Comparison of spray drying, freeze drying and convective hot air drying for the production of a probiotic orange powder

Highlights

• Probiotic orange powders were obtained by spray-, freeze- and hot air drying techniques.

• During drying, cell inactivation was only observed for convective hot air dried cells.

• Reductions of P. acidilactici HA-6111-2 were minimal at most conditions of storage.

• Minimal reductions of L. plantarum 299v were observed during storage at 4 °C.

• Better dissolution, colour and vitamin C retention were obtained by freeze- or spray-drying.

Abstract

Survival of two LAB in orange powders obtained by spray-(SD), freeze-(FD) and convective hot air drying (CD) was investigated during drying and subsequent storage. Colour and vitamin C content of the powders were also evaluated. There was no decrease in the cell number during SD and FD, but a reduction of ~2 log cycles was obtained in CD. During storage at 4 °C no significant differences (p > 0.05) in the survival of Lactobacillus plantarum 299v were observed for the orange powder obtained by the different techniques. However, during storage at room temperature, its survival was better in orange powders prepared by CD. For Pediococcus acidilactici HA-6111-2, reductions during storage were minimal for most of the conditions investigated. Considering the initial cell number obtained after each drying process, SD and FD allowed survival of an increased number of cells after storage period. The best colour retention was obtained by FD, with no maltodextrin, and the higher total vitamin C retention was achieved in SD and FD powders. By the high production costs and long drying times of FD, it can be concluded that SD could be a good method to produce a new functional non-dairy product, such as a probiotic orange juice powder.

Introduction

Orange (Citrus sinensis L. Osbeck) is one of the fresh fruits, following apple, with the highest production in Portugal: 208,000 metric tonnes in the marketing year 2013/2014 (Observatório dos Mercados Agrícolas e das Importações Agro-alimentares, 2011, USDA Foreign Agricultural Service, 2014). Orange is a fruit with a high water content, protein, sugars, fibre, minerals and vitamins such as vitamin C (57 mg per 100 mL) and carotene (120 mg per 100 mL), this last being responsible for the typical colour (Instituto Nacional de Saúde Doutor Ricardo Jorge, 2014). It can be consumed in several ways and plays an important role in national and international gastronomy and nutrition. Orange juice is also highly appreciated, and if probiotics are incorporated, the nutritional content of the juice can improve survival of the added organisms during storage (Ding & Shah, 2008). This is an advantage since a probiotic food should contain viable probiotic microorganisms in amounts of about 10⁶–10⁷ cfu/g or mL until the expiry date of products, and additionally should survive during the passage through the gastrointestinal tract of the consumer (Food and Agriculture Organization/World Health Organization, 2002, Sanz, 2007). Orange juice with probiotics would be an ideal product for consumers who, besides orange juice, also like products which have health benefits and which are not milk-based, such as yoghurts and cheeses (Abadía-García et al, 2013, Senaka Ranadheera et al, 2012). Nonetheless, several disadvantages are found in the production and sale of orange juice with probiotics, such as their short shelf-life, the possible need for refrigeration and high volume and weight of packaging. Conversion of the liquid orange juice into a powdered orange juice with probiotic characteristics will potentially enhance the stability of the product, resulting in a novel and healthy product. For the production of a probiotic orange juice powder, various drying methods could be used, such as spray-, freeze- and convective hot air drying, each having both advantages and disadvantages.

Spray drying allows the transformation of a solution into a dried powder in a single operation. The feed solution is sprayed into a chamber where hot dry air rapidly evaporates the small droplets leaving the spray dried particles (Silva, Freixo, Gibbs, & Teixeira, 2011). Beyond being a rapid drying process, this technique is also inexpensive and its operation is simple and continuous (Duffie & Marshall, 1953).

Nevertheless, the high temperatures in this process may lead to the decrease or loss of vitamin C and carotene, as well as the flavour and aroma of orange (Dziezak, 1988). Also the presence of low molecular weight sugars such as fructose, glucose and sucrose and organic acids in the composition of orange juice affects its drying using this method, since they cause problems of stickiness, resulting in low process yield and operating difficulties (Bhandari, Dutta, & Howes, 1997). Drying agents, such as maltodextrins have been extensively used to reduce the stickiness of sugar-rich fruit juices (Tonon, Brabet, & Hubinger, 2010). Besides being effective during drying, maltodextrins minimize crystallization during storage, are inexpensive, have a mild flavour, and can be used as encapsulating agents; it has also been reported that maltodextrins are effective in preserving carotenoids (Desobry, Netto, & Labuza, 1997). Evidence of their prebiotic properties, conferring beneficial characteristics to the final product, has been provided (Anekella, Orsat, 2013, Slavin, 2013).

Freeze drying is a process in which the water is removed from a frozen solution by sublimation under reduced pressure (Castro, Teixeira, & Kirby, 1997) giving rise to high quality dried products (Ratti, 2001). During the process no significant losses of vitamin C occur (Lin, Durance, & Scaman, 1998) and there is high retention of nutrients and flavourings; freeze dried products can be easily rehydrated before use (Tsami, Krokida, & Drouzas, 1999). However, freeze drying is expensive (about six times more expensive per kg of water removed in comparison with spray drying) and is time-consuming (Castro et al, 1997, Knorr, 1998).

The convective drying technique allows exposure of a solid to a continuous flow of hot air evaporating the moisture (Ratti, 2001). Whilst this fast hot air drying process can cost from 4 to 8 times less than freeze drying (Flink, 1977) and the obtained dried products have a long shelf-life, their quality can be much lower than the original, with a drastic reduction in the volume, with deformation and colour change. In contrast to spray- and freeze drying, rehydration of products dried by hot air drying is poor (Ratti, 2001). Studies with different fruits demonstrated an increase in retention of ascorbic acid and colour or an increase in retention of β-carotene and better rehydration when the fruit was exposed to an intermittent change in temperature or air flux, respectively (Lewicki, 2006).

The present study was carried out to investigate the effect of three different drying methods on the survival of two lactic acid bacteria (LAB) in orange powders during drying and storage, as well as on the colour and vitamin C content of orange powder.