Microorganism preservation by convective air-drying—A review

Abstract

Newcastle University ePrints -eprint.ncl.ac.uk Tan DT, Poh PE, Chin SK. Microorganism preservation by convective air-drying-A review. AbstractAt present, microorganisms are mainly preserved by freeze drying. There is, however, lack of studies conducted on various cheaper yet promising convective air drying alternatives. Convective air drying has been proven to produce dried culture with comparable cell survival and final moisture content to that of freeze drying. This paper aims to draw an understanding to application and suitability of convective air drying which indludes spray-, oven, heat pump, fluidised bed, conveyor, and rotary drying to preserve microorganisms. The paper concludes that drying near ambient temperature and the addition of dehydration protectants are important to obtain satisfactory drying quality. ambient temperature and mild processing conditions [28]. Such condition is hypothesised to be favourable towards high cell survival of microorganisms [29].Majority of studies and reviews, however, largely revolve around freeze-drying and cryogenic preservation which are both costly [8,15,30,31]. Although convective air drying has been used to preserve many foodstuffs, research on its applicability for bacteria preservation is still limited [32][33][34][35][36]. This paper is aimed to critically review work on preservation of microbes; looking into the factors that will affect the cell viability in convective air drying and to suggest future directions on the use of convective air drying for microbial preservation. Methods for microorganism preservationWhile there are many means to preserve a desired culture of microorganisms, some methods might be more suitable than the other depending on the purpose of preservation. Based on applicability of the techniques, preservation methods can be distinguished into those that are limited to just laboratory scale and those that can be applied in industry, as some pilot techniques could be simple and reliable but are not scalable [15]. The key difference between laboratory and industrial scale is the amount of culture that needs to be salvaged [26]. For laboratory purposes, low cell survival is sufficient whereas large quantity (and therefore high cell survival) is required for industrial usage [15]. Typical laboratory preservation techniques include smearing and storing on agar or gelatine, cell immersion in paraffin oil, and the adsorption-desiccation of bacteria culture on filter-paper or on pre-dried plugs of starch or silica gel [15,37,38].High number of successfully preserved viable cells is particularly important to ensure possible direct inoculation to the process fluid where the bacterial culture is to be utilised [11]. The laboratory preservation methods mentioned above are unsuitable due to the complexity of the process, requirement of additives, and low recovery rate [15]. Preservation methods which are recognised to be practical for industrial use are subcultivation, freezing, and drying [39,40].

Role of critical reviews in encouraging appropriate R&D

“…During the last two years (2018-2019), this journal has published 14 review papers covering a wide range of themes and applications in a number of industrial sectors, ranging from agricultural products, [1,2] food, [3] to pharmaceuticals, [4,5] and microorganisms [6] . Both scientific approaches [5,7,8] and industrial applications [9] are covered to meet the needs of industry as well as academia.…”

Kinetic study of the thermal inactivation of Lactobacillus plantarum during bread baking

“…Different studies report that a high drying rate has a positive, negative or no influence on the survival of microorganisms during drying. [17,21,22,30] In previous studies, drying and temperature-rate dependent models were successfully applied to describe the survival data of different microorganisms (Bifidobacterium infantis Bb1, Streptococcus thermophilus St10, and Saccharomyces cerevisiae ATCC 2601) during drying. [23,24] An extensive theoretical appraisal of rate-dependent models can be found in the paper by Chen and Patel.…”

“…[16][17][18][19][20]26] Interestingly, only few studies incorporated the influence of the drying rate and/or the change in temperature to predict the inactivation kinetics of microorganisms. [19,[21][22][23][24]27,28] To the best of our knowledge, no previous work exists in the literature that deals with modeling of the influences of temperature, moisture content and their changes in time on inactivation kinetics of probiotics in the typical temperature range of baking processes. This is, however, of particular interest when the baking conditions need to be optimized to maximize viability of probiotics in final bread products.…”

Modelling approach for analysis of moisture transport in diatomaceous earth pellets

“…The desiccation process to ensure the shelf life of formulated IJs has been extrinsically controlled by proper selection of carrier materials, adjuvants, the manufacturing method and the control of storage conditions [1,6,11]. S. carpocapsae IJs have been formulated into waterdispersible granules (WDGs) [12].…”