Effect of elevated temperature on the microstructure of full fat Cheddar cheese during ripening

Highlights

• Ripening at elevated temperatures was found to alter the microstructure of cheese.

• Protein strands were thicker and pores larger after 33 days at higher temperatures.

• These structural changes coincided with increased proteolysis and altered texture.

• Microscopy and image analysis helped establish a structure-function relationship.

• The concentration of biogenic amines was low, confirming no health concerns.

Abstract

Elevated temperatures have been widely studied as a route to accelerate cheese ripening and decrease energy and storage requirements but the impact of temperature on the underlying microstructure of the cheese during prolonged periods of ripening is poorly understood. In this study, Cheddar cheese was matured at four different ripening temperatures (8 °C, 15 °C, 20 °C or a combination of 8 °C and 15 °C) and the impact on cheese microstructure assessed using confocal laser scanning microscopy, cryo scanning electron microscopy and quantitative image analysis of 3D images. An increase in ripening temperature was shown to alter the microstructure of the cheese protein network after only a few weeks of ripening. Incubation at 20 °C significantly reduced branching within the protein network, leading to thicker protein strands and larger pores after 33 days. These structural changes coincided with increased proteolysis, consistent with solubilisation of the protein network; they also led to a softer, less chewy and less cohesive cheese. While the concentration of biogenic amines tryptamine and tyramine were observed to increase with ripening temperature, the concentrations were generally low, confirming that biogenic amines do not represent a health concern under the conditions examined. This study illustrates how 3D image analysis can be used to observe and quantify the effect of process changes on cheese structure, assisting our understanding of the link between structure and function in Cheddar cheese.

Introduction

Ripening is a slow and lengthy process for rennet coagulated cheeses, typically lasting from three weeks to two or more years (Fox, 2002). During this period microbiological and biochemical changes occur, leading to changes in cheese texture and the development of flavour (McSweeney, 2004). Accelerated ripening of Cheddar can be accomplished by increasing the ripening temperature, the use of additional cultures (derived from starters or non-starter lactic acid bacteria (NSLAB) in live or attenuated form), or the use of enzymes (in many cases products of genetic modification, where legislatively approved) or cultures with high enzyme activities. Such acceleration can reduce the energy required for temperature control during maturation and decrease storage requirements leading to increased profitability. Among these options the application of a higher ripening temperature is the easiest to implement.

Our understanding of the impact of higher temperatures on the microstructure of Cheddar cheese is far from complete. The microbial and biochemical changes induced by high temperatures have been well documented for Cheddar (Aston, Giles, Durward, & Dulley, 1985; Cromie, Giles, & Dulley, 1987; Folkertsma, Fox, & McSweeney, 1996) and the link between accelerated flavour development and higher ripening temperatures established (Aston et al., 1985, Folkertsma et al., 1996, Hannon et al., 2005). Few studies, however, have probed the link between elevated ripening temperature and Cheddar microstructure. The link between this microstructure and changes in cheese properties has also not been studied.

Industrial Cheddar cheese is conventionally ripened at a constant temperature in the range of 6 °C–8 °C. Short periods of 70 h at 25 °C were shown to have little impact on the structure observed in Cheddar cheese by confocal laser scanning microscopy (CLSM) (O’Reilly et al., 2003). The structure observed was characteristic of Cheddar soon after manufacture, which is typically described as containing a continuous protein matrix with irregular shaped fat globules embedded within the protein network; curd junctions may also be present in the structure (Auty, Twomey, Guinee, & Mulvihill, 2001).

Proteolytic activity is known to be higher in Cheddar at elevated ripening temperatures (Aston et al., 1985, Folkertsma et al., 1996), leading to a softer cheese when ripened at 16 °C (Folkertsma et al., 1996). Lipolysis can also be accelerated (Folkertsma et al., 1996). Other textural changes reported include a more brittle and less springy Cheddar cheese when the temperature was increased to 20 °C (Fedrick & Dulley, 1984). These changes all indicate a breakdown in the protein network and potential rearrangement in the underlying microstructure of the cheese that could be visualised using microscopic techniques.

Another potential consequence of ripening at elevated temperatures, not linked directly to the microstructure, is that changes in the bacterial population and proteolysis may affect the concentration of amino acids and biogenic amines, such as putrescine, cadaverine and tyramine. Both amino acids and biogenic amines were observed to significantly increase with temperature and storage in Dutch type cheeses (Komprda et al., 2007; Pachlová, Buňka, Flasarová, Válková, & Buňková, 2012), potentially introducing a health hazard for consumers. The final concentration of amines in Cheddar cheeses ripened at high temperatures at the end of the accelerated ripening period would therefore be valuable data to assess the relative merits of incubation at high temperatures.

The aims of the current study were to investigate the impact of elevated temperature on the microstructure, texture and proteolysis of ripened Cheddar cheese, applying quantitative image analysis to establish structure-function relationships during ripening. Four treatments were applied: ripening at 8 °C, 15 °C, 20 °C, or a two-stage ripening process involving ripening at 15 °C for 33 days followed by ripening at 8 °C for the subsequent ripening period. The presence of biogenic amines was also assessed. This study provides new insights into the microstructural changes that occur within Cheddar during accelerated ripening and the link between changes in cheese structure and functionality.