Elsevier

International Dairy Journal

Volume 61, October 2016, Pages 142-147
International Dairy Journal

Citric acid cross-linking of heat-set whey protein hydrogel influences its textural attributes and caffeine uptake and release behaviour

https://doi.org/10.1016/j.idairyj.2016.05.008Get rights and content

Abstract

Heat-induced whey protein gels were either pre-loaded with caffeine, then cross-linked with citric acid, or vice versa. Based on the results of Fourier transform infrared spectroscopy, the pre-loading then cross-linking procedure caused less cross-linkage formation than the cross-linking then caffeine loading (post-loaded) procedure. The latter also resulted in a firmer gel. The caffeine-to-protein ratio was approximately 8.5 μg mg−1 in the non-cross-linked hydrogel; those of the caffeine-loaded then cross-linked and the cross-linked then caffeine-loaded hydrogels were 4.5 and 2.9 μg mg−1, respectively. Cross-linking decreased swelling of the freeze-dried gels upon subsequent rehydration. It also influenced the extent of caffeine release from the protein hydrogel. Pre-loaded and post-loaded samples retained ∼78% and ∼88%, respectively, of the entrapped caffeine after immersion in water for 4 h; the non-cross-linked gel retained <55% of the initially loaded caffeine. Caffeine loading decreased gel water-holding capacity; subsequent citric acid cross-linking increased water-holding capacity.

Introduction

Whey protein hydrogels are conventionally formed either by heat-induced (Oztop, Rosenberg, Rosenberg, McCarthy, & McCarthy, 2010) or cold-set (Kuhn, Cavallieri, & da Cunha, 2010) gelation processes. Heat gelation of whey proteins involves consecutive events, starting with protein unfolding and exposure of the buried –SH group of β-lactoglobulin, followed by formation of molecular aggregates, interactions among the aggregates to form a network, and, finally, absorption of molecular aggregates and free whey proteins from the bulk to the network (Nicolai and Durand, 2007, Nicolai et al., 2011). Such gels are promising for loading, and subsequent controlled release, of nutraceuticals and drugs in food and the medical sector (Gunasekaran, Xiao, & Ould Eleya, 2006). It is noteworthy that nutraceutical molecules may influence the properties of the host hydrogel, and result in modified techno-functional attributes. Therefore, characterisation of cargo-loaded gels in terms of textural properties, as well as the impact of loaded substance on the subsequent reactivity and chemical functionality of proteins, is of utmost significance.

Caffeine occurs naturally in coffee, tea, cola nuts, mate and guarana (Suzuki & Waller, 1984). It is a purine alkaloid with heterocyclic structure and has important pharmacological function. Caffeine is classified as a member of the methylxanthine family of drugs, and is the most widely consumed behaviourally-active substance in the western world. Caffeine has also impacts on the endocrine, cardiovascular, respiratory, renal and gastrointestinal systems (Horrigan, Kelly, & Connor, 2006). Biologically active compounds, such as caffeine, can be immobilised within hydrogels, then released in response to alterations in microenvironmental conditions. If the cargo uptake and release of whey protein hydrogels are elaborated, a biocompatible delivery system with controllable and predetermined characters can be designed for therapeutic purposes (Graham, Zulfiqar, MacDonald, & McNeill, 1987).

Protein molecules are vulnerable to modification due to the presence of free carboxyl and amino groups. Modification may occur by a variety of means such as pH manipulation, chemical derivatisation, and cross-linking, leading in turn to modified chemical and functional properties (Garti & McClements, 2012). Cross-linking of whey proteins, for example by citric acid, alters the textural and techno-functional properties of resulting gels (Eissa, Bisram, & Khan, 2004). Citric acid is a readily available, nontoxic, generally recognised as safe, and low cost substance and has been employed for alkali-catalysed low-temperature wet cross-linking of proteins (Reddy, Li, & Yang, 2009). In the presence of an alkali, deprotonated amino groups of proteins attack the partially positively charged carbon of carboxyl group of citric acid, resulting in a nucleophilic substitution. If more than one carboxyl group of citric acid (which has three carboxyl groups) and two or three protein molecules involve in the substitution reaction, intermolecular cross-linking happens (Xu, Shen, Xu, & Yang, 2015).

