Tulare County

Introduction

Raisin drying is mostly an "art" based on observation and experience. Many factors influence drying time, including harvest date, tray type, fullness of tray, and type of roll used; however, seasonal weather conditions always have the greatest effect. Historically, growers preferred a 22-pound tray, turned them, and rolled fully-cured raisins in a biscuit roll. Now, more than ever, there is a tendency for pickers to leave lighter trays and for growers to use flop or cigarette rolls to finish the drying process. Last year we initiated a study to begin quantifying some of these factors. For example, we wanted to measure how tray fullness and type of roll affected drying rate. The following are our findings in a 1994 study at the Kearney Ag. Center.

Tray Weight. A light tray can be ready to roll or box several days before a heavy tray. The following study evaluated this effect.

Trays with 16, 18, 20, and 22 pounds of fresh fruit were placed side by side down a row, and this was replicated eight times. Harvest occurred on August 29 using standard sized (24½" x 36"), wet strength paper trays. Trays were turned and then raisins boxed on Sept. 12. Raisin moisture content was measured with a dried-fruit moisture meter.

When raisins were drying, the average daily high and low temperature was 88.6°F and 51° F, respectively. The average daily high and low relative humidity was 75% and 22%, respectively.

Fruit on the 16 and 18 pound trays dried the fastest. When more than 18 pounds of fruit was loaded on trays, it took a little longer to make raisins. The 20 pound trays took a day longer to dry and the 22 pound trays took two days longer. (Table 1)

Two days doesn't seem like much difference, but the moisture content of raisins on open trays can drop from 16% to 12% in two days with normal drying conditions. This is demonstrated in Figure 1 showing the moisture content of fruit on the 20 pound trays from harvest to boxing (Aug. 29 to Sept. 12). Daily high temperatures for this 15 day period are also given. Note that temperatures were mild the last two days before boxing, yet raisin moisture dropped from 15% to 12%.

Type of Roll. The type of roll used to finish raisins affects the rate of moisture loss. Raisins dry slower in a biscuit roll than in a cigarette or flop. This was evaluated last year at the Kearney Agricultural Center.

We evaluated a cigarette, flop, and biscuit roll, and an unrolled or open tray. Harvest occurred on August 29 with 18 pounds of fruit placed on all trays. Paper trays were standard sized, wet strength, and trays were turned prior to rolling.

The cigarette, flop, and biscuit rolls were made on Sept. 8 when raisin moisture was ~20%, a little on the wet side. When trays were rolled, there were many uncured raisins ("frog bellies") and a few green berries.

Trays weights were taken every other day. Raisins were boxed on Sept. 16 and moisture content determined with a dried-fruit moisture meter. The average daily high and low temperature from Sept. 8 to Sept. 16, when raisins were rolled, was 83 °F and 51°F, respectively. The average daily high and low relative humidity was 88% and 28%, respectively.

Raisins in the biscuit roll dried the slowest and had a moisture content of 13% at boxing. Raisins in the flop and the cigarette rolls dried at the same rate. Flop rolls had 11.5% moisture and cigarette rolls had 11.0% moisture when boxed on Sept. 16. (Figure 2)

Note that raisins continue to dry rapidly when in rolls and under good drying conditions. During the eight days raisins were in the flop and cigarette rolls, moisture dropped from 20% to 11%, and in the biscuit roll moisture dropped from 20% to 13%. The moisture content of the open tray dropped from 20% to 9.5% during the same period.

Summary

We are not advocating that growers go to lighter trays or change rolling practices based on this study. Well-filled trays reduce harvest cost especially with increasing paper and labor costs. However, the results point to the potential of overdrying with light tray loads or leaving flop rolls in the field for a week or so under normal drying conditions. Also, it demonstrated the importance of tray load in judging drying rate and when to roll.

The rate of drying, and thus the amount of "curing", that occurred in rolls during this study is almost surprising. Certainly, it demonstrates the amount of "curing" that can be accomplished in the roll, especially when open-ended cigarette or flop rolls are used. Thus, rolling "on the heavy side", especially during rapid drying weather, can help avoid over-drying and caramelization. The amount of finishing needed should dictate which type of roll to use. While these measurements do not break new ground, they may help us fine-tune our judgement calls on field practices.

*Bill Peacock is a University of California Cooperative Extension Farm Advisor in Tulare County, and Pete Christensen is a Specialist with the Department of Viticulture and Enology, University of California, Davis.

Figure 1. Drying rate of raisins at Kearney Ag Center (1994) with corresponding air temperatures.

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Figure 2. Drying of raisins comparing flop, cigarette, and biscuit rolls and an unrolled (open) tray.*

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