BackgroundRaspberry ketone is the primary aroma compound found in raspberries and naturally derived raspberry ketone is a valuable flavoring agent. The economic incentives for the production of raspberry ketone, combined with the very poor yields from plant tissue, therefore make this compound an excellent target for heterologous production in synthetically engineered microbial strains.MethodsA de novo pathway for the production of raspberry ketone was assembled using four heterologous genes, encoding phenylalanine/tyrosine ammonia lyase, cinnamate-4-hydroxlase, coumarate-CoA ligase and benzalacetone synthase, in an industrial strain of Saccharomycescerevisiae. Synthetic protein fusions were also explored as a means of increasing yields of the final product.ResultsThe highest raspberry ketone concentration achieved in minimal media exceeded 7.5 mg/L when strains were fed with 3 mM p-coumaric acid; or 2.8 mg/L for complete de novo synthesis, both of which utilized a coumarate-CoA ligase, benzalacetone synthase synthetic fusion protein that increased yields over fivefold compared to the native enzymes. In addition, this strain was shown to be able to produce significant amounts of raspberry ketone in wine, with a raspberry ketone titer of 3.5 mg/L achieved after aerobic fermentation of Chardonnay juice or 0.68 mg/L under anaerobic winemaking conditions.ConclusionsWe have shown that it is possible to produce sensorially-relevant quantities of raspberry ketone in an industrial heterologous host. This paves the way for further pathway optimization to provide an economical alternative to raspberry ketone derived from plant sources.Electronic supplementary materialThe online version of this article (doi:10.1186/s12934-016-0446-2) contains supplementary material, which is available to authorized users.

Temperature can play a significant role in the development of wine at many stages during its lifetime. Elevated temperature, however, poses a significant risk to the sensory attributes of wine and its resultant shelf-life. Wines often experience difficult environmental conditions during transport and storage, and this can directly impact on the colour, aroma and mouthfeel of the wine. Higher and/or fluctuating temperature can essentially accelerate the ageing process. Unfortunately, these changes often go unnoticed until the wine reaches the consumer. Numerous studies have investigated the impact of elevated temperature on wines, with noticeable effect, such as reduction of sulfur dioxide, colour development (especially browning of white wines) and changes in the profile of volatile compounds, being common. Unfortunately, most of these studies tend to have a narrow scope and tend to focus only on a limited number of wine types or on specific compounds. The chemistry changes involved in heat-affected red wines are generally more complex than they are in white wines, but it is arguable that white wines are more sensitive to the effect of heat and therefore require the same or a greater level of research consideration with respect to temperature effects. The focus of this review is to highlight the common effects that different wine types and styles can experience when subjected to elevated storage temperature that are considered to be beyond the limits that most winemakers and consumers would accept. This review will also summarise the fundamental chemical kinetics that play a significant role in wine development at elevated temperature.

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The fermentations, at a commercial winery, of six different grape musts encompassing the varieties Riesling, Chardonnay, Sauvignon blanc, Shiraz, Grenache, and Pinot noir were monitored for damascenone concentration. In every case, the concentration of damascenone increased during fermentation from low or undetectable levels to concentrations of several parts per billion. Further increases in damascenone concentration were observed during barrel aging of three of these wines. Two ketones, megastigma-4,6,7-triene-3,9-dione (4) and 3-hydroxymegastigma-4,6,7-trien-9-one (5), were synthesized and subjected to fermentation conditions using two yeasts, AWRI 796, and AWRI 1537. In the case of the former compound, 4, synthesis confirmed the original, tentative assignment of the structure and confirmed 4 as a natural product, isolated from honey. Both compounds, under the action of both yeasts, produced appreciable amounts of damascenone (1), with ketone 5 and AWRI 796 yeast yielding the highest concentration of 1.