Freeze-drying Blackcurrant Juice Foam – Measurement and Modelling of Drying Rate and Anthocyanin Retention

Blackcurrants (Ribes nigrum L.) are widely found in the market as juice or processed into spreads, infusions, purees and others. They contain high concentrations of anthocyanins which have been shown to have human health benefits. Anthocyanins are also used as food colorants. However, blackcurrants are highly perishable with a short shelf life, and thus drying is used to reduce harvest losses. A relatively new drying technology has been developed to reduce drying times of juices and other preparations. This technology consists in freeze drying a frozen foam (foam-mat freeze-drying).
This study compared foam-mat freeze-drying and conventional freeze drying to establish the extent to which the former reduces the drying time of blackcurrant juice, retains its anthocyanin content, and preserves its colour intensity after processing. It also analysed the changes of the characteristics of the foams due to anthocyanin and sugar contents and their effects on the drying rate. To address these questions, foams with different anthocyanin concentrations (made with egg albumin as foaming agent and xanthan gum as stabiliser) and non-foamed blackcurrant juice controls, were prepared and compared during the sublimation freeze-drying stage. A mathematical model was developed to describe differences between foamed and non–foamed
blackcurrant juice. In addition, the resulting dried powders were analysed for their anthocyanin content. To do this, a pepsin treatment was developed to separate the egg albumin from the anthocyanin before using light and fluorescence spectrometry to respectively quantify and identify anthocyanins.
Finally, colour spectrophotometry was used to quantify colour changes in dried samples stored for six months.
A faster drying rate in freeze-dried than in foam-mat freeze-dried juice was observed during the first stage of freeze-drying. This was consistent with the predictions of the mathematical model and was explained by the lower free
water content in foams. Foams prepared with different anthocyanin concentrations produced different bubble sizes, densities, air volume fractions and viscosities which influenced the drying rate. The foaming and drying
processes aided the formation of egg albumin–anthocyanin complexes. Foam mat freeze-dried juice with high and low anthocyanin content yielded an anthocyanin recovery of 39 and 70%, respectively, while freeze-dried juice obtained 83 and 93% for the same concentrations. Finally, the egg albumin presence in blackcurrant juice reduced the hue intensity after freeze-drying and during storage.