Abstract
The popularity of craft beer in New Zealand and around the world continues to grow rapidly. In recent years, there has been a trend for increasing the diversity of beers flavour, much of which has been facilitated by the development of hop cultivars with distinct aroma characteristics. Hop aroma characteristics are produced from the volatile compounds present in the essential oils. New Zealand’s hop breeding programme has produced a range of hop cultivars (varieties) with distinct flavour profiles. However, New Zealand produces only 1% of the world’s hops. With distinct flavours and high demand, there is potential for New Zealand’s hop production to expand. For the first time in literature, this PhD thesis investigates the volatile profiles of New Zealand hops, as a function of cultivar, growing location, and harvest date, to provide further knowledge which will support the hop breeding programme and enable further development of hop cultivars in New Zealand.
The volatile compounds were analysed using headspace solid phase micro extraction and gas chromatography mass spectrometry (HS-SPME-GCMS). Data analysis was conducted using chemometrics, PCA, PLSDA or PLSR to classify the different samples and identify discriminating volatile compounds.
The volatile compounds were compared as a function of growing location (terroir) and harvesting period. To study the effect of terroir, this PhD project employed a systematic and comprehensive approach, starting by comparing hop samples sourced from different countries, then from different locations within New Zealand, and finally different commercial farms within the Tasman region in New Zealand. To investigate the effect of maturity on volatile compounds (influenced by advancing harvest date), the evolution of volatile compounds was studied for each of the 2019 and 2020 harvesting periods.
The first study compared the volatile profiles of four hop cultivars grown in the USA, UK and New Zealand. Cultivar (genetics) was the dominating variable over location (environment) that determined the volatile profile. Nevertheless, the New Zealand grown cultivars had a comparable volatile profile in contrast to those grown overseas. The second location study aimed at comparing two New Zealand hop growing locations (Tasman and Central Otago) based on six cultivars (Kohatu, Motueka, Nelson Sauvin, Taiheke, Wai-iti and Wakatu) over two harvest years (2019 and 2020). In line with the first study, cultivar had a dominant effect over location, but some differences between locations were also observed. After further investigation (using significant difference testing), terpenes and esters were found to be the dominant chemical classes that were significantly different between the two locations. As an example, methyl octanoate concentrations were significantly different between the two locations in Motueka, Nelson Sauvin, Wai-iti and Wakatu cultivars. Moreover, hop samples grown in the Tasman region had higher concentrations of methyl octanoate. The third location study focused on comparing hop samples from multiple farms in the primary New Zealand commercial hop region (Tasman). Four hop cultivars (Motueka, Nelson Sauvin, Taiheke and Wakatu) were collected from seven commercial hop farms. Cultivar was again the dominant effect over location. Subtle differences between each of the commercial farms found esters to be the most prevalent chemical class separating samples from the Moutere location farm from those grown in the Motueka area. These differences between farms could be due to several variables, including differences in microclimate and soil differences, but may also be influenced by extrinsic factors such as maturity (i.e. different harvest dates), handling, storage conditions, and processing (e.g. drying).
To understand the effect of maturity on hop volatile profiles, six New Zealand grown hop cultivars (Cascade, Riwaka, Nectaron, Nelson Sauvin, Dr Rudi and Waimea) were analysed over 19 days within the 2019 harvesting period. Results illustrated that the majority of volatiles increased with maturity. However, a number of volatile compounds demonstrated non-linear patterns in their concentration over the harvest period. An expanded harvest date study (over 32 days) was subsequently conducted on two New Zealand cultivars (Nectaron and Nelson Sauvin) during the 2020 harvesting period. Results found that the dominating trend was that volatile compounds increase with maturity of the hop plant, and continued to increase even after the customary harvesting time and maturity for the hops. In both the 2019 and 2020 harvesting periods, esters and terpenes were found to be the dominating chemical class that significantly changed with time of harvest.
Overall, the location experiments concluded that cultivar (genetics) was the dominating factor over growing location influencing the volatile profiles, while location has a subtle effect on some compounds, like esters, which may influence aroma character. Results from the location experiments illustrated that the majority of cultivars grown in different locations should still be recognisable by their volatile profiles, albeit indicting subtle differences. Some cultivars were more affected by location than others, namely Wai-iti and Wakatu. Harvest date experiments illustrated that most volatile compounds increased over the harvest period. This indicates that flavour profiles will likely change with increasing maturity and hops can be harvested at different time points to obtain different flavour profiles.
The information presented in this thesis can be used by hop breeders and farmers to further expand the New Zealand hop industry. Hop breeders can utilise the information to understand the volatile flavour profile of hops which will facilitate targeted breeding to select for specific compounds and aroma characteristics. The hop industry will be able to use this knowledge to understand the variability of hops between different locations and how the cultivars can produce similar volatile profiles when grown outside of the current commercial producing region. Hop farmers can use the knowledge gained to help understand how they can extend their growing area and the potential to diversify the regions where they grow hops. The harvest date results provides data to facilitate growers altering harvest timings to accentuate certain compounds and aroma characteristics.