Elsevier

Food Hydrocolloids

Volume 17, Issue 5, September 2003, Pages 671-678
Food Hydrocolloids

Particle sizes and stability of UHT bovine, cereal and grain milks

https://doi.org/10.1016/S0268-005X(03)00012-2Get rights and content

Abstract

Commercial ultra high temperature (UHT) processed bovine, oat, rice and soy milks were analysed for stability and particle size. Product stability was determined using a turbidity analyser to follow particle migration over time. Changes in the level of backscattering produced by the sample as a function of time were related to the stability of the sample. The results indicated that the level of stability of the milks varied and the type of destabilisation phenomenon (sedimentation or creaming) also varied between the milks. The relative stability of the samples studied was determined to be: rice≪oat<soy<bovine. The milks generally had bi-modal particle size distributions. Particles less than about 3 μm were mainly fat or oil globules, while particles greater than about 3 μm were dense non-fat milk solids. The size and relative amount of fat globules and solid particles varied with milk type. The size of the particles in the milks was highly correlated to the stability of the milks, with the most stable milk having the smallest particles.

Introduction

The concept of cereal and grain milks (aqueous oil-in-water emulsions stabilised with oat, rice or soy solids) and other liquid products produced from cereal and grains have been present in various societies for many years. Examples of the use of cereal beverages include the traditional Japanese non- alcoholic beverage ‘amazake’ prepared from steamed or cooked rice (Matz, 1991); ‘sikhe’ is a traditional Korean rice beverage (Mee, Tae, & Hye, 1999); and ‘kishk’ is a fermented cereal/ milk mixture available in Lebanon (Tamime & O'Conner, 1995). There are also other examples of milks constituted from chickpeas, corn, millet and rye.

The consumption of cereal and grain milks in the Western world has increased over the last 2 decades as a result of increased health consciousness of consumers, the minimisation of off-flavours and the reduction of growth inhibitors (Kwok & Niranjan, 1995). More recently, milks made from oats and rice have emerged on the market. In comparison to bovine milk, the cereal and grain milks have no cholesterol or lactose, which makes them an attractive alternative to bovine milk for people conscious of their health and/or diet, vegetarians, and those who are lactose intolerant. Some of the uses of these bovine milk alternatives include infant formulas (Benward & Benward, 2000); reduced sugar and reduced calorie milks for dietetic and diabetic purposes (Demirag, Elmaci, & Altug, 1999); and non-dairy vegetarian alternatives to dairy and cultured dairy products (Martensson et al., 2000, Shirai et al., 1992).

Bovine milk and the cereal and grain milks are colloidal emulsions and suspensions, that is, dispersions in which there are particles or droplets with at least one linear dimension in the size range of 1 nm to about 1 μm (Dickinson, 1992). The emulsions in milks are the small fat or oil droplets, stabilised with surface-active agents such as proteins and lipids. The suspensions are the insoluble solid particles, such as undissolved protein, starch, fibre and other cellular material, which are suspended in the aqueous solution. In suspensions, the dense particles can aggregate and sediment out. In emulsions the total number of droplets, their size and their arrangement in space can all change over time (Dickinson & Stainsby, 1988). The stability of the emulsion depends upon droplet size and aggregation. Cereal and grain milks are made by disintegration of the plant materials, which means that particle composition and size are not as uniform as in bovine milk. The particle size of the cereals and grains will have a large dependence upon the method by which they are milled. The presence of larger particles will affect the relative stability of the milks and can create problems with wetting the dry material and heat transfer, which could result in inadequate thermal destruction of microbes. Homogenisation is one tool that can be used to increase the stability of bovine and soy milks (Durand & Hosken, 1999).

Bacterial spoilage should not be a limiting factor to the shelf life of UHT processed milks because the heat treatment is designed to produce very low to zero microbial counts (Reuter, 1993). As a result, the physico-chemical reactions that occur in the milk and the extent of these, limit the shelf life of a UHT processed milk. The UHT process may cause changes in the proteins and lipids that stabilise the emulsion. This may affect the stability of the fat droplets and result in droplet growth as a function of time, producing creaming. Changes in the protein chemistry may lead to gelation and/or precipitation of solids and sediment formation. The starch that is naturally present in plants requires heating for dissolution, but heating also causes a great increase in viscosity (Fennema, 1996), which may also have a significant affect on the stability of the cereal and grain milk. The relative stability of the milks will also affect their shelf life. Changes in the level of backscattering can be used to determine the relative stability of the milks and to estimate their shelf life (Durand & Hosken, 1999).

Optimising the UHT process and stabilising the emulsions against separation are essential for the manufacture of long life milk products. The aim of this study was to investigate the stability and shelf life of various cereal and grain milks using back scattering turbidity techniques to follow particle migration over time. These results were compared to the sizes of the solid particles and droplets of the milks to help understand the mechanisms of instability and increase the predictability of shelf life evaluation.

Section snippets

Materials and methods

Four different types of commercial UHT processed milks purchased in Australia were used in this study. The cereal and grain milks were made from oats, rice and soybeans. Lactose free bovine milk was used as a reference product. The ingredients and fat contents of the milks used are presented in Table 1.

The size distributions of the particles in the milks were measured by light scattering techniques with a Malvern Mastersizer E using a 45 mm lens. This instrument requires a nearly transparent

Results

Typical particle size distributions (by volume) of the as-received milk samples, the floating portions and the sinking portions are shown in Fig. 1. The particle size distributions were bi- or tri-modal and typically extended from about 0.1 to 50 μm. Particles generally fell into two populations, greater than about 3 μm (referred to as large particles in this paper) or less than about 3 μm (referred to as small particles in this paper). The as-received bovine milk and soy milk contained only a

Discussion

The fact that sedimentation was the primary mode of destabilisation in the rice and oat milks is consistent with the particle size analysis of the as-received milks. The rice and oat milks had significant fractions of large particles that were more dense than water as shown in Fig. 1c and d. Also these milks had a lower fat content than the bovine and soy milks. In contrast, the bovine and soy milks contained a very low percentage of large particles that were more dense than water (as shown in

Conclusion

The colloidal stability of bovine milk and formulated cereal and grain milks varies, depending upon the ingredients, processing conditions and storage conditions. These results indicate that there is a correlation between the stability and particle size of the milks. Small oil droplets and small protein solids produce stable milks. Larger oil droplets and protein solids produce milks that separate quickly. The measurements of backscattering from a sample over time provided a good, quick

Acknowledgements

This work is funded by the Australian Research Council, Sanitarium Health Food Company and the University of Newcastle.

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