deformation compared to P. ovata, which could be useful in product formulation.
Chapter 4 showed how inclusion of fractions characterised in Chapter 3 affect the quality (colour, freeze-thaw syneresis, rheology, and starch hydrolysis) of rice starch. This study revealed that fibre most likely disrupted the amylose network in gelatinised rice starch, but fibre addition improved viscoelastic properties under stress. Interestingly, Plantago fibre fractions increased starch hydrolysis rate in all but one composite gel, which was identified as promising candidate for the improvement of structure and digestibility of starch gels.
Chapter 5, therefore, explores in-depth microscopic and rheological characterization of the promising fraction identified in Chapter 4 when combined with five starches of varying properties at different storage conditions. In this chapter, we propose different interaction mechanisms between P. ovata fraction and starches, which were mostly dependent on amylose content of the starch.
Preceding chapters were highly fundamental though we were also interested in exploring fractionations with modifications that might make them more industrially relevant. Chapter 6 presents a one-step fractionation of commercial psyllium husk, where our aim was to study fractionated fibre application and its outcomes which would be industrially relevant. The main finding of this chapter was that the simple fractionation produced two psyllium fractions that had highly contrasting effects on corn starch with varying amylose content, and that those effects were also temperature dependent. This may open the door for adjusting food processing based on viscosity of individual ingredients and texture and digestibility properties of the final product.
Continuing the more application-driven component of this thesis, Chapter 7 investigates the addition of P. ovata and P. turrifera fibre, identified in Chapters 3 and 4 as having promising rheological differences, in gluten-free rice bread. We studied the effect of P. ovata and P. turrifera fibre addition on dough behaviour and texture, baking quality of breads, as well as the appearance, texture, storage behaviour of bread and its digestibility in in vitro analysis. Our goal was to see if observations from previous chapters would be transferable in complex food products, such as bread, and if the fibre addition improve quality of gluten-free bread. We showed that comparable fractions from alternative Plantago species might be suitable alternatives to P. ovata, whose supply is becoming more and more volatile.
Finally, Chapter 8, provides an overview of the main findings and avenues for future research.
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