Development and characterization of a novel, edible bigel system with the potential to protect probiotics during in vitro digestion
Bigels are semi-solid biphasic systems. They are composed of an organic phase, called an organogel (or oleogel, if edible), and aqueous phase, called a hydrogel. They have been used to deliver drugs, but their application in the food industry is still relatively new. The two broad objectives of this work include: first, to develop and characterize the structure of an edible bigel, and second, to assess the ability of a bigel to protect probiotics from harsh digestive tract conditions.
For the first broad objective, three main methodologies were used: small angle X-ray scattering, rheology (amplitude sweeps, frequency sweeps, and temperature ramps), and fluorescence microscopy. The developed bigel was made from an oleogel emulsion containing soybean oil, soy lecithin, stearic acid, and water and hydrogel containing whey protein concentrate 80 and water. Two water usage levels within the oleogel emulsion and two protein usage levels within the hydrogel were explored. Moreover, five ratios of oleogel emulsion:hydrogel were examined. The gels were stable for a minimum of five months. Small angle X-ray scattering revealed that the oleogel emulsion retained its basic structural units, a reverse micelle from soy lecithin and bilayer from stearic acid, at every level of hydrogel addition. Rheology affirmed the solid-like behavior of the bigels and showed that a bigel could have improved mechanical properties over a monogel (oleogel emulsion or hydrogel on their own) at certain water usage levels, protein usage levels, and ratio of oleogel emulsion:hydrogel. Rheology furthermore revealed that a bigel has a higher critical strain than a pure oleogel emulsion, which is a major advantage of using a bigel over a pure oleogel emulsion. Fluorescence microscopy showed the continuity and interaction of phases.
For the second broad objective, a standardized in vitro digestion system was used, and the viability of Lactobacillus acidophilus and Bifidobacterium lactis were assessed at various time points throughout digestion. A specific objective of the second phase was to understand the effect, if any, of phospholipids on probiotic survival during digestion. Two gels with similar macro properties, but different in that one had phospholipids (soy lecithin acted as the phospholipid source) and one did not, were used to understand this effect. Gas chromatography affirmed enzyme activity during digestion, and the control, with no gelators, underwent the greatest lipolysis. Additionally, no probiotics in the control survived gelation, but those entrapped within a bigel did survive – affirming the suitability of a bigel to protect probiotics during digestion. Phospholipids did not have a significant effect on probiotic viability, likely because they are broken down by digestive enzymes.
This research has laid the groundwork for bigel implementation into foods. Additional work may need to be done to optimize bigel structure for a particular application, but this work has shown how bigels are assembled, how their phases interact, their ability to protect probiotics during in vitro digestion, and the inability of phospholipids to extend probiotic viability during in vitro digestion.