Farjami, Madadlou, and Labbafi (2015) prepared cold-set hydrogels from whey protein microgels cross-linked with citric acid. It was found that the cross-linking stage, i.e., either before or after microgel formation (by heating at 85 °C, pH 5.8, for 15 min), had a profound impact on the rheological properties of the subsequently formed bulk hydrogel. Cross-linking of whey proteins prior to microgel formation yielded firmer (higher fracture stress and complex modulus) hydrogels. The applicability of citric acid for cross-linking of whey protein gels, rather than whey protein molecules and microgels, remains undefined and no report was found in the literature about citric acid cross-linking of heat-induced whey protein gels. The objective of the present study was therefore to cross-link heat gelled whey proteins either before or after caffeine loading and investigate the effect of such treatment on the textural, functional and caffeine uptake/release behaviour of protein gels.

Section snippets

Materials

Whey protein isolate (WPI) with 90% protein content and 0.1% calcium content was donated by Arla Food Ingredients (Viby J, Denmark). Extra pure sodium hydroxide (NaOH), citric acid monohydrate and sodium azide were purchased from Merck (Darmstadt, Germany). Caffeine of >99.9% purity was procured from FTZ JC Yujie International Inc. (Qingdao Shandong, China). Samples were prepared with distilled water in all cases.

Preparation of whey protein hydrogels

For preparation of protein solution, 1.2 g WPI was dissolved in 10 mL distilled

Chemical interactions

The FTIR spectra of WPI gels are shown in Fig. 2. The bands observed at 3443 cm−1 and 3427 cm−1 are assigned to the Nsingle bondH stretching vibration of proteins (Palaniappan & Pramod, 2011). A shoulder was observed near 3300 cm−1 in the spectra of all three gel samples, but was more intense for the cross-linked gels that for the non-cross-linked sample. This may indicate the non-ionisable Osingle bondH group of the citric acid molecule (Tavakolipour, Bagheri, & Madadlou, 2014). The peak that appeared at 2938 cm−1

Conclusion

Heat-set whey protein hydrogel absorbed caffeine from the surrounding immersion solution and released it when was subsequently immersed in a caffeine-free water. The low-temperature alkali-catalysed cross-linking of whey proteins with citric acid after hydrogel formation increased gel firmness and decreased caffeine release from the gel network. A firmer gel was obtained by the cross-linking then caffeine loading procedure compared with the loading then cross-linking method that suggests that

References (30)

  • H. Xu et al.

    Low-temperature crosslinking of proteins using non-toxic citric acid in neutral aqueous medium: Mechanism and kinetic study

    Industrial Crops and Products

    (2015)
  • H. Zand-Rajabi et al.

    Caffeine-loaded whey protein hydrogels reinforced with gellan and enriched with calcium chloride

    International Dairy Journal

    (2016)
  • A. Abbasi et al.

    Modeling of caffeine release from crosslinked water-swellable gelatin and gelatin-maltodextrin hydrogels

    Drug Delivery

    (2008)
  • T. Begam et al.

    A comparative study of swelling properties of hydrogels based on poly (acrylamide-co-methyl methacrylate) containing physical and chemical crosslinks

    Journal of Applied Polymer Science

    (2003)
  • Z. Cao et al.

    A novel temperature- and pH- responsive polymer-biomolecular conjugate composed of casein and poly (N-isopropylacrylamide)

    Iranian Polymer Journal

    (2010)
  • Cited by (34)

    • Unusual gelation behavior of low-acetyl gellan under microwave field: Changes in rheological and hydration properties

      2022, Carbohydrate Polymers
      Citation Excerpt :

      Overall, the microwave-heated gels demonstrated a better ability to retain water than the water bath-induced gels. Free water was captured between the pores of the network, which resulted from chain bridges formed by calcium ions (Zand-Rajabi et al., 2016). Owing to various polarization mechanisms, hydroxyl groups of the gellan molecule exhibited orientation behavior under the microwave field and the position and orientation of water molecules around gellan molecule were arranged as a water network, which might achieve better water-holding performance (Liu et al., 2017).

    • Heat-induced denaturation and bioactivity changes of whey proteins

      2021, International Dairy Journal
      Citation Excerpt :

      Not only protein denaturation but also protein aggregation happened during heat treatment (Fig. 1). Protein aggregation amongst food are deemed to have industrial applications especially in the context of textural properties, for instance forming gel in yogurt products (Zand-Rajabi & Madadlou, 2016). Furthermore, aggregation of whey proteins may have important implications for the products properties.

    View all citing articles on Scopus
    View full text