Rapid methods for measuring pre-germination in malting barley and assessing its effect on malt quality

Abstract

Objectives

The objectives were threefold; firstly to examine the effectiveness of current methods used to estimate pregermination, secondly to design new rapid methods of identifying and quantifying pregermination and thirdly to assess the value of these methods for their ability to identify the actual problems created by pregermination rather than determining the degree of this phenomenon per se. These will allow an informed judgment to be made concerning the implications of the methods used as indicators of the value of barley for malting. The project has been divided into three stages:

A number of barley samples designated as pregerminated at grain intake sites were collected and analyzed for pregermination by a range of different tests.
The malting performance of these samples was assessed.
The two sets of data were analyzed for correlations

Deliverables

A new, rapid method of detecting pregermination in freshly harvested barley grains.
A comparison between new and existing methods.
A correlation relating the performance of barley grain in the chosen tests with the performance of the same six months later in recognized malting performance tests.
Summary

Barley samples designated as "pregerminated" by a number of grain merchants and handlers have been tested for the extent of this phenomenon using a current IOB Recommended Method (fluoroscein dibutyrate staining), a variation of a previously Recommended Method (a -amylase activity) and two which are used routinely at various malting intake laboratories (tetrazolium and copper sulphate staining). All gave positive results when samples were visually pregerminated, with certain tests (in particular fluoroscein dibutyrate and tetrazolium) being more easy to interpret when intake/rejection were to be considered. Tetrazolium staining is still the cheapest, easiest and most rapid to perform and interpret method to determine signs of pregermination in barley grains. If time and equipment are available staining barley with fluoroscein dibutyrate gives reliable and repeatable results but estimates of pregermination are normally lower than those obtained by tetrazolium staining. In this study a -amylase did not prove a good test for pregermination as it only identified grains in which pregermination was fairly advanced. This was due to the time of development of the enzyme (it is synthesised only after one to two days of germination). In terms of cost and equipment, for early indications, speed and precision, the tetrazolium test was preferred.

New methods for determining pregermination in barley grains have included measuring levels of catalase, peroxidase, acid phosphatase and b -glucanase and examining seeds by NIR, conductivity, density and X-ray transflectance. Of the first two enzymes, catalase was undetectable in raw barley, whether it was pregerminated or not. Peroxidase was measurable but levels did not vary sufficiently for it to be reliably used as an estimate of pregermination. Acid phosphatase did show some correlation with results obtained by tetrazolium staining but more work needs to be carried out to corroborate these findings. Staining of barley endosperm, in particular the area adjacent to the scutellum, as a result of b -glucanase activity has been shown to be a reflect the extent of pregermination but again the repeatability of this method needs to be confirmed.

Prediction of barley pregermination by NIR spectroscopy showed good correlation with tetrazolium and copper sulphate staining and conductivity measurements of whole grains suggested that pregermination gave lower increases in current, particularly over the first 5 minutes of assessment. Grain density measured by flotation in a liquid with a density gradient suggested that the more pregerminated a grain the lower its density. This difference in density was not due to varying levels of total nitrogen between samples but did vary with variety. It was observed however, that the majority of densities from non-pregerminated grain were higher than those of the pregerminated samples again suggesting that pregermination reduced a grain's density. In order to establish whether the difference in density is due to pregermination rather than variety it will be necessary to compare the density of pregerminated grain to a sample of non-pregerminated grain from the same variety. X-ray transflectance clearly showed the development of roots and shoots in pregerminated grains but the cost of the equipment may preclude its routine use by grain handlers. Each of the new methods needs to be validated using a greater number of samples taken from different harvests.

Simulation of pregermination by steeping and drying of dried and undried barley grains cannot be used to generate samples mirroring those which occur naturally in the field. In particular, the degree of variability associated with wetting and heating of the growing ear on the plant, under different growing conditions, were not able to be reproduced in a laboratory-scale trial by steeping and then gently drying the grain.

Storage of small batches of pregerminated barleys at ambient temperatures for up to 6 months had very little effect on their malting performance or malt quality.