Inter-particle space fractions in fried batter coatings as influenced by batter formulation and pre-drying time Jamshid Rahimi, Michael O. Ngadi * Department of Bioresource Engineering, Macdonald Campus, McGill University, Quebec H9X 3V9, Canada article info Article history: Received 19 October 2013 Received in revised form 15 February 2014 Accepted 17 February 2014 Keywords: Batter formulation Pre-drying Porosity Pore fractions abstract Evaluation different fractions of the spaces in a fried product that are occupied by air, moisture, or fat provides useful information for researcher and manufacturer to better understand the mechanism of oil absorption during frying. The objective of this study was to determine the influence of batter formula- tions and pre-drying on the fraction of inter-particle spaces in batter coating. Pre-drying decreased the amount of fat to 2.97 g/100 g compared to non-pre-dried samples which showed 4.37 g/100 g fat content. Higher rice flour content in batter significantly increased the fraction of inter-particle spaces occupied by air (SOA), and decreased the fraction of spaces occupied by moisture (SOM) and fat (SOF). Longer pre- drying times showed greater fraction of spaces occupied by air, and smaller fraction of spaces occu- pied by moisture and fat. SOA, SOM, and SOF of the batter systems ranged from 2.2% to 46.03%, 2.55 to 47.07%, and 0.35% to 11.11%, respectively. Using higher amount of wheat flour in batter and longer pre- drying time showed to significantly decrease fat uptake during frying. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Study of microstructure of fried foods is very important in quality characterization, development, process modeling and optimization of the products. The structure of fried foods highly depends on their initial physicoechemical characteristics, oil properties, pre-treatment, and frying conditions such as tempera- ture and time. During frying, some microstructural changes occur as a result of starch gelatinization and protein denaturation at the high frying temperatures. The effect of microstructural changes on mass transfer during frying has been studied by several authors (Dana & Saguy, 2006; Moreira, Castell-Perez, & Barrufet, 1999; Saguy & Dana, 2003), and it has generally been established that rapid water evaporation and consequently crust formation occur when food is exposed to high frying temperature. High internal pressure, which is generated due to steam, creates cracks, defects, open capillaries and channels in the cellular structures and membranes of the food material (Dana & Saguy, 2006; Saguy & Dana, 2003). After frying and during the cooling phase, water vapor condensation and subsequently pressure drop occur in some products. Consequently, oil which is adhered to the surface of the product is sucked into the large voids and crevices due to a generated vacuum effect (Moreira et al., 1999). It was reported that oil absorption during cooling phase is a pressure-driven flow and involves a balance between the capillary forces and oil drainage (Ziaiifar, Courtois, & Trystram, 2010). Thus, surface microstructure plays a critical role in oil uptake and also in the distribution of oil, water and air through the network of pores and voids during frying (Kassama & Ngadi, 2004, 2005). Kassama and Ngadi (2004) reported three possible types of pores namely interconnected pores (accessible from many directions), isolated pores (inaccessible) and non-connected pores (accessible from one direction). Interconnected pores play more important role in transport of fluid through the porous medium than the non- connected pores, while isolated pores have limited influence on transport phenomena. Interconnected pores and non-connected pores are the reservoirs of moisture and oil in fried products, and the distribution of oil, moisture, and even air through these spaces changes during frying process. The measurement of inter-particle spaces occupied by oil, moisture, or air is therefore necessary for studying the mechanism of oil uptake and moisture loss during frying. Different techniques such as decreasing the initial moisture content by pre-drying the product, surface coating and using different ingredients as the coating elements, and pre-cooking in oven or microwave have been used to reduce fat content of fried foods (Adedeji, Ngadi, & Raghavan, 2009; Ali, Abdel-Razek, & Kamil, * Corresponding author. Tel.: þ1 514 398 7779; fax: þ1 514 398 8387. E-mail addresses: Jamshid.rahimi@mail.mcgill.ca (J. Rahimi), michael.ngadi@ mcgill.ca (M.O. Ngadi). Contents lists available at ScienceDirect LWT - Food Science and Technology journal homepage: www.elsevier.com/locate/lwt http://dx.doi.org/10.1016/j.lwt.2014.02.032 0023-6438/Ó 2014 Elsevier Ltd. All rights reserved. LWT - Food Science and Technology 57 (2014) 486e493