Place, season and drying

The Shennong Bencao Jing (神农本草经, Divine Farmer’s Classic of Materia Medica), compiled around the beginning of the common era and now known from later reconstructions, opens with general rules. One sentence could stand as the epigraph of this book.

Every variable that the earlier chapters of this book have measured is in that list: the land (Chapter 1), the time of harvest (Chapter 2’s age and season), drying in shade or sun (Chapters 3 and 5), raw or processed (Chapter 3’s gingerol and shogaol), genuine or false and old or new (Chapter 6’s identity tests and storage). What the classical texts did not have was a way to measure them.

Shihu: the upper grade

The Shennong Bencao Jing sorts its 365 drugs into three grades. The upper grade holds substances regarded as nourishing and safe for long use. Shihu (石斛, Dendrobium) is among them.

Two later texts add what the classic leaves out: where the plant grows and how it is dried.

In the Bencao Gangmu (本草纲目), completed in 1578, Li Shizhen gathered earlier writers’ notes on each drug. Under shihu he preserved the Song-dynasty scholar Su Song’s list of places where it was found, which includes “Wenzhou and Taizhou” (温州、台州) on the Zhejiang coast (李时珍 著；刘衡如、刘山水 校注 2011; 郑亚倩 et al. 2024). Yueqing, where the Yandang Mountains stand, lies within today’s Wenzhou.

The Mingyi Bielu’s instruction, shade-drying, is interesting in the light of Chapter 5. Shade-drying is slow and cool. It spares heat-sensitive compounds, but a stem that stays moist for weeks risks mould. Later processing texts record another approach: the Leigong Paozhi Lun has the stems “soaked in wine for a night, then steamed with ghee” (用酒浸一宿，却用酥蒸) (谢清华 et al. 2026). Modern producers who twist fresh stems into fengdou spirals and dry them over gentle heat are following a third path.

Dried ginger: the middle grade

Dried ginger (干姜, ganjiang) is in the middle grade of the Shennong Bencao Jing, the group of drugs regarded as useful for treating conditions but needing more care, not in the upper grade.

The last phrase, “the fresh one is especially good”, is a reminder that the classical authors knew fresh and dried ginger were different materials. Li Shizhen describes how dried ginger was made and judged.

“White, clean and firm” is a description of quality that a modern grader could still use: free of soil and mould, not shrivelled or hollow. It also hints that the best dried ginger was not dark, which in modern terms suggests limited browning during drying.

Shade, sun and oven, by calculation

Traditional texts distinguish drying in the shade (阴干), in the sun (晒干 or 暴干) and over a fire or in an oven (焙干). The kinetics of Chapter 3 can put rough numbers on what each does to the balance between gingerol and shogaol.

import math

R = 8.314
# Arrhenius constants fitted in Chapter 3 to Bhattarai et al. (2001), pH 1.
# Ginger tissue is far less acidic, so only the comparison between
# schedules means anything here, not the absolute numbers.
EA_F, LNA_F = 75_050, 22.624            # gingerol → shogaol
EA_R, LNA_R = 51_840, 14.870            # shogaol → gingerol

def shogaol_after(steps, s=0.0):
    """Share of the pair present as shogaol after (°C, hours) steps."""
    for t, hours in steps:
        T = t + 273.15
        kf = math.exp(LNA_F - EA_F / (R * T))
        kr = math.exp(LNA_R - EA_R / (R * T))
        eq = kf / (kf + kr)
        s = eq + (s - eq) * math.exp(-(kf + kr) * hours)
    return s

methods = {
    "shade-dried, 28 °C for 10 days":          [(28, 240)],
    "sun-dried, 4 × (40 °C 8 h + 26 °C 16 h)": [(40, 8), (26, 16)] * 4,
    "oven-dried, 70 °C for 8 h":               [(70, 8)],
    "freeze-dried, −25 °C 20 h + 25 °C 8 h":   [(-25, 20), (25, 8)],
}
for name, steps in methods.items():
    s = shogaol_after(steps)
    print(f"{name:41s} shogaol {100 * s:4.1f} %, "
          f"gingerol : shogaol = {(1 - s) / s:4.0f} : 1")
# shade-dried, 28 °C for 10 days            shogaol 10.4 %, gingerol : shogaol =    9 : 1
# sun-dried, 4 × (40 °C 8 h + 26 °C 16 h)   shogaol  7.8 %, gingerol : shogaol =   12 : 1
# oven-dried, 70 °C for 8 h                 shogaol 15.9 %, gingerol : shogaol =    5 : 1
# freeze-dried, −25 °C 20 h + 25 °C 8 h     shogaol  0.4 %, gingerol : shogaol =  264 : 1

The result is not what a quick guess would give. Ten days of shade-drying at a warm 28 °C produces more shogaol than four days of sun-drying with hotter afternoons, because time counts as well as temperature; this agrees with the experimental finding in Chapter 3 that drying time mattered more than air temperature for gingerol loss. An oven at 70 °C for eight hours converts more still. Freeze-drying converts almost nothing. Each tradition of processing was, in effect, choosing a point on this scale, and the classical preference for different preparations of ginger for different purposes (fresh, dried, and later charred 炮姜) corresponds to real chemical differences.

Daodi: the proper region

The idea of daodi (道地) medicinal materials, those from the regions recognised over centuries as the best sources, is the Chinese counterpart of terroir. Historical research on Dendrobium officinale traces how its recognised regions shifted over time, and how both place and processing were thought to shape quality (郑亚倩 et al. 2024).