Elsevier

International Dairy Journal

Volume 85, October 2018, Pages 105-111
International Dairy Journal

Time consolidation of skim milk powder near the glass transition temperature

https://doi.org/10.1016/j.idairyj.2018.05.005Get rights and content

Abstract

Time consolidation (caking) of skim milk powder was studied at 35% relative humidity and different temperatures and consolidation stresses using a ring shear tester. Unconfined yield strength developed linearly with storage time. The storage time until the powder formed hard lumps (caking time) was lower when storage temperature and consolidation pressure were increased. Even a slight increase in storage temperature near the glass transition temperature resulted in a drastic decrease of storage stability. For example, when the skim milk powder was stored 2.5 °C below the glass transition temperature, the caking time was 110 h. In contrast, an increase in storage temperature of 4 °C reduced the caking time by 80% to only 18 h. The caking time increased markedly when the storage temperature was more than 3 °C below the glass transition temperature of the skim milk powder.

Introduction

As bulk product, skim milk powder is used in a wide range of food industries. Several foods such as soups, sauces, confectionery and bakery products benefit from the functionality of skim milk powder. Further processing of skim milk powder commonly includes various transport, storage and dose processes up to the final customer. Thus, the powders have to exhibit good handling properties, even after long storage periods, to ensure trouble-free processing. However, since skim milk powder contains lactose in its amorphous state (Kelly, 2009), stickiness and caking may occur when stored under unfavourable temperature and humidity conditions. Stickiness and caking reduce significantly the utilisation value of milk powders, and result in product losses, poor quality classification as well as customer complaints.

Stickiness of amorphous food powders develops above the glass transition temperature (Tg) as a result of a decrease in surface viscosity and plasticisation, which allows the material to adhere to another particle or surface (Aguilera et al., 1993, Chuy and Labuza, 1994, Downton et al., 1982, Wallack and King, 1988). The transition from the non-sticky region to the sticky state (sticky point) occurs a few degrees above Tg and is often referred to the difference between powder temperature and glass transition temperature [T-Tg] (Hogan & O'Callaghan, 2010). For skim milk powder, sticky points of 23.3 K (Hennigs, Kockel, & Langrish, 2001), 14–22 K (Ozmen & Langrish, 2003), 38 K (Paterson, Bronlund, Zuo, & Chatterjee, 2007), 33.6 K (Murti, Paterson, Pearce, & Bronlund, 2009) and 29 K (Hogan & O'Callaghan, 2010) have been measured. Knowledge of [T-Tg] where the powder becomes sticky is very important for spray drying to reduce wall depositions, cyclone blockages as well as fire and explosion hazards (Hennigs et al., 2001, Kudra, 2003). However, during storage contact time and contact pressure between particles are much higher in comparison with spray drying and therefore sticky points obtained by stickiness measurements may not always be useful to predict susceptibility to lumping when powders are stored in stacked bags or big bags.

Caking of amorphous substances can be regarded as a process by which a free flowing powder is transformed into lumps due to bridging and agglomeration of sticky particles (Aguilera, del Valle, & Karel, 1995). Since stickiness and caking are time-dependent phenomena (Downton et al., 1982, Paterson et al., 2005), bridging and compaction will result in a solid block, if sufficient time is available.

Considering flowability of skim milk powder, several authors have estimated an easy or free flowing characteristic by using shear cell technique (Fitzpatrick et al., 2005, Fitzpatrick et al., 2004, Fitzpatrick et al., 2007a, Teunou and Fitzpatrick, 1999). Exposure of skim milk powder to 46% relative humidity for 18 h caused an cohesive flow behaviour (Fitzpatrick et al., 2004). Moreover, caking of skim milk powder has been investigated by means of empirical methods. Özkan, Withy, and Chen (2003) used an Instron machine to investigate caking of SMP in the temperature range from 50 °C to 70 °C. Moisture content and [T-Tg] values were not addressed and could have varied during experiments. Furthermore, Fitzpatrick et al. (2007b) and Fitzpatrick, O'Callaghan, and O'Flynn (2008) used an empirical force-displacement caking test to measure caking of skim milk powder at 76% and 100% relative humidity. No significant caking was observed at [T-Tg] < 10 K. However, powder was not consolidated before testing.

To eliminate the influence of stress history (e.g., filling, preceding deformation) preshearing until steady-state flow is recommended prior to measurement of powder or cake strength (Schulze, 2008). Penetration tests, such as introduced by Knight and Johnson (1988), were found to have big scatter from results of identical tests and time consolidation tendencies could hardly be detected (Schwedes, 2003). Because shear cell measurements include a defined consolidation procedure, less scatter in test results can be expected and one obtains the greatest strength at a given consolidation stress (Schulze, 2008). Thus, powder strength is not underestimated and susceptibility to caking can be determined on the safe side.

Since most previous work has focused on stickiness and caking measurement using empirical tests, the aim of this work was to measure time consolidation kinetic of commercial skim milk powder under industrial relevant consolidation stresses using a ring shear tester. Time consolidation was investigated at low [T-Tg] (<10 K) to determine caking times in the near region of the glass transition. Two consolidation stresses were used to simulate storage pressures in the lower and upper part of a pallet of stacked bags or in flexible bulk containers. Information of caking kinetics will be helpful to assess powder state at different storage conditions.

Section snippets

Skim milk powder

Commercial spray dried skim milk powder (SMP) was kindly donated by Prolcatal (Prolactal GmbH, Hartberg, Austria). The powder had a median particle size (d50) of 87 μm and a span (width of size distribution) of 1.44, as measured by dynamic image analysis (Microtrac SIA, Microtrac GmbH, Krefeld, Germany). The moisture, protein, lactose and fat contents of the powder were 4.2, 36, 59, and 1.1% (dry basis), respectively, and the loose bulk density was 518 kg m−1. The SMP was stored in a sealed

Moisture sorption isotherm and glass transition temperature

Fig. 1 shows the measured sorption isotherm and glass transition temperatures as a function of SMP water activity. In the aw range from 0.1 to 0.3 moisture content increased linear with increasing water activity. Moisture sorption in this area is often related to adsorbed water (Reh, Bhat, & Berrut, 2004). At aw < 0.2, casein is supposed to be the main water absorber, whereby in the intermediate region water sorption is dominated by lactose (Schuck, Mejean, Dolivet, Jeantet, & Bhandari, 2007).

Conclusions

In this work, time consolidation of skim milk powder was investigated near the glass transition temperature ([T-Tg] < 10 K) using shear cell measurements. The powder strength increased linearly during storage at temperatures of 18.5 °C–37.5 °C and 35% relative humidity. Higher storage pressures and temperatures resulted in a faster consolidation of the powder samples. The SMP formed hard lumps when the unconfined yield strength reached 20 kPa. Using a sinter kinetic model, a sinter bridge

Acknowledgement

This IGF Project AIF 16624 BR of the FEI was supported via AiF within the programme for promoting the Industrial Collective Research (IGF) of the German Ministry of Economic Affairs and Energy (BMWi), based on a resolution of the German Parliament.

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