Microencapsulation of Lactobacillus reuteri DPC16 using spray-drying : a thesis presented in partial fulfilment of the requirements for the degree of Master of Food Technology at Massey University, Auckland, New Zealand
Probiotic microorganisms and the products containing the beneficial microorganisms are popular due to their ability to confer health benefits on consumer health. The majority of probiotics are delivered in liquid media which limit their shelf life and they are not convenient for the modern lifestyles. Thus, in this study, different wall materials for the microencapsulation of Lactobacillus (L.) reuteri DPC16 were investigated in Stage 1. The shelf-life tests of selected spray-dried powders were carried out in Stage 2 with different packaging materials. In Stage 1, L. reuteri DPC16 was encapsulated in 10% reconstituted skim milk (RSM), 10% gum Arabic, 10% maltodextrin, and 4:1 mixed wall material (2.5% whey protein isolate/ 2.5% gum Arabic/ 2.5% inulin/ 2.5% sucrose), (w/w) then spray-dried at 160 ℃/80 ℃ inlet/outlet temperatures. The spray-dried DPC16 microcapsules were characterised for viable cells of the probiotic, water activity and morphology. Viable cell counts were measured using standard plate count method, water activity using a water activity meter (AquaLab, Series 3, New Zealand) and the morphology of the powder particles was scanned by the electron microscope (FEI Electron Optics, Quanta 200, The Netherlands). Results of Stage 1 showed that at the inlet/outlet temperatures of 160 ℃/80 ℃, the RSM as an encapsulation wall material had the highest cell counts (98.06%±0.86%) with 0.284±0.005, water activity followed by the mixed wall material which contained cells of 93.97%±1.49% log CFU/g with water activity of 0.196±0.010. The powder made from gum Arabic had the lowest viable cells (90.63%±3.08%) with 0.170±0.005, water activity. Thus, RSM showed good potential to maintain high cell viability during spray-drying although the water activity was higher than the expected range of <0.25. For all the treatments, particle sizes of the powders were well below 100 μm which is ideal for addition to food products as they do not affect mouthfeel. Most of the powder particles were spherical with variable sizes and dented surfaces. Thus, RSM and the mixed wall materials were selected for encapsulating DPC16 in the storage trials. In stage 2, DPC16 were encapsulated using selected wall materials (RSM and the mixed wall material) and vacuum-packed in PET/EVOH/PE co-ex topweb FOC films (Multivac New Zealand Ltd) and aluminium foil bags (ALFW5-18, PBAG, China), then stored at 25 ℃ and 55 ℃ for four weeks. During storage, viable cells of the DPC16, water activity, colour, moisture content, and morphology of the powder were determined. Colour was measured by the Minolta Colourimeter (Minolta, Japan), moisture content was determined by the oven-dry method, bulk density was determined by the measuring cylinder method and the other characteristics of the powders were determined as previously described. The survival of DPC16 cells encapsulated in skim milk and vacuum-packed in aluminium bags were higher and more stable during storage at 25 ℃. Water activity, moisture content, bulk density, colour and morphology of the powder were all relatively more stable than in other treatments. Water activity (mean) and moisture content (mean) were within the expected ranges for the product. When stored at 55 ℃, the viable cell counts of DPC16 encapsulated in RSM and vacuum-packed powder in PET/EVOJ/PE co-ex topweb FOC film decreased to <106 CFU/g by end storage which was below the FAO/WHO, 2003 recommended level. The moisture content (0.0246±0.0003) was also below recommended levels (0.028 – 0.056), although water activity (0.102±0.007) was within expected levels (<0.25). Low moisture levels are critically important for the survival of encapsulated spray-dried probiotic microorganisms. High moisture initiates chemical reactions within the carrier materials leading to cell death and also affects colour stability. However, storage temperature is also important to cell survival. In conclusion, the present study showed that spray-drying encapsulated L. reuteri DPC16 in 10% RSM at 160°C/80°C, followed by vacuum-packaging in aluminium bags showed potential to maintain cell viability during storage (25 °C) for four weeks. It is desirable to check the performance of the encapsulated DPC16 powders in the simulated gastrointestinal tract and its ability to target-release the cells in the colon.