Rheological characterization of coloured oil-in-water food emulsions with lutein and phycocyanin added to the oil and aqueous phases

Abstract

The use of natural colourings in food products presents nutritional advantages, and certain pigments are associated with functional properties, e.g. antioxidant effects. This can be very advantageous in food products with high fat contents like mayonnaises. The aim of this work was to study the effect of adding natural pigments, lutein and phycocyanin, to the water and oil phases, respectively, of oil-in-water pea protein-stabilized emulsions, beyond the desirable and expected development of a green colour. The emulsions were prepared with 3% (w/w) pea protein, 65% (w/w) vegetable oil, and varying concentrations of lutein and phycocyanin (0.25–1.25% w/w). Emulsions containing both pigments, in different proportions (total pigment concentration of 0.5%, w/w) were also prepared. Oscillatory and steady-state measurements, as well as droplet size distribution and textural profile tests, were performed. The addition of lutein to the disperse oil phase promoted a decrease of the emulsions' rheological and textural parameters, while the addition of phycocyanin to the water (continuous) phase resulted in an increase of these properties. When using combinations of both pigments, an increase of the rheological and textural parameters with phycocyanin proportion was apparent, and a synergetic effect due to the presence of both pigments was found.

Introduction

Consumer's demand for more natural food products, presenting health benefits, has increased over the years. Besides the nutritional aspects, an appealing appearance with positive sensorial characteristics is also desired. As a result of this trend, the use of natural colourings in food products is preferred, while the synthetic colourings are perceived as undesirable and harmful and some are considered to be responsible for allergenic and intolerance reactions.

Many natural pigments currently used primarily for colouring purposes are phytochemicals, which have been associated to a nutraceutical effect (e.g. Guhr and Lachance, 1998, Pszczola, 1998, Mazza, 1998), namely antioxidant activity (Downham & Collins, 2000). Natural antioxidants are now thought to possibly prevent the incidence of many diseases such as cancers, cardiovascular and age related diseases, by protecting cells against oxidative damage.

Carotenoids are a main source of natural colourings, widely distributed in nature, and over 600 pigments with varied structures have been identified (Olson & Krinsky, 1995). Carotenoids are terpenoid compounds with a long conjugated polyene chain, which determines the light absorption properties and consequently the colour (yellow to red) and phytochemical properties (Astorg, 1997). These compounds are biosynthesised by bacteria, algae, fungi and plants (Armstrong & Hearst, 1996), but not by animals, who have to obtain them from their food (Astorg, 1997). Carotenoids have been used by the food industry to colour foods and beverages (e.g. orange juice) and are precursors of many important chemicals responsible for the flavour of foods and the fragrance of flowers (Gouveia & Empis, 2003).

Much scientific research has been completed on the health benefits of carotenoids that lie in two main areas: antioxidant and pro-vitamin A activity (e.g. from β-carotene). Several epidemiologic studies have demonstrated a correlation between increased carotenoid intake and reduced incidence of cardiovascular disease (Halliwell, 2000) and certain types of cancer, such as breast and lung cancer (De Stefani et al., 1999, Tavani et al., 1999, Reddy et al., 2003), atherosclerosis, cataracts, macular degeneration and other major degenerative diseases (Cooper, Eldridge, & Peters, 1999) as well as an increased resistance to viral, bacterial, fungal and parasitic infection (Gouveia & Empis, 2003).

Carotenoids can be classified in two types of compounds: carotenes, which are unsaturated hydrocarbons, e.g. licopene, α- and β-carotene; and xanthophylls, more abundant in nature, which present one or more functional groups containing oxygen, e.g. lutein, astaxanthin and cantaxanthin.

Lutein and zeaxhantin (3,3′-dihydroxy-α-carotene) isomers (they differ by the position of one double bound) are xanthophylls, carotenoids with two hydroxyl groups in the conjugated polyene chain (Krinsky, Landrum, & Bone, 2003).

Lutein, which is the most abundant of these two isomers, can be found in green leaf vegetables like broccoli and spinach and are commercially extracted from the Marigold Flower (Tagetes erecta) for the production of orange colourings for the food and beverages industry and production of dietary supplements. Epidemiological studies provide evidence to suggest that lutein may protect against Age-related Macular Degeneration (AMD), a leading cause of blindness in people over 65 (Seddon et al., 1994, Pratt, 1999, Mozaffarieh et al., 2003). This is related to the fact that lutein and zeaxanthin are the only carotenoids quantitatively identified in the human macula (Bone, Landrum, Fernandez, & Tarsis, 1998), and absorb the short wavelength light preventing damaging effects on the retina (Ham, 1983).

