MAGNETIC FIELD-BASED SUPERCOOLING: CELL VIABILITY ENHANCEMENT OF FREEZE-DRIED LACTOBACILLUS SPP. AND INHIBITORY ICE NUCLEATION OF IRONOXIDE NANOPARTICLES IN AN AGAR MODEL SYSTEM
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2023
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Food Science
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Supercooling is the phenomenon of lowering the temperature of a solution or food material past its equilibrium freezing point without the formation of ice crystals. It is a novel food preservation technology that can extend the shelf-life of fresh and perishable foods, while still maintaining food quality aspects. Freezing can extend the shelf-life of foods for extended periods of time for months and in some cases for up to a year. However, the food quality can be degraded due to the formation of ice crystals, in which the texture and nutrient losses can be comprised. Supercooling preservation can contribute to shelf-life extension without destroying food membranes and tissues by inhibiting the formation of ice crystals. A method to achieve this supercooling status is the use of magnetic fields paired with the conventional freezing methods.
Oscillating magnetic fields (OMFs) have been employed with freezing as a novel preservation technique that has been utilized in biomedical and food applications. Additionally, this
technology is still in the developing stages and remains controversial. Probiotics are live microorganisms that confer health benefits to the host when taken in
adequate amounts. These microorganisms may provide several benefits to overall human health and contribute to the gastrointestinal tract (GIT) as they can aid in digestion, immune health, and various GIT diseases, such as irritable bowel syndrome (IBS), constipation, and diarrhea. Probiotics can be administered in various forms such as fermented foods, beverages, and
supplements in the form of powder, capsules, gels, and tablets. Probiotic supplement processing usually involves a drying step to preserve the cell viability and extend the shelf-life. Common
drying steps include spray-, freeze-, vacuum-, or fluidized bed-drying; however, these methods can lead to detrimental effects on the probiotic cells due to drastic temperature changes and
dehydration, which poses a threat to the viability of these cells and may have an impact to the potential benefits that can be administered to the host. Freeze-drying, also known as lyophilization, is the preferred method of drying since most probiotic strains are heat sensitive and the use of cryoprotectants can be applied to the cells to minimize the detrimental effects caused by the initial freezing step. Cryoprotectants are additives used to maintain high cell viability during freeze-drying processing and are also used as bulking agents for probiotic material. After the initial freezing step, cells undergo a primary drying step, in which water or other solvents are removed via sublimation, allowing ice to directly change from solid to vapor, and a secondary drying step is applied to remove any additional water. This method is highly preferred for long-term preservation and the cells keep their biological and structural integrity. However, the freeze-drying process does have an effect on the cell viability due to the ice formation initial freezing step. In Chapter 3, OMF-assisted freezing was used as a pretreatment to probiotics before freeze-drying to investigate the effects on the cell viability of Lactobacillus acidophilus (L. acidophilus). The OMF-assisted freezing parameters for this experiment were 10 mT for the magnetic field strength and 10 Hz for the frequency and maintained a supercooled status at -5°C for 6 h. It was found that supercooled cells before freeze-drying compared to cells that were not(control) had a lower reduction of cell viability of 0.5 and 1-log reduction, respectively, to the initial cell count, which were cells before freeze-drying. A preliminary study by Wang (2021) found similar results, in which supercooling achieved the highest cell viability of 78% compared to fast and slow freezing, and refrigeration storage, which can be explained by the formation of surface layer proteins (SLPs) that are formed when L. acidophilus was exposed to cold temperatures. The subzero temperatures allow L. acidophilus to strengthen its natural defense mechanism by protecting itself from cold stress. This current research study was a continuation
of the preliminary study that changed several parameters to focus on practices that can be implemented for industrial purposes. A case study done with a collaborative partnership company used our supercooling system with OMF as a pretreatment for L. salivarius before freeze-drying to investigate the effects of the supercooling technology on the cell viability and shelf-life after 0-, 3-, and 8 days of storage. Supercooling technology was able to maintain high cell viability of supercooled cells even without the use of cryoprotectants after 8 days of storage. This technology has great potential in maintaining a high cell viability in probiotics, though there still needs to be further research on the effects of different supercooling parameters such as temperatures, magnetic field strengths, frequencies, exposure time, and different probiotic strains. Furthermore, it should be pointed out that OMF-assisted supercooling is still highly controversial and remains a topic of debate and the mechanisms regarding this technology are still unknown. In Chapter 4, the effects of OMF-assisted freezing on the supercooling behavior of magnetic nanoparticles (MNPs), particularly iron-oxide nanoparticles (IONPs) were also investigated. The current literature about this topic of magnetic field freezing is a controversial topic due to the variations in results of many researchers. The mechanisms involved are still not fully understood as this technology is still in the beginning and developing stages. Proposed mechanisms for this technology include the reorientation or vibration of the water molecules within solutions and/or food samples, due to the diamagnetic properties of water, as well as hydrogen bonds breaking between water molecules, allowing for the inhibition of ice
crystallization. However, some researchers disagree with this proposed mechanism, and it has been postulated that there is ferromagnetic material naturally present in biomaterials, which could be the reason for supercooling to occur under OMF during freezing without the formation of ice crystals. It is difficult to conclude that supercooling is successful in all foods, since foods are composed of complex matrices with varying constituents of proteins, lipids, carbohydrates, water, and minerals, which lead to the varying and contradictory results reported in the current literature. Due to this reason, food models are employed to exclude the factors that will have an influence on supercooling. Nevertheless, the mechanisms still need to be further investigated to understand the interactions between supercooling and foods and/or biomaterials under magnetic fields. The effects of OMF-assisted freezing on the supercooling behavior of magnetic nanoparticles were investigated in an agar food model system. The IONPs at different concentrations of 3, 6, 12, and 15 mg were used to investigate the supercooling behavior at 10 mT as the magnetic field strength and 10 Hz as the frequency at -8°C for 24 h treatment time. Samples without any nanoparticles were used as a control and compared with 12 mg IONP and 12 mg zinc nanoparticle (ZNP) agar samples, as well. For each sample, a supercooling probability was calculated, which is the number of times that a sample successfully supercooled divided by the total amount of outcomes or trials (n=20). 12 mg IONPs exhibited a high supercooling probability of 90%, while the 3, 6, and 15 mg IONP samples had a supercooling probability of 75%. The control samples had a supercooling probability of 60% and ZNP
samples had a 55% supercooling probability, which can possibly be due to the stochastic nature of ice nucleation during the supercooling status. This research demonstrates that magnetic nanoparticles do affect the supercooling behavior under OMF during freezing. This study can contribute to some understanding of the interactions or mechanisms within foods under OMFs during freezing to improve the current technology and systems to maintain supercooling and potentially implement this technology in the industry. In any case, further investigation on the supercooling parameters such as the magnetic field strength, frequency, temperature, and exposure time, as well as different types of NPs and concentrations.
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Food science
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115 pages
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