All carotenoid molecules present a hydrophobic character, and consequently have affinity to the disperse oil phase of the emulsion. Nevertheless, it should be considered that terminal hydroxyl groups of xantophylls, like lutein, can provide a certain polar character (Khachatryan, 2003) and potential interfacial activity, as well as the ability to bind with proteins through intermolecular hydrogen bonds (Bassi et al., 1993, Moros et al., 2002).

Another source of natural colourings are phycobiliproteins, which are water soluble fluorescent proteins derived from cyanobacteria and eukaryotic algae. In these organisms, they are used as accessory or antenna pigments for photosynthetic light collection. They absorb energy in portions of the visible spectrum (450–650 nm) that are poorly utilized by chlorophyll and, through fluorescence energy transfer, convey the energy to chlorophyll at the photosynthetic reaction center (Arad & Yaron, 1992).

Phycobiliprotein structure includes linear tetrapyrrole chromophores (bilins) covalently linked via one or two tioether links to the cystein residues of apoproteins. In many algae the phycobiliproteins are arranged in subcellular structures called phycobilisomes, which allow the pigments to be arranged geometrically in a manner which helps to optimize the capture of light and transfer of energy (Arad and Yaron, 1992, Borowitzka, 1989).

Phycocyanin is a blue phycobiliprotein (absorption maximum at 610–650 nm), from Spirulina maxima (Arthrospira) widely used as a fluorescence marker in clinical diagnosis, and also as colouring in the pharmaceutical and cosmetics industry. Using phycocyanin as food colouring is not yet permitted within Europe or the USA but it is authorized in Japan, where it has been used in products like chewing gums, candies and dairy products (e.g. yoghurts) (Kawasaki and Kaneko, 1978, Ogawa et al., 1979, Shoten and Kogyo, 1981). Antioxidant, anti-inflammatory, neuroprotective and hepatoprotective effects have been attributed to phycocyanin (Romay et al., 1998, Romay et al., 2001, Romay et al., 2003).

The use of these natural colourings in food systems is limited by the nature of the food matrix in which the pigment is to be dispersed and to the interactions with the other food components (proteins, polysaccharides, lipids, sugars, salts, etc.). In food emulsions, the addition of colouring pigments can affect the stability of the system according to their distribution between the oil, water, and interfacial regions and to specific interactions with the local environment, especially with emulsifier molecules. The molecules in an emulsion distribute themselves among these three regions according to their concentration and polarity (Wedzicha, 1988). The molecules that have significant amounts of both polar and non-polar groups tend to be surface active (Dickinson, 1992, Dalgleish, 1996, Damodaran, 1996), while polar molecules tend to be located primarily in the aqueous phase and non-polar molecules in the oil phase.

The constituents in a food emulsion interact with each other, either physically or chemically, to determine the overall physicochemical and organoleptic properties of the final product (Dickinson and Stainsby, 1982, Wedzicha et al., 1991, Coupland and McClements, 1996, Landy et al., 1996). The efficient production of high quality food emulsions therefore, depends on knowledge of the contribution that each individual constituent makes to the overall properties and how this contribution is influenced by the presence of the other constituents (McClements, 1999).

Oil-in-water food emulsions are often stabilized by proteins. Previous studies have showed that vegetable proteins from leguminous seeds, like lupin (Raymundo et al., 1998, Raymundo et al., 1999, Raymundo et al., 2002, Franco et al., 1998), pea (Bower et al., 1999, Franco et al., 2000, Tomoskozi et al., 2001) and soy (Bogracheva et al., 1994, Eliazalde et al., 1996, Roesch and Corredig, 2002, Floury et al., 2002) can be good emulsifiers. These proteins present some additional advantages such as no cholesterol, positive agricultural impact and expectable superior microbiological quality.

The aim of this work was to study the effect of adding natural pigments, phycocyanin and lutein, to the aqueous and oil phases, respectively, of pea protein-stabilized oil-in-water emulsions. Linear viscoelasticity, flow behaviour, texture analysis and droplet size distribution of the emulsions were evaluated for various pigment concentrations. The evolution of the emulsions colour parameters (Lab) with pigment concentration and over time has been previously studied (Raymundo, Osório, Reis, Sousa, & Empis, 2002).